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<div class="section" id="modules">
<span id="tut-modules"></span><h1>6. Modules<a class="headerlink" href="#modules" title="Permalink to this headline">¶</a></h1>
<p>If you quit from the Python interpreter and enter it again, the definitions you
have made (functions and variables) are lost. Therefore, if you want to write a
somewhat longer program, you are better off using a text editor to prepare the
input for the interpreter and running it with that file as input instead. This
is known as creating a <em>script</em>. As your program gets longer, you may want to
split it into several files for easier maintenance. You may also want to use a
handy function that you’ve written in several programs without copying its
definition into each program.</p>
<p>To support this, Python has a way to put definitions in a file and use them in a
script or in an interactive instance of the interpreter. Such a file is called a
<em>module</em>; definitions from a module can be <em>imported</em> into other modules or into
the <em>main</em> module (the collection of variables that you have access to in a
script executed at the top level and in calculator mode).</p>
<p>A module is a file containing Python definitions and statements. The file name
is the module name with the suffix <code class="file docutils literal"><span class="pre">.py</span></code> appended. Within a module, the
module’s name (as a string) is available as the value of the global variable
<code class="docutils literal"><span class="pre">__name__</span></code>. For instance, use your favorite text editor to create a file
called <code class="file docutils literal"><span class="pre">fibo.py</span></code> in the current directory with the following contents:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="c1"># Fibonacci numbers module</span>
<span class="k">def</span> <span class="nf">fib</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c1"># write Fibonacci series up to n</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
<span class="k">while</span> <span class="n">b</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="nb">print</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">fib2</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c1"># return Fibonacci series up to n</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
<span class="k">while</span> <span class="n">b</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span>
<span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="k">return</span> <span class="n">result</span>
</pre></div>
</div>
<p>Now enter the Python interpreter and import this module with the following
command:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span>
</pre></div>
</div>
<p>This does not enter the names of the functions defined in <code class="docutils literal"><span class="pre">fibo</span></code> directly in
the current symbol table; it only enters the module name <code class="docutils literal"><span class="pre">fibo</span></code> there. Using
the module name you can access the functions:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fibo</span><span class="o">.</span><span class="n">fib</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987</span>
<span class="gp">>>> </span><span class="n">fibo</span><span class="o">.</span><span class="n">fib2</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
<span class="go">[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]</span>
<span class="gp">>>> </span><span class="n">fibo</span><span class="o">.</span><span class="n">__name__</span>
<span class="go">'fibo'</span>
</pre></div>
</div>
<p>If you intend to use a function often you can assign it to a local name:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fib</span> <span class="o">=</span> <span class="n">fibo</span><span class="o">.</span><span class="n">fib</span>
<span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">500</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377</span>
</pre></div>
</div>
<div class="section" id="more-on-modules">
<span id="tut-moremodules"></span><h2>6.1. More on Modules<a class="headerlink" href="#more-on-modules" title="Permalink to this headline">¶</a></h2>
<p>A module can contain executable statements as well as function definitions.
These statements are intended to initialize the module. They are executed only
the <em>first</em> time the module name is encountered in an import statement. <a class="footnote-reference" href="#id2" id="id1">[1]</a>
(They are also run if the file is executed as a script.)</p>
<p>Each module has its own private symbol table, which is used as the global symbol
table by all functions defined in the module. Thus, the author of a module can
use global variables in the module without worrying about accidental clashes
with a user’s global variables. On the other hand, if you know what you are
doing you can touch a module’s global variables with the same notation used to
refer to its functions, <code class="docutils literal"><span class="pre">modname.itemname</span></code>.</p>
<p>Modules can import other modules. It is customary but not required to place all
<a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><span class="pre">import</span></code></a> statements at the beginning of a module (or script, for that
matter). The imported module names are placed in the importing module’s global
symbol table.</p>
<p>There is a variant of the <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><span class="pre">import</span></code></a> statement that imports names from a
module directly into the importing module’s symbol table. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">fibo</span> <span class="k">import</span> <span class="n">fib</span><span class="p">,</span> <span class="n">fib2</span>
<span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">500</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377</span>
</pre></div>
</div>
<p>This does not introduce the module name from which the imports are taken in the
local symbol table (so in the example, <code class="docutils literal"><span class="pre">fibo</span></code> is not defined).</p>
<p>There is even a variant to import all names that a module defines:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">fibo</span> <span class="k">import</span> <span class="o">*</span>
<span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">500</span><span class="p">)</span>
<span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377</span>
</pre></div>
</div>
<p>This imports all names except those beginning with an underscore (<code class="docutils literal"><span class="pre">_</span></code>).
In most cases Python programmers do not use this facility since it introduces
an unknown set of names into the interpreter, possibly hiding some things
you have already defined.</p>
<p>Note that in general the practice of importing <code class="docutils literal"><span class="pre">*</span></code> from a module or package is
frowned upon, since it often causes poorly readable code. However, it is okay to
use it to save typing in interactive sessions.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">For efficiency reasons, each module is only imported once per interpreter
session. Therefore, if you change your modules, you must restart the
interpreter – or, if it’s just one module you want to test interactively,
use <a class="reference internal" href="../library/importlib.html#importlib.reload" title="importlib.reload"><code class="xref py py-func docutils literal"><span class="pre">importlib.reload()</span></code></a>, e.g. <code class="docutils literal"><span class="pre">import</span> <span class="pre">importlib;</span>
<span class="pre">importlib.reload(modulename)</span></code>.</p>
</div>
<div class="section" id="executing-modules-as-scripts">
<span id="tut-modulesasscripts"></span><h3>6.1.1. Executing modules as scripts<a class="headerlink" href="#executing-modules-as-scripts" title="Permalink to this headline">¶</a></h3>
<p>When you run a Python module with</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="n">fibo</span><span class="o">.</span><span class="n">py</span> <span class="o"><</span><span class="n">arguments</span><span class="o">></span>
</pre></div>
</div>
<p>the code in the module will be executed, just as if you imported it, but with
the <code class="docutils literal"><span class="pre">__name__</span></code> set to <code class="docutils literal"><span class="pre">"__main__"</span></code>. That means that by adding this code at
the end of your module:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s2">"__main__"</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">fib</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]))</span>
</pre></div>
</div>
<p>you can make the file usable as a script as well as an importable module,
because the code that parses the command line only runs if the module is
executed as the “main” file:</p>
<div class="highlight-shell-session"><div class="highlight"><pre><span></span><span class="gp">$</span> python fibo.py 50
<span class="go">1 1 2 3 5 8 13 21 34</span>
</pre></div>
</div>
<p>If the module is imported, the code is not run:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>This is often used either to provide a convenient user interface to a module, or
for testing purposes (running the module as a script executes a test suite).</p>
</div>
<div class="section" id="the-module-search-path">
<span id="tut-searchpath"></span><h3>6.1.2. The Module Search Path<a class="headerlink" href="#the-module-search-path" title="Permalink to this headline">¶</a></h3>
<p id="index-0">When a module named <code class="xref py py-mod docutils literal"><span class="pre">spam</span></code> is imported, the interpreter first searches for
a built-in module with that name. If not found, it then searches for a file
named <code class="file docutils literal"><span class="pre">spam.py</span></code> in a list of directories given by the variable
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>. <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a> is initialized from these locations:</p>
<ul class="simple">
<li>The directory containing the input script (or the current directory when no
file is specified).</li>
<li><span class="target" id="index-1"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a> (a list of directory names, with the same syntax as the
shell variable <span class="target" id="index-2"></span><code class="xref std std-envvar docutils literal"><span class="pre">PATH</span></code>).</li>
<li>The installation-dependent default.</li>
</ul>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">On file systems which support symlinks, the directory containing the input
script is calculated after the symlink is followed. In other words the
directory containing the symlink is <strong>not</strong> added to the module search path.</p>
</div>
<p>After initialization, Python programs can modify <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>. The
directory containing the script being run is placed at the beginning of the
search path, ahead of the standard library path. This means that scripts in that
directory will be loaded instead of modules of the same name in the library
directory. This is an error unless the replacement is intended. See section
<a class="reference internal" href="#tut-standardmodules"><span>Standard Modules</span></a> for more information.</p>
</div>
<div class="section" id="compiled-python-files">
<h3>6.1.3. “Compiled” Python files<a class="headerlink" href="#compiled-python-files" title="Permalink to this headline">¶</a></h3>
<p>To speed up loading modules, Python caches the compiled version of each module
in the <code class="docutils literal"><span class="pre">__pycache__</span></code> directory under the name <code class="file docutils literal"><span class="pre">module.</span><em><span class="pre">version</span></em><span class="pre">.pyc</span></code>,
where the version encodes the format of the compiled file; it generally contains
the Python version number. For example, in CPython release 3.3 the compiled
version of spam.py would be cached as <code class="docutils literal"><span class="pre">__pycache__/spam.cpython-33.pyc</span></code>. This
naming convention allows compiled modules from different releases and different
versions of Python to coexist.</p>
<p>Python checks the modification date of the source against the compiled version
to see if it’s out of date and needs to be recompiled. This is a completely
automatic process. Also, the compiled modules are platform-independent, so the
same library can be shared among systems with different architectures.</p>
<p>Python does not check the cache in two circumstances. First, it always
recompiles and does not store the result for the module that’s loaded directly
from the command line. Second, it does not check the cache if there is no
source module. To support a non-source (compiled only) distribution, the
compiled module must be in the source directory, and there must not be a source
module.</p>
<p>Some tips for experts:</p>
<ul class="simple">
<li>You can use the <a class="reference internal" href="../using/cmdline.html#cmdoption-O"><code class="xref std std-option docutils literal"><span class="pre">-O</span></code></a> or <a class="reference internal" href="../using/cmdline.html#cmdoption-OO"><code class="xref std std-option docutils literal"><span class="pre">-OO</span></code></a> switches on the Python command
to reduce the size of a compiled module. The <code class="docutils literal"><span class="pre">-O</span></code> switch removes assert
statements, the <code class="docutils literal"><span class="pre">-OO</span></code> switch removes both assert statements and __doc__
strings. Since some programs may rely on having these available, you should
only use this option if you know what you’re doing. “Optimized” modules have
an <code class="docutils literal"><span class="pre">opt-</span></code> tag and are usually smaller. Future releases may
change the effects of optimization.</li>
<li>A program doesn’t run any faster when it is read from a <code class="docutils literal"><span class="pre">.pyc</span></code>
file than when it is read from a <code class="docutils literal"><span class="pre">.py</span></code> file; the only thing that’s faster
about <code class="docutils literal"><span class="pre">.pyc</span></code> files is the speed with which they are loaded.</li>
<li>The module <a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><code class="xref py py-mod docutils literal"><span class="pre">compileall</span></code></a> can create .pyc files for all modules in a
directory.</li>
<li>There is more detail on this process, including a flow chart of the
decisions, in PEP 3147.</li>
</ul>
</div>
</div>
<div class="section" id="standard-modules">
<span id="tut-standardmodules"></span><h2>6.2. Standard Modules<a class="headerlink" href="#standard-modules" title="Permalink to this headline">¶</a></h2>
<p id="index-3">Python comes with a library of standard modules, described in a separate
document, the Python Library Reference (“Library Reference” hereafter). Some
modules are built into the interpreter; these provide access to operations that
are not part of the core of the language but are nevertheless built in, either
for efficiency or to provide access to operating system primitives such as
system calls. The set of such modules is a configuration option which also
depends on the underlying platform. For example, the <a class="reference internal" href="../library/winreg.html#module-winreg" title="winreg: Routines and objects for manipulating the Windows registry. (Windows)"><code class="xref py py-mod docutils literal"><span class="pre">winreg</span></code></a> module is only
provided on Windows systems. One particular module deserves some attention:
<a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal"><span class="pre">sys</span></code></a>, which is built into every Python interpreter. The variables
<code class="docutils literal"><span class="pre">sys.ps1</span></code> and <code class="docutils literal"><span class="pre">sys.ps2</span></code> define the strings used as primary and secondary
prompts:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">ps1</span>
<span class="go">'>>> '</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">ps2</span>
<span class="go">'... '</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">ps1</span> <span class="o">=</span> <span class="s1">'C> '</span>
<span class="go">C> print('Yuck!')</span>
<span class="go">Yuck!</span>
<span class="go">C></span>
</pre></div>
</div>
<p>These two variables are only defined if the interpreter is in interactive mode.</p>
<p>The variable <code class="docutils literal"><span class="pre">sys.path</span></code> is a list of strings that determines the interpreter’s
search path for modules. It is initialized to a default path taken from the
environment variable <span class="target" id="index-4"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a>, or from a built-in default if
<span class="target" id="index-5"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a> is not set. You can modify it using standard list
operations:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s1">'/ufs/guido/lib/python'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="the-dir-function">
<span id="tut-dir"></span><h2>6.3. The <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal"><span class="pre">dir()</span></code></a> Function<a class="headerlink" href="#the-dir-function" title="Permalink to this headline">¶</a></h2>
<p>The built-in function <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal"><span class="pre">dir()</span></code></a> is used to find out which names a module
defines. It returns a sorted list of strings:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span><span class="o">,</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">fibo</span><span class="p">)</span>
<span class="go">['__name__', 'fib', 'fib2']</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">sys</span><span class="p">)</span>
<span class="go">['__displayhook__', '__doc__', '__excepthook__', '__loader__', '__name__',</span>
<span class="go"> '__package__', '__stderr__', '__stdin__', '__stdout__',</span>
<span class="go"> '_clear_type_cache', '_current_frames', '_debugmallocstats', '_getframe',</span>
<span class="go"> '_home', '_mercurial', '_xoptions', 'abiflags', 'api_version', 'argv',</span>
<span class="go"> 'base_exec_prefix', 'base_prefix', 'builtin_module_names', 'byteorder',</span>
<span class="go"> 'call_tracing', 'callstats', 'copyright', 'displayhook',</span>
<span class="go"> 'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix',</span>
<span class="go"> 'executable', 'exit', 'flags', 'float_info', 'float_repr_style',</span>
<span class="go"> 'getcheckinterval', 'getdefaultencoding', 'getdlopenflags',</span>
<span class="go"> 'getfilesystemencoding', 'getobjects', 'getprofile', 'getrecursionlimit',</span>
<span class="go"> 'getrefcount', 'getsizeof', 'getswitchinterval', 'gettotalrefcount',</span>
<span class="go"> 'gettrace', 'hash_info', 'hexversion', 'implementation', 'int_info',</span>
<span class="go"> 'intern', 'maxsize', 'maxunicode', 'meta_path', 'modules', 'path',</span>
<span class="go"> 'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'ps1',</span>
<span class="go"> 'setcheckinterval', 'setdlopenflags', 'setprofile', 'setrecursionlimit',</span>
<span class="go"> 'setswitchinterval', 'settrace', 'stderr', 'stdin', 'stdout',</span>
<span class="go"> 'thread_info', 'version', 'version_info', 'warnoptions']</span>
</pre></div>
</div>
<p>Without arguments, <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal"><span class="pre">dir()</span></code></a> lists the names you have defined currently:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fibo</span>
<span class="gp">>>> </span><span class="n">fib</span> <span class="o">=</span> <span class="n">fibo</span><span class="o">.</span><span class="n">fib</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">()</span>
<span class="go">['__builtins__', '__name__', 'a', 'fib', 'fibo', 'sys']</span>
</pre></div>
</div>
<p>Note that it lists all types of names: variables, modules, functions, etc.</p>
<p id="index-6"><a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal"><span class="pre">dir()</span></code></a> does not list the names of built-in functions and variables. If you
want a list of those, they are defined in the standard module
<a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal"><span class="pre">builtins</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">builtins</span>
<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">builtins</span><span class="p">)</span>
<span class="go">['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException',</span>
<span class="go"> 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning',</span>
<span class="go"> 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError',</span>
<span class="go"> 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning',</span>
<span class="go"> 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',</span>
<span class="go"> 'FileExistsError', 'FileNotFoundError', 'FloatingPointError',</span>
<span class="go"> 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError',</span>
<span class="go"> 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError',</span>
<span class="go"> 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError',</span>
<span class="go"> 'MemoryError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented',</span>
<span class="go"> 'NotImplementedError', 'OSError', 'OverflowError',</span>
<span class="go"> 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError',</span>
<span class="go"> 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning',</span>
<span class="go"> 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError',</span>
<span class="go"> 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError',</span>
<span class="go"> 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError',</span>
<span class="go"> 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning',</span>
<span class="go"> 'ValueError', 'Warning', 'ZeroDivisionError', '_', '__build_class__',</span>
<span class="go"> '__debug__', '__doc__', '__import__', '__name__', '__package__', 'abs',</span>
<span class="go"> 'all', 'any', 'ascii', 'bin', 'bool', 'bytearray', 'bytes', 'callable',</span>
<span class="go"> 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits',</span>
<span class="go"> 'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit',</span>
<span class="go"> 'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr',</span>
<span class="go"> 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass',</span>
<span class="go"> 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview',</span>
<span class="go"> 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property',</span>
<span class="go"> 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice',</span>
<span class="go"> 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars',</span>
<span class="go"> 'zip']</span>
</pre></div>
</div>
</div>
<div class="section" id="packages">
<span id="tut-packages"></span><h2>6.4. Packages<a class="headerlink" href="#packages" title="Permalink to this headline">¶</a></h2>
<p>Packages are a way of structuring Python’s module namespace by using “dotted
module names”. For example, the module name <code class="xref py py-mod docutils literal"><span class="pre">A.B</span></code> designates a submodule
named <code class="docutils literal"><span class="pre">B</span></code> in a package named <code class="docutils literal"><span class="pre">A</span></code>. Just like the use of modules saves the
authors of different modules from having to worry about each other’s global
variable names, the use of dotted module names saves the authors of multi-module
packages like NumPy or the Python Imaging Library from having to worry about
each other’s module names.</p>
<p>Suppose you want to design a collection of modules (a “package”) for the uniform
handling of sound files and sound data. There are many different sound file
formats (usually recognized by their extension, for example: <code class="file docutils literal"><span class="pre">.wav</span></code>,
<code class="file docutils literal"><span class="pre">.aiff</span></code>, <code class="file docutils literal"><span class="pre">.au</span></code>), so you may need to create and maintain a growing
collection of modules for the conversion between the various file formats.
There are also many different operations you might want to perform on sound data
(such as mixing, adding echo, applying an equalizer function, creating an
artificial stereo effect), so in addition you will be writing a never-ending
stream of modules to perform these operations. Here’s a possible structure for
your package (expressed in terms of a hierarchical filesystem):</p>
<div class="highlight-text"><div class="highlight"><pre><span></span>sound/ Top-level package
__init__.py Initialize the sound package
formats/ Subpackage for file format conversions
__init__.py
wavread.py
wavwrite.py
aiffread.py
aiffwrite.py
auread.py
auwrite.py
...
effects/ Subpackage for sound effects
__init__.py
echo.py
surround.py
reverse.py
...
filters/ Subpackage for filters
__init__.py
equalizer.py
vocoder.py
karaoke.py
...
</pre></div>
</div>
<p>When importing the package, Python searches through the directories on
<code class="docutils literal"><span class="pre">sys.path</span></code> looking for the package subdirectory.</p>
<p>The <code class="file docutils literal"><span class="pre">__init__.py</span></code> files are required to make Python treat the directories
as containing packages; this is done to prevent directories with a common name,
such as <code class="docutils literal"><span class="pre">string</span></code>, from unintentionally hiding valid modules that occur later
on the module search path. In the simplest case, <code class="file docutils literal"><span class="pre">__init__.py</span></code> can just be
an empty file, but it can also execute initialization code for the package or
set the <code class="docutils literal"><span class="pre">__all__</span></code> variable, described later.</p>
<p>Users of the package can import individual modules from the package, for
example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">sound.effects.echo</span>
</pre></div>
</div>
<p>This loads the submodule <code class="xref py py-mod docutils literal"><span class="pre">sound.effects.echo</span></code>. It must be referenced with
its full name.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">sound</span><span class="o">.</span><span class="n">effects</span><span class="o">.</span><span class="n">echo</span><span class="o">.</span><span class="n">echofilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>
</div>
<p>An alternative way of importing the submodule is:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sound.effects</span> <span class="k">import</span> <span class="n">echo</span>
</pre></div>
</div>
<p>This also loads the submodule <code class="xref py py-mod docutils literal"><span class="pre">echo</span></code>, and makes it available without its
package prefix, so it can be used as follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">echo</span><span class="o">.</span><span class="n">echofilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>
</div>
<p>Yet another variation is to import the desired function or variable directly:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sound.effects.echo</span> <span class="k">import</span> <span class="n">echofilter</span>
</pre></div>
</div>
<p>Again, this loads the submodule <code class="xref py py-mod docutils literal"><span class="pre">echo</span></code>, but this makes its function
<code class="xref py py-func docutils literal"><span class="pre">echofilter()</span></code> directly available:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">echofilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that when using <code class="docutils literal"><span class="pre">from</span> <span class="pre">package</span> <span class="pre">import</span> <span class="pre">item</span></code>, the item can be either a
submodule (or subpackage) of the package, or some other name defined in the
package, like a function, class or variable. The <code class="docutils literal"><span class="pre">import</span></code> statement first
tests whether the item is defined in the package; if not, it assumes it is a
module and attempts to load it. If it fails to find it, an <a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal"><span class="pre">ImportError</span></code></a>
exception is raised.</p>
<p>Contrarily, when using syntax like <code class="docutils literal"><span class="pre">import</span> <span class="pre">item.subitem.subsubitem</span></code>, each item
except for the last must be a package; the last item can be a module or a
package but can’t be a class or function or variable defined in the previous
item.</p>
<div class="section" id="importing-from-a-package">
<span id="tut-pkg-import-star"></span><h3>6.4.1. Importing * From a Package<a class="headerlink" href="#importing-from-a-package" title="Permalink to this headline">¶</a></h3>
<p id="index-7">Now what happens when the user writes <code class="docutils literal"><span class="pre">from</span> <span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">*</span></code>? Ideally,
one would hope that this somehow goes out to the filesystem, finds which
submodules are present in the package, and imports them all. This could take a
long time and importing sub-modules might have unwanted side-effects that should
only happen when the sub-module is explicitly imported.</p>
<p>The only solution is for the package author to provide an explicit index of the
package. The <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><span class="pre">import</span></code></a> statement uses the following convention: if a package’s
<code class="file docutils literal"><span class="pre">__init__.py</span></code> code defines a list named <code class="docutils literal"><span class="pre">__all__</span></code>, it is taken to be the
list of module names that should be imported when <code class="docutils literal"><span class="pre">from</span> <span class="pre">package</span> <span class="pre">import</span> <span class="pre">*</span></code> is
encountered. It is up to the package author to keep this list up-to-date when a
new version of the package is released. Package authors may also decide not to
support it, if they don’t see a use for importing * from their package. For
example, the file <code class="file docutils literal"><span class="pre">sound/effects/__init__.py</span></code> could contain the following
code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">__all__</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"echo"</span><span class="p">,</span> <span class="s2">"surround"</span><span class="p">,</span> <span class="s2">"reverse"</span><span class="p">]</span>
</pre></div>
</div>
<p>This would mean that <code class="docutils literal"><span class="pre">from</span> <span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">*</span></code> would import the three
named submodules of the <code class="xref py py-mod docutils literal"><span class="pre">sound</span></code> package.</p>
<p>If <code class="docutils literal"><span class="pre">__all__</span></code> is not defined, the statement <code class="docutils literal"><span class="pre">from</span> <span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">*</span></code>
does <em>not</em> import all submodules from the package <code class="xref py py-mod docutils literal"><span class="pre">sound.effects</span></code> into the
current namespace; it only ensures that the package <code class="xref py py-mod docutils literal"><span class="pre">sound.effects</span></code> has
been imported (possibly running any initialization code in <code class="file docutils literal"><span class="pre">__init__.py</span></code>)
and then imports whatever names are defined in the package. This includes any
names defined (and submodules explicitly loaded) by <code class="file docutils literal"><span class="pre">__init__.py</span></code>. It
also includes any submodules of the package that were explicitly loaded by
previous <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><span class="pre">import</span></code></a> statements. Consider this code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">sound.effects.echo</span>
<span class="kn">import</span> <span class="nn">sound.effects.surround</span>
<span class="kn">from</span> <span class="nn">sound.effects</span> <span class="k">import</span> <span class="o">*</span>
</pre></div>
</div>
<p>In this example, the <code class="xref py py-mod docutils literal"><span class="pre">echo</span></code> and <code class="xref py py-mod docutils literal"><span class="pre">surround</span></code> modules are imported in the
current namespace because they are defined in the <code class="xref py py-mod docutils literal"><span class="pre">sound.effects</span></code> package
when the <code class="docutils literal"><span class="pre">from...import</span></code> statement is executed. (This also works when
<code class="docutils literal"><span class="pre">__all__</span></code> is defined.)</p>
<p>Although certain modules are designed to export only names that follow certain
patterns when you use <code class="docutils literal"><span class="pre">import</span> <span class="pre">*</span></code>, it is still considered bad practice in
production code.</p>
<p>Remember, there is nothing wrong with using <code class="docutils literal"><span class="pre">from</span> <span class="pre">Package</span> <span class="pre">import</span>
<span class="pre">specific_submodule</span></code>! In fact, this is the recommended notation unless the
importing module needs to use submodules with the same name from different
packages.</p>
</div>
<div class="section" id="intra-package-references">
<h3>6.4.2. Intra-package References<a class="headerlink" href="#intra-package-references" title="Permalink to this headline">¶</a></h3>
<p>When packages are structured into subpackages (as with the <code class="xref py py-mod docutils literal"><span class="pre">sound</span></code> package
in the example), you can use absolute imports to refer to submodules of siblings
packages. For example, if the module <code class="xref py py-mod docutils literal"><span class="pre">sound.filters.vocoder</span></code> needs to use
the <code class="xref py py-mod docutils literal"><span class="pre">echo</span></code> module in the <code class="xref py py-mod docutils literal"><span class="pre">sound.effects</span></code> package, it can use <code class="docutils literal"><span class="pre">from</span>
<span class="pre">sound.effects</span> <span class="pre">import</span> <span class="pre">echo</span></code>.</p>
<p>You can also write relative imports, with the <code class="docutils literal"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">name</span></code> form
of import statement. These imports use leading dots to indicate the current and
parent packages involved in the relative import. From the <code class="xref py py-mod docutils literal"><span class="pre">surround</span></code>
module for example, you might use:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">.</span> <span class="k">import</span> <span class="n">echo</span>
<span class="kn">from</span> <span class="nn">..</span> <span class="k">import</span> <span class="n">formats</span>
<span class="kn">from</span> <span class="nn">..filters</span> <span class="k">import</span> <span class="n">equalizer</span>
</pre></div>
</div>
<p>Note that relative imports are based on the name of the current module. Since
the name of the main module is always <code class="docutils literal"><span class="pre">"__main__"</span></code>, modules intended for use
as the main module of a Python application must always use absolute imports.</p>
</div>
<div class="section" id="packages-in-multiple-directories">
<h3>6.4.3. Packages in Multiple Directories<a class="headerlink" href="#packages-in-multiple-directories" title="Permalink to this headline">¶</a></h3>
<p>Packages support one more special attribute, <a class="reference internal" href="../reference/import.html#__path__" title="__path__"><code class="xref py py-attr docutils literal"><span class="pre">__path__</span></code></a>. This is
initialized to be a list containing the name of the directory holding the
package’s <code class="file docutils literal"><span class="pre">__init__.py</span></code> before the code in that file is executed. This
variable can be modified; doing so affects future searches for modules and
subpackages contained in the package.</p>
<p>While this feature is not often needed, it can be used to extend the set of
modules found in a package.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>In fact function definitions are also ‘statements’ that are ‘executed’; the
execution of a module-level function definition enters the function name in
the module’s global symbol table.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">6. Modules</a><ul>
<li><a class="reference internal" href="#more-on-modules">6.1. More on Modules</a><ul>
<li><a class="reference internal" href="#executing-modules-as-scripts">6.1.1. Executing modules as scripts</a></li>
<li><a class="reference internal" href="#the-module-search-path">6.1.2. The Module Search Path</a></li>
<li><a class="reference internal" href="#compiled-python-files">6.1.3. “Compiled” Python files</a></li>
</ul>
</li>
<li><a class="reference internal" href="#standard-modules">6.2. Standard Modules</a></li>
<li><a class="reference internal" href="#the-dir-function">6.3. The <code class="docutils literal"><span class="pre">dir()</span></code> Function</a></li>
<li><a class="reference internal" href="#packages">6.4. Packages</a><ul>
<li><a class="reference internal" href="#importing-from-a-package">6.4.1. Importing * From a Package</a></li>
<li><a class="reference internal" href="#intra-package-references">6.4.2. Intra-package References</a></li>
<li><a class="reference internal" href="#packages-in-multiple-directories">6.4.3. Packages in Multiple Directories</a></li>
</ul>
</li>
</ul>
</li>
</ul>
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<h4>Next topic</h4>
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title="next chapter">7. Input and Output</a></p>
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