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<div class="section" id="command-line-and-environment">
<span id="using-on-general"></span><h1><span class="section-number">1. </span>Command line and environment<a class="headerlink" href="#command-line-and-environment" title="Permalink to this headline">¶</a></h1>
<p>The CPython interpreter scans the command line and the environment for various
settings.</p>
<div class="impl-detail compound">
<p><strong>CPython implementation detail:</strong> Other implementations’ command line schemes may differ. See
<a class="reference internal" href="../reference/introduction.html#implementations"><span class="std std-ref">Alternate Implementations</span></a> for further resources.</p>
</div>
<div class="section" id="command-line">
<span id="using-on-cmdline"></span><h2><span class="section-number">1.1. </span>Command line<a class="headerlink" href="#command-line" title="Permalink to this headline">¶</a></h2>
<p>When invoking Python, you may specify any of these options:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>python <span class="o">[</span>-bBdEhiIOqsSuvVWx?<span class="o">]</span> <span class="o">[</span>-c <span class="nb">command</span> <span class="p">|</span> -m module-name <span class="p">|</span> script <span class="p">|</span> - <span class="o">]</span> <span class="o">[</span>args<span class="o">]</span>
</pre></div>
</div>
<p>The most common use case is, of course, a simple invocation of a script:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>python myscript.py
</pre></div>
</div>
<div class="section" id="interface-options">
<span id="using-on-interface-options"></span><h3><span class="section-number">1.1.1. </span>Interface options<a class="headerlink" href="#interface-options" title="Permalink to this headline">¶</a></h3>
<p>The interpreter interface resembles that of the UNIX shell, but provides some
additional methods of invocation:</p>
<ul class="simple">
<li><p>When called with standard input connected to a tty device, it prompts for
commands and executes them until an EOF (an end-of-file character, you can
produce that with <kbd class="kbd docutils literal notranslate">Ctrl-D</kbd> on UNIX or <kbd class="kbd docutils literal notranslate">Ctrl-Z, Enter</kbd> on Windows) is read.</p></li>
<li><p>When called with a file name argument or with a file as standard input, it
reads and executes a script from that file.</p></li>
<li><p>When called with a directory name argument, it reads and executes an
appropriately named script from that directory.</p></li>
<li><p>When called with <code class="docutils literal notranslate"><span class="pre">-c</span> <span class="pre">command</span></code>, it executes the Python statement(s) given as
<em>command</em>. Here <em>command</em> may contain multiple statements separated by
newlines. Leading whitespace is significant in Python statements!</p></li>
<li><p>When called with <code class="docutils literal notranslate"><span class="pre">-m</span> <span class="pre">module-name</span></code>, the given module is located on the
Python module path and executed as a script.</p></li>
</ul>
<p>In non-interactive mode, the entire input is parsed before it is executed.</p>
<p>An interface option terminates the list of options consumed by the interpreter,
all consecutive arguments will end up in <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a> – note that the first
element, subscript zero (<code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code>), is a string reflecting the program’s
source.</p>
<dl class="cmdoption">
<dt id="cmdoption-c">
<code class="sig-name descname">-c</code><code class="sig-prename descclassname"> <command></code><a class="headerlink" href="#cmdoption-c" title="Permalink to this definition">¶</a></dt>
<dd><p>Execute the Python code in <em>command</em>. <em>command</em> can be one or more
statements separated by newlines, with significant leading whitespace as in
normal module code.</p>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a> will be
<code class="docutils literal notranslate"><span class="pre">"-c"</span></code> and the current directory will be added to the start of
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> (allowing modules in that directory to be imported as top
level modules).</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-m">
<code class="sig-name descname">-m</code><code class="sig-prename descclassname"> <module-name></code><a class="headerlink" href="#cmdoption-m" title="Permalink to this definition">¶</a></dt>
<dd><p>Search <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> for the named module and execute its contents as
the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module.</p>
<p>Since the argument is a <em>module</em> name, you must not give a file extension
(<code class="docutils literal notranslate"><span class="pre">.py</span></code>). The module name should be a valid absolute Python module name, but
the implementation may not always enforce this (e.g. it may allow you to
use a name that includes a hyphen).</p>
<p>Package names (including namespace packages) are also permitted. When a
package name is supplied instead
of a normal module, the interpreter will execute <code class="docutils literal notranslate"><span class="pre"><pkg>.__main__</span></code> as
the main module. This behaviour is deliberately similar to the handling
of directories and zipfiles that are passed to the interpreter as the
script argument.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This option cannot be used with built-in modules and extension modules
written in C, since they do not have Python module files. However, it
can still be used for precompiled modules, even if the original source
file is not available.</p>
</div>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a> will be the
full path to the module file (while the module file is being located, the
first element will be set to <code class="docutils literal notranslate"><span class="pre">"-m"</span></code>). As with the <a class="reference internal" href="#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> option,
the current directory will be added to the start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>.</p>
<p><a class="reference internal" href="#id2"><code class="xref std std-option docutils literal notranslate"><span class="pre">-I</span></code></a> option can be used to run the script in isolated mode where
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> contains neither the current directory nor the user’s
site-packages directory. All <span class="target" id="index-0"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHON*</span></code> environment variables are
ignored, too.</p>
<p>Many standard library modules contain code that is invoked on their execution
as a script. An example is the <a class="reference internal" href="../library/timeit.html#module-timeit" title="timeit: Measure the execution time of small code snippets."><code class="xref py py-mod docutils literal notranslate"><span class="pre">timeit</span></code></a> module:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>python -m timeit -s <span class="s1">'setup here'</span> <span class="s1">'benchmarked code here'</span>
python -m timeit -h <span class="c1"># for details</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference internal" href="../library/runpy.html#runpy.run_module" title="runpy.run_module"><code class="xref py py-func docutils literal notranslate"><span class="pre">runpy.run_module()</span></code></a></dt><dd><p>Equivalent functionality directly available to Python code</p>
</dd>
</dl>
<p><span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0338"><strong>PEP 338</strong></a> – Executing modules as scripts</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.1: </span>Supply the package name to run a <code class="docutils literal notranslate"><span class="pre">__main__</span></code> submodule.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.4: </span>namespace packages are also supported</p>
</div>
</dd></dl>
<span class="target" id="cmdarg-dash"></span><dl class="describe">
<dt>
<code class="sig-name descname">-</code></dt>
<dd><p>Read commands from standard input (<a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdin</span></code></a>). If standard input is
a terminal, <a class="reference internal" href="#cmdoption-i"><code class="xref std std-option docutils literal notranslate"><span class="pre">-i</span></code></a> is implied.</p>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a> will be
<code class="docutils literal notranslate"><span class="pre">"-"</span></code> and the current directory will be added to the start of
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>.</p>
</dd></dl>
<span class="target" id="cmdarg-script"></span><dl class="describe">
<dt>
<code class="sig-name descname"><script></code></dt>
<dd><p>Execute the Python code contained in <em>script</em>, which must be a filesystem
path (absolute or relative) referring to either a Python file, a directory
containing a <code class="docutils literal notranslate"><span class="pre">__main__.py</span></code> file, or a zipfile containing a
<code class="docutils literal notranslate"><span class="pre">__main__.py</span></code> file.</p>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a> will be the
script name as given on the command line.</p>
<p>If the script name refers directly to a Python file, the directory
containing that file is added to the start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>, and the
file is executed as the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module.</p>
<p>If the script name refers to a directory or zipfile, the script name is
added to the start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> and the <code class="docutils literal notranslate"><span class="pre">__main__.py</span></code> file in
that location is executed as the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module.</p>
<p><a class="reference internal" href="#id2"><code class="xref std std-option docutils literal notranslate"><span class="pre">-I</span></code></a> option can be used to run the script in isolated mode where
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> contains neither the script’s directory nor the user’s
site-packages directory. All <span class="target" id="index-2"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHON*</span></code> environment variables are
ignored, too.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference internal" href="../library/runpy.html#runpy.run_path" title="runpy.run_path"><code class="xref py py-func docutils literal notranslate"><span class="pre">runpy.run_path()</span></code></a></dt><dd><p>Equivalent functionality directly available to Python code</p>
</dd>
</dl>
</div>
</dd></dl>
<p>If no interface option is given, <a class="reference internal" href="#cmdoption-i"><code class="xref std std-option docutils literal notranslate"><span class="pre">-i</span></code></a> is implied, <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> is
an empty string (<code class="docutils literal notranslate"><span class="pre">""</span></code>) and the current directory will be added to the
start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>. Also, tab-completion and history editing is
automatically enabled, if available on your platform (see
<a class="reference internal" href="../library/site.html#rlcompleter-config"><span class="std std-ref">Readline configuration</span></a>).</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="../tutorial/interpreter.html#tut-invoking"><span class="std std-ref">Invoking the Interpreter</span></a></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.4: </span>Automatic enabling of tab-completion and history editing.</p>
</div>
</div>
<div class="section" id="generic-options">
<h3><span class="section-number">1.1.2. </span>Generic options<a class="headerlink" href="#generic-options" title="Permalink to this headline">¶</a></h3>
<dl class="cmdoption">
<dt id="cmdoption">
<code class="sig-name descname">-?</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption" title="Permalink to this definition">¶</a></dt>
<dt id="cmdoption-h">
<code class="sig-name descname">-h</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-h" title="Permalink to this definition">¶</a></dt>
<dt id="cmdoption-help">
<code class="sig-name descname">--help</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-help" title="Permalink to this definition">¶</a></dt>
<dd><p>Print a short description of all command line options.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-v">
<code class="sig-name descname">-V</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-v" title="Permalink to this definition">¶</a></dt>
<dt id="cmdoption-version">
<code class="sig-name descname">--version</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-version" title="Permalink to this definition">¶</a></dt>
<dd><p>Print the Python version number and exit. Example output could be:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Python 3.7.0b2+
</pre></div>
</div>
<p>When given twice, print more information about the build, like:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Python 3.7.0b2+ (3.7:0c076caaa8, Sep 22 2018, 12:04:24)
[GCC 6.2.0 20161005]
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6: </span>The <code class="docutils literal notranslate"><span class="pre">-VV</span></code> option.</p>
</div>
</dd></dl>
</div>
<div class="section" id="miscellaneous-options">
<span id="using-on-misc-options"></span><h3><span class="section-number">1.1.3. </span>Miscellaneous options<a class="headerlink" href="#miscellaneous-options" title="Permalink to this headline">¶</a></h3>
<dl class="cmdoption">
<dt id="cmdoption-b">
<code class="sig-name descname">-b</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-b" title="Permalink to this definition">¶</a></dt>
<dd><p>Issue a warning when comparing <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> or <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytearray</span></code></a> with
<a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> or <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> with <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>. Issue an error when the
option is given twice (<code class="xref std std-option docutils literal notranslate"><span class="pre">-bb</span></code>).</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>Affects comparisons of <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> with <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>.</p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="id1">
<code class="sig-name descname">-B</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#id1" title="Permalink to this definition">¶</a></dt>
<dd><p>If given, Python won’t try to write <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files on the
import of source modules. See also <span class="target" id="index-3"></span><a class="reference internal" href="#envvar-PYTHONDONTWRITEBYTECODE"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONDONTWRITEBYTECODE</span></code></a>.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-check-hash-based-pycs">
<code class="sig-name descname">--check-hash-based-pycs</code><code class="sig-prename descclassname"> default|always|never</code><a class="headerlink" href="#cmdoption-check-hash-based-pycs" title="Permalink to this definition">¶</a></dt>
<dd><p>Control the validation behavior of hash-based <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files. See
<a class="reference internal" href="../reference/import.html#pyc-invalidation"><span class="std std-ref">Cached bytecode invalidation</span></a>. When set to <code class="docutils literal notranslate"><span class="pre">default</span></code>, checked and unchecked
hash-based bytecode cache files are validated according to their default
semantics. When set to <code class="docutils literal notranslate"><span class="pre">always</span></code>, all hash-based <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files, whether
checked or unchecked, are validated against their corresponding source
file. When set to <code class="docutils literal notranslate"><span class="pre">never</span></code>, hash-based <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files are not validated
against their corresponding source files.</p>
<p>The semantics of timestamp-based <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files are unaffected by this
option.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-d">
<code class="sig-name descname">-d</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-d" title="Permalink to this definition">¶</a></dt>
<dd><p>Turn on parser debugging output (for expert only, depending on compilation
options). See also <span class="target" id="index-4"></span><a class="reference internal" href="#envvar-PYTHONDEBUG"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONDEBUG</span></code></a>.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-e">
<code class="sig-name descname">-E</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-e" title="Permalink to this definition">¶</a></dt>
<dd><p>Ignore all <span class="target" id="index-5"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHON*</span></code> environment variables, e.g.
<span class="target" id="index-6"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPATH</span></code></a> and <span class="target" id="index-7"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHOME</span></code></a>, that might be set.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-i">
<code class="sig-name descname">-i</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-i" title="Permalink to this definition">¶</a></dt>
<dd><p>When a script is passed as first argument or the <a class="reference internal" href="#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> option is used,
enter interactive mode after executing the script or the command, even when
<a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdin</span></code></a> does not appear to be a terminal. The
<span class="target" id="index-8"></span><a class="reference internal" href="#envvar-PYTHONSTARTUP"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONSTARTUP</span></code></a> file is not read.</p>
<p>This can be useful to inspect global variables or a stack trace when a script
raises an exception. See also <span class="target" id="index-9"></span><a class="reference internal" href="#envvar-PYTHONINSPECT"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONINSPECT</span></code></a>.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="id2">
<code class="sig-name descname">-I</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#id2" title="Permalink to this definition">¶</a></dt>
<dd><p>Run Python in isolated mode. This also implies -E and -s.
In isolated mode <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> contains neither the script’s directory nor
the user’s site-packages directory. All <span class="target" id="index-10"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHON*</span></code> environment
variables are ignored, too. Further restrictions may be imposed to prevent
the user from injecting malicious code.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-o">
<code class="sig-name descname">-O</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-o" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove assert statements and any code conditional on the value of
<a class="reference internal" href="../library/constants.html#__debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a>. Augment the filename for compiled
(<a class="reference internal" href="../glossary.html#term-bytecode"><span class="xref std std-term">bytecode</span></a>) files by adding <code class="docutils literal notranslate"><span class="pre">.opt-1</span></code> before the <code class="docutils literal notranslate"><span class="pre">.pyc</span></code>
extension (see <span class="target" id="index-11"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0488"><strong>PEP 488</strong></a>). See also <span class="target" id="index-12"></span><a class="reference internal" href="#envvar-PYTHONOPTIMIZE"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONOPTIMIZE</span></code></a>.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>Modify <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> filenames according to <span class="target" id="index-13"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0488"><strong>PEP 488</strong></a>.</p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-oo">
<code class="sig-name descname">-OO</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-oo" title="Permalink to this definition">¶</a></dt>
<dd><p>Do <a class="reference internal" href="#cmdoption-o"><code class="xref std std-option docutils literal notranslate"><span class="pre">-O</span></code></a> and also discard docstrings. Augment the filename
for compiled (<a class="reference internal" href="../glossary.html#term-bytecode"><span class="xref std std-term">bytecode</span></a>) files by adding <code class="docutils literal notranslate"><span class="pre">.opt-2</span></code> before the
<code class="docutils literal notranslate"><span class="pre">.pyc</span></code> extension (see <span class="target" id="index-14"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0488"><strong>PEP 488</strong></a>).</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>Modify <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> filenames according to <span class="target" id="index-15"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0488"><strong>PEP 488</strong></a>.</p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-q">
<code class="sig-name descname">-q</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-q" title="Permalink to this definition">¶</a></dt>
<dd><p>Don’t display the copyright and version messages even in interactive mode.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-r">
<code class="sig-name descname">-R</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-r" title="Permalink to this definition">¶</a></dt>
<dd><p>Turn on hash randomization. This option only has an effect if the
<span class="target" id="index-16"></span><a class="reference internal" href="#envvar-PYTHONHASHSEED"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHASHSEED</span></code></a> environment variable is set to <code class="docutils literal notranslate"><span class="pre">0</span></code>, since hash
randomization is enabled by default.</p>
<p>On previous versions of Python, this option turns on hash randomization,
so that the <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> values of str, bytes and datetime
are “salted” with an unpredictable random value. Although they remain
constant within an individual Python process, they are not predictable
between repeated invocations of Python.</p>
<p>Hash randomization is intended to provide protection against a
denial-of-service caused by carefully-chosen inputs that exploit the worst
case performance of a dict construction, O(n^2) complexity. See
<a class="reference external" href="http://www.ocert.org/advisories/ocert-2011-003.html">http://www.ocert.org/advisories/ocert-2011-003.html</a> for details.</p>
<p><span class="target" id="index-17"></span><a class="reference internal" href="#envvar-PYTHONHASHSEED"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHASHSEED</span></code></a> allows you to set a fixed value for the hash
seed secret.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7: </span>The option is no longer ignored.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.3.</span></p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-s">
<code class="sig-name descname">-s</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-s" title="Permalink to this definition">¶</a></dt>
<dd><p>Don’t add the <a class="reference internal" href="../library/site.html#site.USER_SITE" title="site.USER_SITE"><code class="xref py py-data docutils literal notranslate"><span class="pre">user</span> <span class="pre">site-packages</span> <span class="pre">directory</span></code></a> to
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><span class="target" id="index-18"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0370"><strong>PEP 370</strong></a> – Per user site-packages directory</p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="id3">
<code class="sig-name descname">-S</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#id3" title="Permalink to this definition">¶</a></dt>
<dd><p>Disable the import of the module <a class="reference internal" href="../library/site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal notranslate"><span class="pre">site</span></code></a> and the site-dependent
manipulations of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> that it entails. Also disable these
manipulations if <a class="reference internal" href="../library/site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal notranslate"><span class="pre">site</span></code></a> is explicitly imported later (call
<a class="reference internal" href="../library/site.html#site.main" title="site.main"><code class="xref py py-func docutils literal notranslate"><span class="pre">site.main()</span></code></a> if you want them to be triggered).</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-u">
<code class="sig-name descname">-u</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-u" title="Permalink to this definition">¶</a></dt>
<dd><p>Force the stdout and stderr streams to be unbuffered. This option has no
effect on the stdin stream.</p>
<p>See also <span class="target" id="index-19"></span><a class="reference internal" href="#envvar-PYTHONUNBUFFERED"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONUNBUFFERED</span></code></a>.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7: </span>The text layer of the stdout and stderr streams now is unbuffered.</p>
</div>
</dd></dl>
<dl class="cmdoption">
<dt id="id4">
<code class="sig-name descname">-v</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#id4" title="Permalink to this definition">¶</a></dt>
<dd><p>Print a message each time a module is initialized, showing the place
(filename or built-in module) from which it is loaded. When given twice
(<code class="xref std std-option docutils literal notranslate"><span class="pre">-vv</span></code>), print a message for each file that is checked for when
searching for a module. Also provides information on module cleanup at exit.
See also <span class="target" id="index-20"></span><a class="reference internal" href="#envvar-PYTHONVERBOSE"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONVERBOSE</span></code></a>.</p>
</dd></dl>
<span class="target" id="using-on-warnings"></span><dl class="cmdoption">
<dt id="cmdoption-w">
<code class="sig-name descname">-W</code><code class="sig-prename descclassname"> arg</code><a class="headerlink" href="#cmdoption-w" title="Permalink to this definition">¶</a></dt>
<dd><p>Warning control. Python’s warning machinery by default prints warning
messages to <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a>. A typical warning message has the following
form:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>file:line: category: message
</pre></div>
</div>
<p>By default, each warning is printed once for each source line where it
occurs. This option controls how often warnings are printed.</p>
<p>Multiple <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> options may be given; when a warning matches more than
one option, the action for the last matching option is performed. Invalid
<a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> options are ignored (though, a warning message is printed about
invalid options when the first warning is issued).</p>
<p>Warnings can also be controlled using the <span class="target" id="index-21"></span><a class="reference internal" href="#envvar-PYTHONWARNINGS"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONWARNINGS</span></code></a>
environment variable and from within a Python program using the
<a class="reference internal" href="../library/warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><code class="xref py py-mod docutils literal notranslate"><span class="pre">warnings</span></code></a> module.</p>
<p>The simplest settings apply a particular action unconditionally to all
warnings emitted by a process (even those that are otherwise ignored by
default):</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>-Wdefault <span class="c1"># Warn once per call location</span>
-Werror <span class="c1"># Convert to exceptions</span>
-Walways <span class="c1"># Warn every time</span>
-Wmodule <span class="c1"># Warn once per calling module</span>
-Wonce <span class="c1"># Warn once per Python process</span>
-Wignore <span class="c1"># Never warn</span>
</pre></div>
</div>
<p>The action names can be abbreviated as desired (e.g. <code class="docutils literal notranslate"><span class="pre">-Wi</span></code>, <code class="docutils literal notranslate"><span class="pre">-Wd</span></code>,
<code class="docutils literal notranslate"><span class="pre">-Wa</span></code>, <code class="docutils literal notranslate"><span class="pre">-We</span></code>) and the interpreter will resolve them to the appropriate
action name.</p>
<p>See <a class="reference internal" href="../library/warnings.html#warning-filter"><span class="std std-ref">The Warnings Filter</span></a> and <a class="reference internal" href="../library/warnings.html#describing-warning-filters"><span class="std std-ref">Describing Warning Filters</span></a> for more
details.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="cmdoption-x">
<code class="sig-name descname">-x</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-x" title="Permalink to this definition">¶</a></dt>
<dd><p>Skip the first line of the source, allowing use of non-Unix forms of
<code class="docutils literal notranslate"><span class="pre">#!cmd</span></code>. This is intended for a DOS specific hack only.</p>
</dd></dl>
<dl class="cmdoption">
<dt id="id5">
<code class="sig-name descname">-X</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#id5" title="Permalink to this definition">¶</a></dt>
<dd><p>Reserved for various implementation-specific options. CPython currently
defines the following possible values:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">faulthandler</span></code> to enable <a class="reference internal" href="../library/faulthandler.html#module-faulthandler" title="faulthandler: Dump the Python traceback."><code class="xref py py-mod docutils literal notranslate"><span class="pre">faulthandler</span></code></a>;</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">showrefcount</span></code> to output the total reference count and number of used
memory blocks when the program finishes or after each statement in the
interactive interpreter. This only works on debug builds.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">tracemalloc</span></code> to start tracing Python memory allocations using the
<a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> module. By default, only the most recent frame is
stored in a traceback of a trace. Use <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">tracemalloc=NFRAME</span></code> to start
tracing with a traceback limit of <em>NFRAME</em> frames. See the
<a class="reference internal" href="../library/tracemalloc.html#tracemalloc.start" title="tracemalloc.start"><code class="xref py py-func docutils literal notranslate"><span class="pre">tracemalloc.start()</span></code></a> for more information.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">showalloccount</span></code> to output the total count of allocated objects for
each type when the program finishes. This only works when Python was built with
<code class="docutils literal notranslate"><span class="pre">COUNT_ALLOCS</span></code> defined.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">importtime</span></code> to show how long each import takes. It shows module
name, cumulative time (including nested imports) and self time (excluding
nested imports). Note that its output may be broken in multi-threaded
application. Typical usage is <code class="docutils literal notranslate"><span class="pre">python3</span> <span class="pre">-X</span> <span class="pre">importtime</span> <span class="pre">-c</span> <span class="pre">'import</span>
<span class="pre">asyncio'</span></code>. See also <span class="target" id="index-22"></span><a class="reference internal" href="#envvar-PYTHONPROFILEIMPORTTIME"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPROFILEIMPORTTIME</span></code></a>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">dev</span></code>: enable CPython’s “development mode”, introducing additional
runtime checks which are too expensive to be enabled by default. It should
not be more verbose than the default if the code is correct: new warnings
are only emitted when an issue is detected. Effect of the developer mode:</p>
<ul>
<li><p>Add <code class="docutils literal notranslate"><span class="pre">default</span></code> warning filter, as <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> <code class="docutils literal notranslate"><span class="pre">default</span></code>.</p></li>
<li><p>Install debug hooks on memory allocators: see the
<a class="reference internal" href="../c-api/memory.html#c.PyMem_SetupDebugHooks" title="PyMem_SetupDebugHooks"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_SetupDebugHooks()</span></code></a> C function.</p></li>
<li><p>Enable the <a class="reference internal" href="../library/faulthandler.html#module-faulthandler" title="faulthandler: Dump the Python traceback."><code class="xref py py-mod docutils literal notranslate"><span class="pre">faulthandler</span></code></a> module to dump the Python traceback
on a crash.</p></li>
<li><p>Enable <a class="reference internal" href="../library/asyncio-dev.html#asyncio-debug-mode"><span class="std std-ref">asyncio debug mode</span></a>.</p></li>
<li><p>Set the <code class="xref py py-attr docutils literal notranslate"><span class="pre">dev_mode</span></code> attribute of <a class="reference internal" href="../library/sys.html#sys.flags" title="sys.flags"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.flags</span></code></a> to
<code class="docutils literal notranslate"><span class="pre">True</span></code>.</p></li>
</ul>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">utf8</span></code> enables UTF-8 mode for operating system interfaces, overriding
the default locale-aware mode. <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">utf8=0</span></code> explicitly disables UTF-8
mode (even when it would otherwise activate automatically).
See <span class="target" id="index-23"></span><a class="reference internal" href="#envvar-PYTHONUTF8"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONUTF8</span></code></a> for more details.</p></li>
</ul>
<p>It also allows passing arbitrary values and retrieving them through the
<a class="reference internal" href="../library/sys.html#sys._xoptions" title="sys._xoptions"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys._xoptions</span></code></a> dictionary.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.2: </span>The <a class="reference internal" href="#id5"><code class="xref std std-option docutils literal notranslate"><span class="pre">-X</span></code></a> option was added.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.3: </span>The <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">faulthandler</span></code> option.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4: </span>The <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">showrefcount</span></code> and <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">tracemalloc</span></code> options.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6: </span>The <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">showalloccount</span></code> option.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7: </span>The <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">importtime</span></code>, <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">dev</span></code> and <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">utf8</span></code> options.</p>
</div>
</dd></dl>
</div>
<div class="section" id="options-you-shouldn-t-use">
<h3><span class="section-number">1.1.4. </span>Options you shouldn’t use<a class="headerlink" href="#options-you-shouldn-t-use" title="Permalink to this headline">¶</a></h3>
<dl class="cmdoption">
<dt id="cmdoption-j">
<code class="sig-name descname">-J</code><code class="sig-prename descclassname"></code><a class="headerlink" href="#cmdoption-j" title="Permalink to this definition">¶</a></dt>
<dd><p>Reserved for use by <a class="reference external" href="http://www.jython.org/">Jython</a>.</p>
</dd></dl>
</div>
</div>
<div class="section" id="environment-variables">
<span id="using-on-envvars"></span><h2><span class="section-number">1.2. </span>Environment variables<a class="headerlink" href="#environment-variables" title="Permalink to this headline">¶</a></h2>
<p>These environment variables influence Python’s behavior, they are processed
before the command-line switches other than -E or -I. It is customary that
command-line switches override environmental variables where there is a
conflict.</p>
<dl class="envvar">
<dt id="envvar-PYTHONHOME">
<code class="sig-name descname">PYTHONHOME</code><a class="headerlink" href="#envvar-PYTHONHOME" title="Permalink to this definition">¶</a></dt>
<dd><p>Change the location of the standard Python libraries. By default, the
libraries are searched in <code class="file docutils literal notranslate"><em><span class="pre">prefix</span></em><span class="pre">/lib/python</span><em><span class="pre">version</span></em></code> and
<code class="file docutils literal notranslate"><em><span class="pre">exec_prefix</span></em><span class="pre">/lib/python</span><em><span class="pre">version</span></em></code>, where <code class="file docutils literal notranslate"><em><span class="pre">prefix</span></em></code> and
<code class="file docutils literal notranslate"><em><span class="pre">exec_prefix</span></em></code> are installation-dependent directories, both defaulting
to <code class="file docutils literal notranslate"><span class="pre">/usr/local</span></code>.</p>
<p>When <span class="target" id="index-24"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHOME</span></code></a> is set to a single directory, its value replaces
both <code class="file docutils literal notranslate"><em><span class="pre">prefix</span></em></code> and <code class="file docutils literal notranslate"><em><span class="pre">exec_prefix</span></em></code>. To specify different values
for these, set <span class="target" id="index-25"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHOME</span></code></a> to <code class="file docutils literal notranslate"><em><span class="pre">prefix</span></em><span class="pre">:</span><em><span class="pre">exec_prefix</span></em></code>.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONPATH">
<code class="sig-name descname">PYTHONPATH</code><a class="headerlink" href="#envvar-PYTHONPATH" title="Permalink to this definition">¶</a></dt>
<dd><p>Augment the default search path for module files. The format is the same as
the shell’s <span class="target" id="index-26"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PATH</span></code>: one or more directory pathnames separated by
<a class="reference internal" href="../library/os.html#os.pathsep" title="os.pathsep"><code class="xref py py-data docutils literal notranslate"><span class="pre">os.pathsep</span></code></a> (e.g. colons on Unix or semicolons on Windows).
Non-existent directories are silently ignored.</p>
<p>In addition to normal directories, individual <span class="target" id="index-27"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPATH</span></code></a> entries
may refer to zipfiles containing pure Python modules (in either source or
compiled form). Extension modules cannot be imported from zipfiles.</p>
<p>The default search path is installation dependent, but generally begins with
<code class="file docutils literal notranslate"><em><span class="pre">prefix</span></em><span class="pre">/lib/python</span><em><span class="pre">version</span></em></code> (see <span class="target" id="index-28"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHOME</span></code></a> above). It
is <em>always</em> appended to <span class="target" id="index-29"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPATH</span></code></a>.</p>
<p>An additional directory will be inserted in the search path in front of
<span class="target" id="index-30"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPATH</span></code></a> as described above under
<a class="reference internal" href="#using-on-interface-options"><span class="std std-ref">Interface options</span></a>. The search path can be manipulated from
within a Python program as the variable <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONSTARTUP">
<code class="sig-name descname">PYTHONSTARTUP</code><a class="headerlink" href="#envvar-PYTHONSTARTUP" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is the name of a readable file, the Python commands in that file are
executed before the first prompt is displayed in interactive mode. The file
is executed in the same namespace where interactive commands are executed so
that objects defined or imported in it can be used without qualification in
the interactive session. You can also change the prompts <a class="reference internal" href="../library/sys.html#sys.ps1" title="sys.ps1"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.ps1</span></code></a> and
<a class="reference internal" href="../library/sys.html#sys.ps2" title="sys.ps2"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.ps2</span></code></a> and the hook <a class="reference internal" href="../library/sys.html#sys.__interactivehook__" title="sys.__interactivehook__"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.__interactivehook__</span></code></a> in this file.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONOPTIMIZE">
<code class="sig-name descname">PYTHONOPTIMIZE</code><a class="headerlink" href="#envvar-PYTHONOPTIMIZE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-o"><code class="xref std std-option docutils literal notranslate"><span class="pre">-O</span></code></a> option. If set to an integer, it is equivalent to specifying
<a class="reference internal" href="#cmdoption-o"><code class="xref std std-option docutils literal notranslate"><span class="pre">-O</span></code></a> multiple times.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONBREAKPOINT">
<code class="sig-name descname">PYTHONBREAKPOINT</code><a class="headerlink" href="#envvar-PYTHONBREAKPOINT" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set, it names a callable using dotted-path notation. The module
containing the callable will be imported and then the callable will be run
by the default implementation of <a class="reference internal" href="../library/sys.html#sys.breakpointhook" title="sys.breakpointhook"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.breakpointhook()</span></code></a> which itself is
called by built-in <a class="reference internal" href="../library/functions.html#breakpoint" title="breakpoint"><code class="xref py py-func docutils literal notranslate"><span class="pre">breakpoint()</span></code></a>. If not set, or set to the empty
string, it is equivalent to the value “pdb.set_trace”. Setting this to the
string “0” causes the default implementation of <a class="reference internal" href="../library/sys.html#sys.breakpointhook" title="sys.breakpointhook"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.breakpointhook()</span></code></a>
to do nothing but return immediately.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONDEBUG">
<code class="sig-name descname">PYTHONDEBUG</code><a class="headerlink" href="#envvar-PYTHONDEBUG" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-d"><code class="xref std std-option docutils literal notranslate"><span class="pre">-d</span></code></a> option. If set to an integer, it is equivalent to specifying
<a class="reference internal" href="#cmdoption-d"><code class="xref std std-option docutils literal notranslate"><span class="pre">-d</span></code></a> multiple times.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONINSPECT">
<code class="sig-name descname">PYTHONINSPECT</code><a class="headerlink" href="#envvar-PYTHONINSPECT" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-i"><code class="xref std std-option docutils literal notranslate"><span class="pre">-i</span></code></a> option.</p>
<p>This variable can also be modified by Python code using <a class="reference internal" href="../library/os.html#os.environ" title="os.environ"><code class="xref py py-data docutils literal notranslate"><span class="pre">os.environ</span></code></a>
to force inspect mode on program termination.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONUNBUFFERED">
<code class="sig-name descname">PYTHONUNBUFFERED</code><a class="headerlink" href="#envvar-PYTHONUNBUFFERED" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-u"><code class="xref std std-option docutils literal notranslate"><span class="pre">-u</span></code></a> option.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONVERBOSE">
<code class="sig-name descname">PYTHONVERBOSE</code><a class="headerlink" href="#envvar-PYTHONVERBOSE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#id4"><code class="xref std std-option docutils literal notranslate"><span class="pre">-v</span></code></a> option. If set to an integer, it is equivalent to specifying
<a class="reference internal" href="#id4"><code class="xref std std-option docutils literal notranslate"><span class="pre">-v</span></code></a> multiple times.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONCASEOK">
<code class="sig-name descname">PYTHONCASEOK</code><a class="headerlink" href="#envvar-PYTHONCASEOK" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set, Python ignores case in <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statements. This
only works on Windows and OS X.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONDONTWRITEBYTECODE">
<code class="sig-name descname">PYTHONDONTWRITEBYTECODE</code><a class="headerlink" href="#envvar-PYTHONDONTWRITEBYTECODE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string, Python won’t try to write <code class="docutils literal notranslate"><span class="pre">.pyc</span></code>
files on the import of source modules. This is equivalent to
specifying the <a class="reference internal" href="#id1"><code class="xref std std-option docutils literal notranslate"><span class="pre">-B</span></code></a> option.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONHASHSEED">
<code class="sig-name descname">PYTHONHASHSEED</code><a class="headerlink" href="#envvar-PYTHONHASHSEED" title="Permalink to this definition">¶</a></dt>
<dd><p>If this variable is not set or set to <code class="docutils literal notranslate"><span class="pre">random</span></code>, a random value is used
to seed the hashes of str, bytes and datetime objects.</p>
<p>If <span class="target" id="index-31"></span><a class="reference internal" href="#envvar-PYTHONHASHSEED"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHASHSEED</span></code></a> is set to an integer value, it is used as a fixed
seed for generating the hash() of the types covered by the hash
randomization.</p>
<p>Its purpose is to allow repeatable hashing, such as for selftests for the
interpreter itself, or to allow a cluster of python processes to share hash
values.</p>
<p>The integer must be a decimal number in the range [0,4294967295]. Specifying
the value 0 will disable hash randomization.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.3.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONIOENCODING">
<code class="sig-name descname">PYTHONIOENCODING</code><a class="headerlink" href="#envvar-PYTHONIOENCODING" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set before running the interpreter, it overrides the encoding used
for stdin/stdout/stderr, in the syntax <code class="docutils literal notranslate"><span class="pre">encodingname:errorhandler</span></code>. Both
the <code class="docutils literal notranslate"><span class="pre">encodingname</span></code> and the <code class="docutils literal notranslate"><span class="pre">:errorhandler</span></code> parts are optional and have
the same meaning as in <a class="reference internal" href="../library/stdtypes.html#str.encode" title="str.encode"><code class="xref py py-func docutils literal notranslate"><span class="pre">str.encode()</span></code></a>.</p>
<p>For stderr, the <code class="docutils literal notranslate"><span class="pre">:errorhandler</span></code> part is ignored; the handler will always be
<code class="docutils literal notranslate"><span class="pre">'backslashreplace'</span></code>.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.4: </span>The <code class="docutils literal notranslate"><span class="pre">encodingname</span></code> part is now optional.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.6: </span>On Windows, the encoding specified by this variable is ignored for interactive
console buffers unless <span class="target" id="index-32"></span><a class="reference internal" href="#envvar-PYTHONLEGACYWINDOWSSTDIO"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONLEGACYWINDOWSSTDIO</span></code></a> is also specified.
Files and pipes redirected through the standard streams are not affected.</p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONNOUSERSITE">
<code class="sig-name descname">PYTHONNOUSERSITE</code><a class="headerlink" href="#envvar-PYTHONNOUSERSITE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set, Python won’t add the <a class="reference internal" href="../library/site.html#site.USER_SITE" title="site.USER_SITE"><code class="xref py py-data docutils literal notranslate"><span class="pre">user</span> <span class="pre">site-packages</span> <span class="pre">directory</span></code></a> to <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><span class="target" id="index-33"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0370"><strong>PEP 370</strong></a> – Per user site-packages directory</p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONUSERBASE">
<code class="sig-name descname">PYTHONUSERBASE</code><a class="headerlink" href="#envvar-PYTHONUSERBASE" title="Permalink to this definition">¶</a></dt>
<dd><p>Defines the <a class="reference internal" href="../library/site.html#site.USER_BASE" title="site.USER_BASE"><code class="xref py py-data docutils literal notranslate"><span class="pre">user</span> <span class="pre">base</span> <span class="pre">directory</span></code></a>, which is used to
compute the path of the <a class="reference internal" href="../library/site.html#site.USER_SITE" title="site.USER_SITE"><code class="xref py py-data docutils literal notranslate"><span class="pre">user</span> <span class="pre">site-packages</span> <span class="pre">directory</span></code></a>
and <a class="reference internal" href="../install/index.html#inst-alt-install-user"><span class="std std-ref">Distutils installation paths</span></a> for
<code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">setup.py</span> <span class="pre">install</span> <span class="pre">--user</span></code>.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><span class="target" id="index-34"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0370"><strong>PEP 370</strong></a> – Per user site-packages directory</p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONEXECUTABLE">
<code class="sig-name descname">PYTHONEXECUTABLE</code><a class="headerlink" href="#envvar-PYTHONEXECUTABLE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set, <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> will be set to its
value instead of the value got through the C runtime. Only works on
Mac OS X.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONWARNINGS">
<code class="sig-name descname">PYTHONWARNINGS</code><a class="headerlink" href="#envvar-PYTHONWARNINGS" title="Permalink to this definition">¶</a></dt>
<dd><p>This is equivalent to the <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> option. If set to a comma
separated string, it is equivalent to specifying <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> multiple
times, with filters later in the list taking precedence over those earlier
in the list.</p>
<p>The simplest settings apply a particular action unconditionally to all
warnings emitted by a process (even those that are otherwise ignored by
default):</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span><span class="nv">PYTHONWARNINGS</span><span class="o">=</span>default <span class="c1"># Warn once per call location</span>
<span class="nv">PYTHONWARNINGS</span><span class="o">=</span>error <span class="c1"># Convert to exceptions</span>
<span class="nv">PYTHONWARNINGS</span><span class="o">=</span>always <span class="c1"># Warn every time</span>
<span class="nv">PYTHONWARNINGS</span><span class="o">=</span>module <span class="c1"># Warn once per calling module</span>
<span class="nv">PYTHONWARNINGS</span><span class="o">=</span>once <span class="c1"># Warn once per Python process</span>
<span class="nv">PYTHONWARNINGS</span><span class="o">=</span>ignore <span class="c1"># Never warn</span>
</pre></div>
</div>
<p>See <a class="reference internal" href="../library/warnings.html#warning-filter"><span class="std std-ref">The Warnings Filter</span></a> and <a class="reference internal" href="../library/warnings.html#describing-warning-filters"><span class="std std-ref">Describing Warning Filters</span></a> for more
details.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONFAULTHANDLER">
<code class="sig-name descname">PYTHONFAULTHANDLER</code><a class="headerlink" href="#envvar-PYTHONFAULTHANDLER" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string,
<a class="reference internal" href="../library/faulthandler.html#faulthandler.enable" title="faulthandler.enable"><code class="xref py py-func docutils literal notranslate"><span class="pre">faulthandler.enable()</span></code></a> is called at startup: install a handler for
<code class="xref py py-const docutils literal notranslate"><span class="pre">SIGSEGV</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGFPE</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGABRT</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGBUS</span></code> and
<code class="xref py py-const docutils literal notranslate"><span class="pre">SIGILL</span></code> signals to dump the Python traceback. This is equivalent to
<a class="reference internal" href="#id5"><code class="xref std std-option docutils literal notranslate"><span class="pre">-X</span></code></a> <code class="docutils literal notranslate"><span class="pre">faulthandler</span></code> option.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONTRACEMALLOC">
<code class="sig-name descname">PYTHONTRACEMALLOC</code><a class="headerlink" href="#envvar-PYTHONTRACEMALLOC" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string, start tracing
Python memory allocations using the <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> module. The value of
the variable is the maximum number of frames stored in a traceback of a
trace. For example, <code class="docutils literal notranslate"><span class="pre">PYTHONTRACEMALLOC=1</span></code> stores only the most recent
frame. See the <a class="reference internal" href="../library/tracemalloc.html#tracemalloc.start" title="tracemalloc.start"><code class="xref py py-func docutils literal notranslate"><span class="pre">tracemalloc.start()</span></code></a> for more information.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONPROFILEIMPORTTIME">
<code class="sig-name descname">PYTHONPROFILEIMPORTTIME</code><a class="headerlink" href="#envvar-PYTHONPROFILEIMPORTTIME" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string, Python will
show how long each import takes. This is exactly equivalent to setting
<code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">importtime</span></code> on the command line.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONASYNCIODEBUG">
<code class="sig-name descname">PYTHONASYNCIODEBUG</code><a class="headerlink" href="#envvar-PYTHONASYNCIODEBUG" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string, enable the
<a class="reference internal" href="../library/asyncio-dev.html#asyncio-debug-mode"><span class="std std-ref">debug mode</span></a> of the <a class="reference internal" href="../library/asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O."><code class="xref py py-mod docutils literal notranslate"><span class="pre">asyncio</span></code></a> module.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONMALLOC">
<code class="sig-name descname">PYTHONMALLOC</code><a class="headerlink" href="#envvar-PYTHONMALLOC" title="Permalink to this definition">¶</a></dt>
<dd><p>Set the Python memory allocators and/or install debug hooks.</p>
<p>Set the family of memory allocators used by Python:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">default</span></code>: use the <a class="reference internal" href="../c-api/memory.html#default-memory-allocators"><span class="std std-ref">default memory allocators</span></a>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code>: use the <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> function of the C library
for all domains (<a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_RAW" title="PYMEM_DOMAIN_RAW"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_RAW</span></code></a>, <a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a>,
<a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a>).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">pymalloc</span></code>: use the <a class="reference internal" href="../c-api/memory.html#pymalloc"><span class="std std-ref">pymalloc allocator</span></a> for
<a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a> and <a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a> domains and use
the <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> function for the <a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_RAW" title="PYMEM_DOMAIN_RAW"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_RAW</span></code></a> domain.</p></li>
</ul>
<p>Install debug hooks:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">debug</span></code>: install debug hooks on top of the <a class="reference internal" href="../c-api/memory.html#default-memory-allocators"><span class="std std-ref">default memory
allocators</span></a>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">malloc_debug</span></code>: same as <code class="docutils literal notranslate"><span class="pre">malloc</span></code> but also install debug hooks.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">pymalloc_debug</span></code>: same as <code class="docutils literal notranslate"><span class="pre">pymalloc</span></code> but also install debug hooks.</p></li>
</ul>
<p>See the <a class="reference internal" href="../c-api/memory.html#default-memory-allocators"><span class="std std-ref">default memory allocators</span></a> and the
<a class="reference internal" href="../c-api/memory.html#c.PyMem_SetupDebugHooks" title="PyMem_SetupDebugHooks"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_SetupDebugHooks()</span></code></a> function (install debug hooks on Python
memory allocators).</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7: </span>Added the <code class="docutils literal notranslate"><span class="pre">"default"</span></code> allocator.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONMALLOCSTATS">
<code class="sig-name descname">PYTHONMALLOCSTATS</code><a class="headerlink" href="#envvar-PYTHONMALLOCSTATS" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to a non-empty string, Python will print statistics of the
<a class="reference internal" href="../c-api/memory.html#pymalloc"><span class="std std-ref">pymalloc memory allocator</span></a> every time a new pymalloc object
arena is created, and on shutdown.</p>
<p>This variable is ignored if the <span class="target" id="index-35"></span><a class="reference internal" href="#envvar-PYTHONMALLOC"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONMALLOC</span></code></a> environment variable
is used to force the <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> allocator of the C library, or if
Python is configured without <code class="docutils literal notranslate"><span class="pre">pymalloc</span></code> support.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.6: </span>This variable can now also be used on Python compiled in release mode.
It now has no effect if set to an empty string.</p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONLEGACYWINDOWSFSENCODING">
<code class="sig-name descname">PYTHONLEGACYWINDOWSFSENCODING</code><a class="headerlink" href="#envvar-PYTHONLEGACYWINDOWSFSENCODING" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to a non-empty string, the default filesystem encoding and errors mode
will revert to their pre-3.6 values of ‘mbcs’ and ‘replace’, respectively.
Otherwise, the new defaults ‘utf-8’ and ‘surrogatepass’ are used.</p>
<p>This may also be enabled at runtime with
<a class="reference internal" href="../library/sys.html#sys._enablelegacywindowsfsencoding" title="sys._enablelegacywindowsfsencoding"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys._enablelegacywindowsfsencoding()</span></code></a>.</p>
<p class="availability"><a class="reference internal" href="../library/intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6: </span>See <span class="target" id="index-36"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0529"><strong>PEP 529</strong></a> for more details.</p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONLEGACYWINDOWSSTDIO">
<code class="sig-name descname">PYTHONLEGACYWINDOWSSTDIO</code><a class="headerlink" href="#envvar-PYTHONLEGACYWINDOWSSTDIO" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to a non-empty string, does not use the new console reader and
writer. This means that Unicode characters will be encoded according to
the active console code page, rather than using utf-8.</p>
<p>This variable is ignored if the standard streams are redirected (to files
or pipes) rather than referring to console buffers.</p>
<p class="availability"><a class="reference internal" href="../library/intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONCOERCECLOCALE">
<code class="sig-name descname">PYTHONCOERCECLOCALE</code><a class="headerlink" href="#envvar-PYTHONCOERCECLOCALE" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to the value <code class="docutils literal notranslate"><span class="pre">0</span></code>, causes the main Python command line application
to skip coercing the legacy ASCII-based C and POSIX locales to a more
capable UTF-8 based alternative.</p>
<p>If this variable is <em>not</em> set (or is set to a value other than <code class="docutils literal notranslate"><span class="pre">0</span></code>), the
<code class="docutils literal notranslate"><span class="pre">LC_ALL</span></code> locale override environment variable is also not set, and the
current locale reported for the <code class="docutils literal notranslate"><span class="pre">LC_CTYPE</span></code> category is either the default
<code class="docutils literal notranslate"><span class="pre">C</span></code> locale, or else the explicitly ASCII-based <code class="docutils literal notranslate"><span class="pre">POSIX</span></code> locale, then the
Python CLI will attempt to configure the following locales for the
<code class="docutils literal notranslate"><span class="pre">LC_CTYPE</span></code> category in the order listed before loading the interpreter
runtime:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">C.UTF-8</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">C.utf8</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">UTF-8</span></code></p></li>
</ul>
<p>If setting one of these locale categories succeeds, then the <code class="docutils literal notranslate"><span class="pre">LC_CTYPE</span></code>
environment variable will also be set accordingly in the current process
environment before the Python runtime is initialized. This ensures that in
addition to being seen by both the interpreter itself and other locale-aware
components running in the same process (such as the GNU <code class="docutils literal notranslate"><span class="pre">readline</span></code>
library), the updated setting is also seen in subprocesses (regardless of
whether or not those processes are running a Python interpreter), as well as
in operations that query the environment rather than the current C locale
(such as Python’s own <a class="reference internal" href="../library/locale.html#locale.getdefaultlocale" title="locale.getdefaultlocale"><code class="xref py py-func docutils literal notranslate"><span class="pre">locale.getdefaultlocale()</span></code></a>).</p>
<p>Configuring one of these locales (either explicitly or via the above
implicit locale coercion) automatically enables the <code class="docutils literal notranslate"><span class="pre">surrogateescape</span></code>
<a class="reference internal" href="../library/codecs.html#error-handlers"><span class="std std-ref">error handler</span></a> for <a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdin</span></code></a> and
<a class="reference internal" href="../library/sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> (<a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a> continues to use <code class="docutils literal notranslate"><span class="pre">backslashreplace</span></code>
as it does in any other locale). This stream handling behavior can be
overridden using <span class="target" id="index-37"></span><a class="reference internal" href="#envvar-PYTHONIOENCODING"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONIOENCODING</span></code></a> as usual.</p>
<p>For debugging purposes, setting <code class="docutils literal notranslate"><span class="pre">PYTHONCOERCECLOCALE=warn</span></code> will cause
Python to emit warning messages on <code class="docutils literal notranslate"><span class="pre">stderr</span></code> if either the locale coercion
activates, or else if a locale that <em>would</em> have triggered coercion is
still active when the Python runtime is initialized.</p>
<p>Also note that even when locale coercion is disabled, or when it fails to
find a suitable target locale, <span class="target" id="index-38"></span><a class="reference internal" href="#envvar-PYTHONUTF8"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONUTF8</span></code></a> will still activate by
default in legacy ASCII-based locales. Both features must be disabled in
order to force the interpreter to use <code class="docutils literal notranslate"><span class="pre">ASCII</span></code> instead of <code class="docutils literal notranslate"><span class="pre">UTF-8</span></code> for
system interfaces.</p>
<p class="availability"><a class="reference internal" href="../library/intro.html#availability"><span class="std std-ref">Availability</span></a>: *nix.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7: </span>See <span class="target" id="index-39"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0538"><strong>PEP 538</strong></a> for more details.</p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONDEVMODE">
<code class="sig-name descname">PYTHONDEVMODE</code><a class="headerlink" href="#envvar-PYTHONDEVMODE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string, enable the
CPython “development mode”. See the <a class="reference internal" href="#id5"><code class="xref std std-option docutils literal notranslate"><span class="pre">-X</span></code></a> <code class="docutils literal notranslate"><span class="pre">dev</span></code> option.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONUTF8">
<code class="sig-name descname">PYTHONUTF8</code><a class="headerlink" href="#envvar-PYTHONUTF8" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to <code class="docutils literal notranslate"><span class="pre">1</span></code>, enables the interpreter’s UTF-8 mode, where <code class="docutils literal notranslate"><span class="pre">UTF-8</span></code> is
used as the text encoding for system interfaces, regardless of the
current locale setting.</p>
<p>This means that:</p>
<blockquote>
<div><ul class="simple">
<li><p><a class="reference internal" href="../library/sys.html#sys.getfilesystemencoding" title="sys.getfilesystemencoding"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.getfilesystemencoding()</span></code></a> returns <code class="docutils literal notranslate"><span class="pre">'UTF-8'</span></code> (the locale
encoding is ignored).</p></li>
<li><p><a class="reference internal" href="../library/locale.html#locale.getpreferredencoding" title="locale.getpreferredencoding"><code class="xref py py-func docutils literal notranslate"><span class="pre">locale.getpreferredencoding()</span></code></a> returns <code class="docutils literal notranslate"><span class="pre">'UTF-8'</span></code> (the locale
encoding is ignored, and the function’s <code class="docutils literal notranslate"><span class="pre">do_setlocale</span></code> parameter has no
effect).</p></li>
<li><p><a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdin</span></code></a>, <a class="reference internal" href="../library/sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a>, and <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a> all use
UTF-8 as their text encoding, with the <code class="docutils literal notranslate"><span class="pre">surrogateescape</span></code>
<a class="reference internal" href="../library/codecs.html#error-handlers"><span class="std std-ref">error handler</span></a> being enabled for <a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdin</span></code></a>
and <a class="reference internal" href="../library/sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> (<a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a> continues to use
<code class="docutils literal notranslate"><span class="pre">backslashreplace</span></code> as it does in the default locale-aware mode)</p></li>
</ul>
</div></blockquote>
<p>As a consequence of the changes in those lower level APIs, other higher
level APIs also exhibit different default behaviours:</p>
<blockquote>
<div><ul class="simple">
<li><p>Command line arguments, environment variables and filenames are decoded
to text using the UTF-8 encoding.</p></li>
<li><p><a class="reference internal" href="../library/os.html#os.fsdecode" title="os.fsdecode"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fsdecode()</span></code></a> and <a class="reference internal" href="../library/os.html#os.fsencode" title="os.fsencode"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fsencode()</span></code></a> use the UTF-8 encoding.</p></li>
<li><p><a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a>, <a class="reference internal" href="../library/io.html#io.open" title="io.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">io.open()</span></code></a>, and <a class="reference internal" href="../library/codecs.html#codecs.open" title="codecs.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">codecs.open()</span></code></a> use the UTF-8
encoding by default. However, they still use the strict error handler by
default so that attempting to open a binary file in text mode is likely
to raise an exception rather than producing nonsense data.</p></li>
</ul>
</div></blockquote>
<p>Note that the standard stream settings in UTF-8 mode can be overridden by
<span class="target" id="index-40"></span><a class="reference internal" href="#envvar-PYTHONIOENCODING"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONIOENCODING</span></code></a> (just as they can be in the default locale-aware
mode).</p>
<p>If set to <code class="docutils literal notranslate"><span class="pre">0</span></code>, the interpreter runs in its default locale-aware mode.</p>
<p>Setting any other non-empty string causes an error during interpreter
initialisation.</p>
<p>If this environment variable is not set at all, then the interpreter defaults
to using the current locale settings, <em>unless</em> the current locale is
identified as a legacy ASCII-based locale
(as described for <span class="target" id="index-41"></span><a class="reference internal" href="#envvar-PYTHONCOERCECLOCALE"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONCOERCECLOCALE</span></code></a>), and locale coercion is
either disabled or fails. In such legacy locales, the interpreter will
default to enabling UTF-8 mode unless explicitly instructed not to do so.</p>
<p>Also available as the <a class="reference internal" href="#id5"><code class="xref std std-option docutils literal notranslate"><span class="pre">-X</span></code></a> <code class="docutils literal notranslate"><span class="pre">utf8</span></code> option.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7: </span>See <span class="target" id="index-42"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0540"><strong>PEP 540</strong></a> for more details.</p>
</div>
</dd></dl>
<div class="section" id="debug-mode-variables">
<h3><span class="section-number">1.2.1. </span>Debug-mode variables<a class="headerlink" href="#debug-mode-variables" title="Permalink to this headline">¶</a></h3>
<p>Setting these variables only has an effect in a debug build of Python, that is,
if Python was configured with the <code class="docutils literal notranslate"><span class="pre">--with-pydebug</span></code> build option.</p>
<dl class="envvar">
<dt id="envvar-PYTHONTHREADDEBUG">
<code class="sig-name descname">PYTHONTHREADDEBUG</code><a class="headerlink" href="#envvar-PYTHONTHREADDEBUG" title="Permalink to this definition">¶</a></dt>
<dd><p>If set, Python will print threading debug info.</p>
</dd></dl>
<dl class="envvar">
<dt id="envvar-PYTHONDUMPREFS">
<code class="sig-name descname">PYTHONDUMPREFS</code><a class="headerlink" href="#envvar-PYTHONDUMPREFS" title="Permalink to this definition">¶</a></dt>
<dd><p>If set, Python will dump objects and reference counts still alive after
shutting down the interpreter.</p>
</dd></dl>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">1. Command line and environment</a><ul>
<li><a class="reference internal" href="#command-line">1.1. Command line</a><ul>
<li><a class="reference internal" href="#interface-options">1.1.1. Interface options</a></li>
<li><a class="reference internal" href="#generic-options">1.1.2. Generic options</a></li>
<li><a class="reference internal" href="#miscellaneous-options">1.1.3. Miscellaneous options</a></li>
<li><a class="reference internal" href="#options-you-shouldn-t-use">1.1.4. Options you shouldn’t use</a></li>
</ul>
</li>
<li><a class="reference internal" href="#environment-variables">1.2. Environment variables</a><ul>
<li><a class="reference internal" href="#debug-mode-variables">1.2.1. Debug-mode variables</a></li>
</ul>
</li>
</ul>
</li>
</ul>
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