<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>27.5. warnings — Warning control — Python v3.2.2 documentation</title>
<link rel="stylesheet" href="../_static/default.css" type="text/css" />
<link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT: '../',
VERSION: '3.2.2',
COLLAPSE_INDEX: false,
FILE_SUFFIX: '.html',
HAS_SOURCE: true
};
</script>
<script type="text/javascript" src="../_static/jquery.js"></script>
<script type="text/javascript" src="../_static/underscore.js"></script>
<script type="text/javascript" src="../_static/doctools.js"></script>
<script type="text/javascript" src="../_static/sidebar.js"></script>
<link rel="search" type="application/opensearchdescription+xml"
title="Search within Python v3.2.2 documentation"
href="../_static/opensearch.xml"/>
<link rel="author" title="About these documents" href="../about.html" />
<link rel="copyright" title="Copyright" href="../copyright.html" />
<link rel="top" title="Python v3.2.2 documentation" href="../index.html" />
<link rel="up" title="27. Python Runtime Services" href="python.html" />
<link rel="next" title="27.6. contextlib — Utilities for with-statement contexts" href="contextlib.html" />
<link rel="prev" title="27.4. __main__ — Top-level script environment" href="__main__.html" />
<link rel="shortcut icon" type="image/png" href="../_static/py.png" />
</head>
<body>
<div class="related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="../genindex.html" title="General Index"
accesskey="I">index</a></li>
<li class="right" >
<a href="../py-modindex.html" title="Python Module Index"
>modules</a> |</li>
<li class="right" >
<a href="contextlib.html" title="27.6. contextlib — Utilities for with-statement contexts"
accesskey="N">next</a> |</li>
<li class="right" >
<a href="__main__.html" title="27.4. __main__ — Top-level script environment"
accesskey="P">previous</a> |</li>
<li><img src="../_static/py.png" alt=""
style="vertical-align: middle; margin-top: -1px"/></li>
<li><a href="../index.html">Python v3.2.2 documentation</a> »</li>
<li><a href="index.html" >The Python Standard Library</a> »</li>
<li><a href="python.html" accesskey="U">27. Python Runtime Services</a> »</li>
</ul>
</div>
<div class="document">
<div class="documentwrapper">
<div class="bodywrapper">
<div class="body">
<div class="section" id="warnings-warning-control">
<h1>27.5. <a class="reference internal" href="#module-warnings" title="warnings: Issue warning messages and control their disposition."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> — Warning control<a class="headerlink" href="#warnings-warning-control" title="Permalink to this headline">¶</a></h1>
<span class="target" id="module-warnings"><span id="index-0"></span></span><p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/3.2/Lib/warnings.py">Lib/warnings.py</a></p>
<hr class="docutils" />
<p>Warning messages are typically issued in situations where it is useful to alert
the user of some condition in a program, where that condition (normally) doesn’t
warrant raising an exception and terminating the program. For example, one
might want to issue a warning when a program uses an obsolete module.</p>
<p>Python programmers issue warnings by calling the <a class="reference internal" href="#warnings.warn" title="warnings.warn"><tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt></a> function defined
in this module. (C programmers use <a class="reference internal" href="../c-api/exceptions.html#PyErr_WarnEx" title="PyErr_WarnEx"><tt class="xref c c-func docutils literal"><span class="pre">PyErr_WarnEx()</span></tt></a>; see
<a class="reference internal" href="../c-api/exceptions.html#exceptionhandling"><em>Exception Handling</em></a> for details).</p>
<p>Warning messages are normally written to <tt class="docutils literal"><span class="pre">sys.stderr</span></tt>, but their disposition
can be changed flexibly, from ignoring all warnings to turning them into
exceptions. The disposition of warnings can vary based on the warning category
(see below), the text of the warning message, and the source location where it
is issued. Repetitions of a particular warning for the same source location are
typically suppressed.</p>
<p>There are two stages in warning control: first, each time a warning is issued, a
determination is made whether a message should be issued or not; next, if a
message is to be issued, it is formatted and printed using a user-settable hook.</p>
<p>The determination whether to issue a warning message is controlled by the
warning filter, which is a sequence of matching rules and actions. Rules can be
added to the filter by calling <a class="reference internal" href="#warnings.filterwarnings" title="warnings.filterwarnings"><tt class="xref py py-func docutils literal"><span class="pre">filterwarnings()</span></tt></a> and reset to its default
state by calling <a class="reference internal" href="#warnings.resetwarnings" title="warnings.resetwarnings"><tt class="xref py py-func docutils literal"><span class="pre">resetwarnings()</span></tt></a>.</p>
<p>The printing of warning messages is done by calling <a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a>, which
may be overridden; the default implementation of this function formats the
message by calling <a class="reference internal" href="#warnings.formatwarning" title="warnings.formatwarning"><tt class="xref py py-func docutils literal"><span class="pre">formatwarning()</span></tt></a>, which is also available for use by
custom implementations.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="logging.html#logging.captureWarnings" title="logging.captureWarnings"><tt class="xref py py-func docutils literal"><span class="pre">logging.captureWarnings()</span></tt></a> allows you to handle all warnings with
the standard logging infrastructure.</p>
</div>
<div class="section" id="warning-categories">
<span id="id1"></span><h2>27.5.1. Warning Categories<a class="headerlink" href="#warning-categories" title="Permalink to this headline">¶</a></h2>
<p>There are a number of built-in exceptions that represent warning categories.
This categorization is useful to be able to filter out groups of warnings. The
following warnings category classes are currently defined:</p>
<table border="1" class="docutils">
<colgroup>
<col width="42%" />
<col width="58%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Class</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a></td>
<td>This is the base class of all warning
category classes. It is a subclass of
<a class="reference internal" href="exceptions.html#Exception" title="Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a>.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#UserWarning" title="UserWarning"><tt class="xref py py-exc docutils literal"><span class="pre">UserWarning</span></tt></a></td>
<td>The default category for <a class="reference internal" href="#warnings.warn" title="warnings.warn"><tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt></a>.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a></td>
<td>Base category for warnings about deprecated
features (ignored by default).</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#SyntaxWarning" title="SyntaxWarning"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxWarning</span></tt></a></td>
<td>Base category for warnings about dubious
syntactic features.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#RuntimeWarning" title="RuntimeWarning"><tt class="xref py py-exc docutils literal"><span class="pre">RuntimeWarning</span></tt></a></td>
<td>Base category for warnings about dubious
runtime features.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#FutureWarning" title="FutureWarning"><tt class="xref py py-exc docutils literal"><span class="pre">FutureWarning</span></tt></a></td>
<td>Base category for warnings about constructs
that will change semantically in the future.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">PendingDeprecationWarning</span></tt></a></td>
<td>Base category for warnings about features
that will be deprecated in the future
(ignored by default).</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#ImportWarning" title="ImportWarning"><tt class="xref py py-exc docutils literal"><span class="pre">ImportWarning</span></tt></a></td>
<td>Base category for warnings triggered during
the process of importing a module (ignored by
default).</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#UnicodeWarning" title="UnicodeWarning"><tt class="xref py py-exc docutils literal"><span class="pre">UnicodeWarning</span></tt></a></td>
<td>Base category for warnings related to
Unicode.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#BytesWarning" title="BytesWarning"><tt class="xref py py-exc docutils literal"><span class="pre">BytesWarning</span></tt></a></td>
<td>Base category for warnings related to
<a class="reference internal" href="functions.html#bytes" title="bytes"><tt class="xref py py-class docutils literal"><span class="pre">bytes</span></tt></a> and <tt class="xref py py-class docutils literal"><span class="pre">buffer</span></tt>.</td>
</tr>
<tr><td><a class="reference internal" href="exceptions.html#ResourceWarning" title="ResourceWarning"><tt class="xref py py-exc docutils literal"><span class="pre">ResourceWarning</span></tt></a></td>
<td>Base category for warnings related to
resource usage.</td>
</tr>
</tbody>
</table>
<p>While these are technically built-in exceptions, they are documented here,
because conceptually they belong to the warnings mechanism.</p>
<p>User code can define additional warning categories by subclassing one of the
standard warning categories. A warning category must always be a subclass of
the <a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a> class.</p>
</div>
<div class="section" id="the-warnings-filter">
<span id="warning-filter"></span><h2>27.5.2. The Warnings Filter<a class="headerlink" href="#the-warnings-filter" title="Permalink to this headline">¶</a></h2>
<p>The warnings filter controls whether warnings are ignored, displayed, or turned
into errors (raising an exception).</p>
<p>Conceptually, the warnings filter maintains an ordered list of filter
specifications; any specific warning is matched against each filter
specification in the list in turn until a match is found; the match determines
the disposition of the match. Each entry is a tuple of the form (<em>action</em>,
<em>message</em>, <em>category</em>, <em>module</em>, <em>lineno</em>), where:</p>
<ul>
<li><p class="first"><em>action</em> is one of the following strings:</p>
<table border="1" class="docutils">
<colgroup>
<col width="25%" />
<col width="75%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head"><p class="first last">Value</p>
</th>
<th class="head"><p class="first last">Disposition</p>
</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><p class="first last"><tt class="docutils literal"><span class="pre">"error"</span></tt></p>
</td>
<td><p class="first last">turn matching warnings into exceptions</p>
</td>
</tr>
<tr><td><p class="first last"><tt class="docutils literal"><span class="pre">"ignore"</span></tt></p>
</td>
<td><p class="first last">never print matching warnings</p>
</td>
</tr>
<tr><td><p class="first last"><tt class="docutils literal"><span class="pre">"always"</span></tt></p>
</td>
<td><p class="first last">always print matching warnings</p>
</td>
</tr>
<tr><td><p class="first last"><tt class="docutils literal"><span class="pre">"default"</span></tt></p>
</td>
<td><p class="first last">print the first occurrence of matching
warnings for each location where the warning
is issued</p>
</td>
</tr>
<tr><td><p class="first last"><tt class="docutils literal"><span class="pre">"module"</span></tt></p>
</td>
<td><p class="first last">print the first occurrence of matching
warnings for each module where the warning
is issued</p>
</td>
</tr>
<tr><td><p class="first last"><tt class="docutils literal"><span class="pre">"once"</span></tt></p>
</td>
<td><p class="first last">print only the first occurrence of matching
warnings, regardless of location</p>
</td>
</tr>
</tbody>
</table>
</li>
<li><p class="first"><em>message</em> is a string containing a regular expression that the warning message
must match (the match is compiled to always be case-insensitive).</p>
</li>
<li><p class="first"><em>category</em> is a class (a subclass of <a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a>) of which the warning
category must be a subclass in order to match.</p>
</li>
<li><p class="first"><em>module</em> is a string containing a regular expression that the module name must
match (the match is compiled to be case-sensitive).</p>
</li>
<li><p class="first"><em>lineno</em> is an integer that the line number where the warning occurred must
match, or <tt class="docutils literal"><span class="pre">0</span></tt> to match all line numbers.</p>
</li>
</ul>
<p>Since the <a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a> class is derived from the built-in <a class="reference internal" href="exceptions.html#Exception" title="Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a>
class, to turn a warning into an error we simply raise <tt class="docutils literal"><span class="pre">category(message)</span></tt>.</p>
<p>The warnings filter is initialized by <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><em class="xref std std-option">-W</em></a> options passed to the Python
interpreter command line. The interpreter saves the arguments for all
<a class="reference internal" href="../using/cmdline.html#cmdoption-W"><em class="xref std std-option">-W</em></a> options without interpretation in <tt class="docutils literal"><span class="pre">sys.warnoptions</span></tt>; the
<a class="reference internal" href="#module-warnings" title="warnings: Issue warning messages and control their disposition."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> module parses these when it is first imported (invalid options
are ignored, after printing a message to <tt class="docutils literal"><span class="pre">sys.stderr</span></tt>).</p>
<div class="section" id="default-warning-filters">
<h3>27.5.2.1. Default Warning Filters<a class="headerlink" href="#default-warning-filters" title="Permalink to this headline">¶</a></h3>
<p>By default, Python installs several warning filters, which can be overridden by
the command-line options passed to <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><em class="xref std std-option">-W</em></a> and calls to
<a class="reference internal" href="#warnings.filterwarnings" title="warnings.filterwarnings"><tt class="xref py py-func docutils literal"><span class="pre">filterwarnings()</span></tt></a>.</p>
<ul class="simple">
<li><a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a> and <a class="reference internal" href="exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">PendingDeprecationWarning</span></tt></a>, and
<a class="reference internal" href="exceptions.html#ImportWarning" title="ImportWarning"><tt class="xref py py-exc docutils literal"><span class="pre">ImportWarning</span></tt></a> are ignored.</li>
<li><a class="reference internal" href="exceptions.html#BytesWarning" title="BytesWarning"><tt class="xref py py-exc docutils literal"><span class="pre">BytesWarning</span></tt></a> is ignored unless the <a class="reference internal" href="../using/cmdline.html#cmdoption-b"><em class="xref std std-option">-b</em></a> option is given once or
twice; in this case this warning is either printed (<tt class="docutils literal"><span class="pre">-b</span></tt>) or turned into an
exception (<tt class="docutils literal"><span class="pre">-bb</span></tt>).</li>
<li><a class="reference internal" href="exceptions.html#ResourceWarning" title="ResourceWarning"><tt class="xref py py-exc docutils literal"><span class="pre">ResourceWarning</span></tt></a> is ignored unless Python was built in debug mode.</li>
</ul>
<p class="versionchanged">
<span class="versionmodified">Changed in version 3.2: </span><a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a> is now ignored by default in addition to
<a class="reference internal" href="exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">PendingDeprecationWarning</span></tt></a>.</p>
</div>
</div>
<div class="section" id="temporarily-suppressing-warnings">
<span id="warning-suppress"></span><h2>27.5.3. Temporarily Suppressing Warnings<a class="headerlink" href="#temporarily-suppressing-warnings" title="Permalink to this headline">¶</a></h2>
<p>If you are using code that you know will raise a warning, such as a deprecated
function, but do not want to see the warning, then it is possible to suppress
the warning using the <a class="reference internal" href="#warnings.catch_warnings" title="warnings.catch_warnings"><tt class="xref py py-class docutils literal"><span class="pre">catch_warnings</span></tt></a> context manager:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="k">def</span> <span class="nf">fxn</span><span class="p">():</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">"deprecated"</span><span class="p">,</span> <span class="ne">DeprecationWarning</span><span class="p">)</span>
<span class="k">with</span> <span class="n">warnings</span><span class="o">.</span><span class="n">catch_warnings</span><span class="p">():</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">simplefilter</span><span class="p">(</span><span class="s">"ignore"</span><span class="p">)</span>
<span class="n">fxn</span><span class="p">()</span>
</pre></div>
</div>
<p>While within the context manager all warnings will simply be ignored. This
allows you to use known-deprecated code without having to see the warning while
not suppressing the warning for other code that might not be aware of its use
of deprecated code. Note: this can only be guaranteed in a single-threaded
application. If two or more threads use the <a class="reference internal" href="#warnings.catch_warnings" title="warnings.catch_warnings"><tt class="xref py py-class docutils literal"><span class="pre">catch_warnings</span></tt></a> context
manager at the same time, the behavior is undefined.</p>
</div>
<div class="section" id="testing-warnings">
<span id="warning-testing"></span><h2>27.5.4. Testing Warnings<a class="headerlink" href="#testing-warnings" title="Permalink to this headline">¶</a></h2>
<p>To test warnings raised by code, use the <a class="reference internal" href="#warnings.catch_warnings" title="warnings.catch_warnings"><tt class="xref py py-class docutils literal"><span class="pre">catch_warnings</span></tt></a> context
manager. With it you can temporarily mutate the warnings filter to facilitate
your testing. For instance, do the following to capture all raised warnings to
check:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="k">def</span> <span class="nf">fxn</span><span class="p">():</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">"deprecated"</span><span class="p">,</span> <span class="ne">DeprecationWarning</span><span class="p">)</span>
<span class="k">with</span> <span class="n">warnings</span><span class="o">.</span><span class="n">catch_warnings</span><span class="p">(</span><span class="n">record</span><span class="o">=</span><span class="k">True</span><span class="p">)</span> <span class="k">as</span> <span class="n">w</span><span class="p">:</span>
<span class="c"># Cause all warnings to always be triggered.</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">simplefilter</span><span class="p">(</span><span class="s">"always"</span><span class="p">)</span>
<span class="c"># Trigger a warning.</span>
<span class="n">fxn</span><span class="p">()</span>
<span class="c"># Verify some things</span>
<span class="k">assert</span> <span class="nb">len</span><span class="p">(</span><span class="n">w</span><span class="p">)</span> <span class="o">==</span> <span class="mi">1</span>
<span class="k">assert</span> <span class="nb">issubclass</span><span class="p">(</span><span class="n">w</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span><span class="o">.</span><span class="n">category</span><span class="p">,</span> <span class="ne">DeprecationWarning</span><span class="p">)</span>
<span class="k">assert</span> <span class="s">"deprecated"</span> <span class="ow">in</span> <span class="nb">str</span><span class="p">(</span><span class="n">w</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span><span class="o">.</span><span class="n">message</span><span class="p">)</span>
</pre></div>
</div>
<p>One can also cause all warnings to be exceptions by using <tt class="docutils literal"><span class="pre">error</span></tt> instead of
<tt class="docutils literal"><span class="pre">always</span></tt>. One thing to be aware of is that if a warning has already been
raised because of a <tt class="docutils literal"><span class="pre">once</span></tt>/<tt class="docutils literal"><span class="pre">default</span></tt> rule, then no matter what filters are
set the warning will not be seen again unless the warnings registry related to
the warning has been cleared.</p>
<p>Once the context manager exits, the warnings filter is restored to its state
when the context was entered. This prevents tests from changing the warnings
filter in unexpected ways between tests and leading to indeterminate test
results. The <a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a> function in the module is also restored to
its original value. Note: this can only be guaranteed in a single-threaded
application. If two or more threads use the <a class="reference internal" href="#warnings.catch_warnings" title="warnings.catch_warnings"><tt class="xref py py-class docutils literal"><span class="pre">catch_warnings</span></tt></a> context
manager at the same time, the behavior is undefined.</p>
<p>When testing multiple operations that raise the same kind of warning, it
is important to test them in a manner that confirms each operation is raising
a new warning (e.g. set warnings to be raised as exceptions and check the
operations raise exceptions, check that the length of the warning list
continues to increase after each operation, or else delete the previous
entries from the warnings list before each new operation).</p>
</div>
<div class="section" id="updating-code-for-new-versions-of-python">
<span id="warning-ignored"></span><h2>27.5.5. Updating Code For New Versions of Python<a class="headerlink" href="#updating-code-for-new-versions-of-python" title="Permalink to this headline">¶</a></h2>
<p>Warnings that are only of interest to the developer are ignored by default. As
such you should make sure to test your code with typically ignored warnings
made visible. You can do this from the command-line by passing <em class="xref std std-option">-Wd</em>
to the interpreter (this is shorthand for <em class="xref std std-option">-W default</em>). This enables
default handling for all warnings, including those that are ignored by default.
To change what action is taken for encountered warnings you simply change what
argument is passed to <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><em class="xref std std-option">-W</em></a>, e.g. <em class="xref std std-option">-W error</em>. See the
<a class="reference internal" href="../using/cmdline.html#cmdoption-W"><em class="xref std std-option">-W</em></a> flag for more details on what is possible.</p>
<p>To programmatically do the same as <em class="xref std std-option">-Wd</em>, use:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">warnings</span><span class="o">.</span><span class="n">simplefilter</span><span class="p">(</span><span class="s">'default'</span><span class="p">)</span>
</pre></div>
</div>
<p>Make sure to execute this code as soon as possible. This prevents the
registering of what warnings have been raised from unexpectedly influencing how
future warnings are treated.</p>
<p>Having certain warnings ignored by default is done to prevent a user from
seeing warnings that are only of interest to the developer. As you do not
necessarily have control over what interpreter a user uses to run their code,
it is possible that a new version of Python will be released between your
release cycles. The new interpreter release could trigger new warnings in your
code that were not there in an older interpreter, e.g.
<a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a> for a module that you are using. While you as a
developer want to be notified that your code is using a deprecated module, to a
user this information is essentially noise and provides no benefit to them.</p>
<p>The <a class="reference internal" href="unittest.html#module-unittest" title="unittest: Unit testing framework for Python."><tt class="xref py py-mod docutils literal"><span class="pre">unittest</span></tt></a> module has been also updated to use the <tt class="docutils literal"><span class="pre">'default'</span></tt>
filter while running tests.</p>
</div>
<div class="section" id="available-functions">
<span id="warning-functions"></span><h2>27.5.6. Available Functions<a class="headerlink" href="#available-functions" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="warnings.warn">
<tt class="descclassname">warnings.</tt><tt class="descname">warn</tt><big>(</big><em>message</em>, <em>category=None</em>, <em>stacklevel=1</em><big>)</big><a class="headerlink" href="#warnings.warn" title="Permalink to this definition">¶</a></dt>
<dd><p>Issue a warning, or maybe ignore it or raise an exception. The <em>category</em>
argument, if given, must be a warning category class (see above); it defaults to
<a class="reference internal" href="exceptions.html#UserWarning" title="UserWarning"><tt class="xref py py-exc docutils literal"><span class="pre">UserWarning</span></tt></a>. Alternatively <em>message</em> can be a <a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a> instance,
in which case <em>category</em> will be ignored and <tt class="docutils literal"><span class="pre">message.__class__</span></tt> will be used.
In this case the message text will be <tt class="docutils literal"><span class="pre">str(message)</span></tt>. This function raises an
exception if the particular warning issued is changed into an error by the
warnings filter see above. The <em>stacklevel</em> argument can be used by wrapper
functions written in Python, like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">def</span> <span class="nf">deprecation</span><span class="p">(</span><span class="n">message</span><span class="p">):</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="ne">DeprecationWarning</span><span class="p">,</span> <span class="n">stacklevel</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
<p>This makes the warning refer to <tt class="xref py py-func docutils literal"><span class="pre">deprecation()</span></tt>‘s caller, rather than to the
source of <tt class="xref py py-func docutils literal"><span class="pre">deprecation()</span></tt> itself (since the latter would defeat the purpose
of the warning message).</p>
</dd></dl>
<dl class="function">
<dt id="warnings.warn_explicit">
<tt class="descclassname">warnings.</tt><tt class="descname">warn_explicit</tt><big>(</big><em>message</em>, <em>category</em>, <em>filename</em>, <em>lineno</em>, <em>module=None</em>, <em>registry=None</em>, <em>module_globals=None</em><big>)</big><a class="headerlink" href="#warnings.warn_explicit" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a low-level interface to the functionality of <a class="reference internal" href="#warnings.warn" title="warnings.warn"><tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt></a>, passing in
explicitly the message, category, filename and line number, and optionally the
module name and the registry (which should be the <tt class="docutils literal"><span class="pre">__warningregistry__</span></tt>
dictionary of the module). The module name defaults to the filename with
<tt class="docutils literal"><span class="pre">.py</span></tt> stripped; if no registry is passed, the warning is never suppressed.
<em>message</em> must be a string and <em>category</em> a subclass of <a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a> or
<em>message</em> may be a <a class="reference internal" href="exceptions.html#Warning" title="Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a> instance, in which case <em>category</em> will be
ignored.</p>
<p><em>module_globals</em>, if supplied, should be the global namespace in use by the code
for which the warning is issued. (This argument is used to support displaying
source for modules found in zipfiles or other non-filesystem import
sources).</p>
</dd></dl>
<dl class="function">
<dt id="warnings.showwarning">
<tt class="descclassname">warnings.</tt><tt class="descname">showwarning</tt><big>(</big><em>message</em>, <em>category</em>, <em>filename</em>, <em>lineno</em>, <em>file=None</em>, <em>line=None</em><big>)</big><a class="headerlink" href="#warnings.showwarning" title="Permalink to this definition">¶</a></dt>
<dd><p>Write a warning to a file. The default implementation calls
<tt class="docutils literal"><span class="pre">formatwarning(message,</span> <span class="pre">category,</span> <span class="pre">filename,</span> <span class="pre">lineno,</span> <span class="pre">line)</span></tt> and writes the
resulting string to <em>file</em>, which defaults to <tt class="docutils literal"><span class="pre">sys.stderr</span></tt>. You may replace
this function with an alternative implementation by assigning to
<tt class="docutils literal"><span class="pre">warnings.showwarning</span></tt>.
<em>line</em> is a line of source code to be included in the warning
message; if <em>line</em> is not supplied, <a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a> will
try to read the line specified by <em>filename</em> and <em>lineno</em>.</p>
</dd></dl>
<dl class="function">
<dt id="warnings.formatwarning">
<tt class="descclassname">warnings.</tt><tt class="descname">formatwarning</tt><big>(</big><em>message</em>, <em>category</em>, <em>filename</em>, <em>lineno</em>, <em>line=None</em><big>)</big><a class="headerlink" href="#warnings.formatwarning" title="Permalink to this definition">¶</a></dt>
<dd><p>Format a warning the standard way. This returns a string which may contain
embedded newlines and ends in a newline. <em>line</em> is a line of source code to
be included in the warning message; if <em>line</em> is not supplied,
<a class="reference internal" href="#warnings.formatwarning" title="warnings.formatwarning"><tt class="xref py py-func docutils literal"><span class="pre">formatwarning()</span></tt></a> will try to read the line specified by <em>filename</em> and
<em>lineno</em>.</p>
</dd></dl>
<dl class="function">
<dt id="warnings.filterwarnings">
<tt class="descclassname">warnings.</tt><tt class="descname">filterwarnings</tt><big>(</big><em>action</em>, <em>message=''</em>, <em>category=Warning</em>, <em>module=''</em>, <em>lineno=0</em>, <em>append=False</em><big>)</big><a class="headerlink" href="#warnings.filterwarnings" title="Permalink to this definition">¶</a></dt>
<dd><p>Insert an entry into the list of <a class="reference internal" href="#warning-filter"><em>warnings filter specifications</em></a>. The entry is inserted at the front by default; if
<em>append</em> is true, it is inserted at the end. This checks the types of the
arguments, compiles the <em>message</em> and <em>module</em> regular expressions, and
inserts them as a tuple in the list of warnings filters. Entries closer to
the front of the list override entries later in the list, if both match a
particular warning. Omitted arguments default to a value that matches
everything.</p>
</dd></dl>
<dl class="function">
<dt id="warnings.simplefilter">
<tt class="descclassname">warnings.</tt><tt class="descname">simplefilter</tt><big>(</big><em>action</em>, <em>category=Warning</em>, <em>lineno=0</em>, <em>append=False</em><big>)</big><a class="headerlink" href="#warnings.simplefilter" title="Permalink to this definition">¶</a></dt>
<dd><p>Insert a simple entry into the list of <a class="reference internal" href="#warning-filter"><em>warnings filter specifications</em></a>. The meaning of the function parameters is as for
<a class="reference internal" href="#warnings.filterwarnings" title="warnings.filterwarnings"><tt class="xref py py-func docutils literal"><span class="pre">filterwarnings()</span></tt></a>, but regular expressions are not needed as the filter
inserted always matches any message in any module as long as the category and
line number match.</p>
</dd></dl>
<dl class="function">
<dt id="warnings.resetwarnings">
<tt class="descclassname">warnings.</tt><tt class="descname">resetwarnings</tt><big>(</big><big>)</big><a class="headerlink" href="#warnings.resetwarnings" title="Permalink to this definition">¶</a></dt>
<dd><p>Reset the warnings filter. This discards the effect of all previous calls to
<a class="reference internal" href="#warnings.filterwarnings" title="warnings.filterwarnings"><tt class="xref py py-func docutils literal"><span class="pre">filterwarnings()</span></tt></a>, including that of the <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><em class="xref std std-option">-W</em></a> command line options
and calls to <a class="reference internal" href="#warnings.simplefilter" title="warnings.simplefilter"><tt class="xref py py-func docutils literal"><span class="pre">simplefilter()</span></tt></a>.</p>
</dd></dl>
</div>
<div class="section" id="available-context-managers">
<h2>27.5.7. Available Context Managers<a class="headerlink" href="#available-context-managers" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="warnings.catch_warnings">
<em class="property">class </em><tt class="descclassname">warnings.</tt><tt class="descname">catch_warnings</tt><big>(</big><em>*</em>, <em>record=False</em>, <em>module=None</em><big>)</big><a class="headerlink" href="#warnings.catch_warnings" title="Permalink to this definition">¶</a></dt>
<dd><p>A context manager that copies and, upon exit, restores the warnings filter
and the <a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a> function.
If the <em>record</em> argument is <a class="reference internal" href="constants.html#False" title="False"><tt class="xref py py-const xref docutils literal"><span class="pre">False</span></tt></a> (the default) the context manager
returns <a class="reference internal" href="constants.html#None" title="None"><tt class="xref py py-class xref docutils literal"><span class="pre">None</span></tt></a> on entry. If <em>record</em> is <a class="reference internal" href="constants.html#True" title="True"><tt class="xref py py-const xref docutils literal"><span class="pre">True</span></tt></a>, a list is
returned that is progressively populated with objects as seen by a custom
<a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a> function (which also suppresses output to <tt class="docutils literal"><span class="pre">sys.stdout</span></tt>).
Each object in the list has attributes with the same names as the arguments to
<a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a>.</p>
<p>The <em>module</em> argument takes a module that will be used instead of the
module returned when you import <a class="reference internal" href="#module-warnings" title="warnings: Issue warning messages and control their disposition."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> whose filter will be
protected. This argument exists primarily for testing the <a class="reference internal" href="#module-warnings" title="warnings: Issue warning messages and control their disposition."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a>
module itself.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference internal" href="#warnings.catch_warnings" title="warnings.catch_warnings"><tt class="xref py py-class docutils literal"><span class="pre">catch_warnings</span></tt></a> manager works by replacing and
then later restoring the module’s
<a class="reference internal" href="#warnings.showwarning" title="warnings.showwarning"><tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt></a> function and internal list of filter
specifications. This means the context manager is modifying
global state and therefore is not thread-safe.</p>
</div>
</dd></dl>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">27.5. <tt class="docutils literal"><span class="pre">warnings</span></tt> — Warning control</a><ul>
<li><a class="reference internal" href="#warning-categories">27.5.1. Warning Categories</a></li>
<li><a class="reference internal" href="#the-warnings-filter">27.5.2. The Warnings Filter</a><ul>
<li><a class="reference internal" href="#default-warning-filters">27.5.2.1. Default Warning Filters</a></li>
</ul>
</li>
<li><a class="reference internal" href="#temporarily-suppressing-warnings">27.5.3. Temporarily Suppressing Warnings</a></li>
<li><a class="reference internal" href="#testing-warnings">27.5.4. Testing Warnings</a></li>
<li><a class="reference internal" href="#updating-code-for-new-versions-of-python">27.5.5. Updating Code For New Versions of Python</a></li>
<li><a class="reference internal" href="#available-functions">27.5.6. Available Functions</a></li>
<li><a class="reference internal" href="#available-context-managers">27.5.7. Available Context Managers</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="__main__.html"
title="previous chapter">27.4. <tt class="docutils literal docutils literal docutils literal"><span class="pre">__main__</span></tt> — Top-level script environment</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="contextlib.html"
title="next chapter">27.6. <tt class="docutils literal docutils literal docutils literal"><span class="pre">contextlib</span></tt> — Utilities for <tt class="docutils literal docutils literal docutils literal"><span class="pre">with</span></tt>-statement contexts</a></p>
<h3>This Page</h3>
<ul class="this-page-menu">
<li><a href="../bugs.html">Report a Bug</a></li>
<li><a href="../_sources/library/warnings.txt"
rel="nofollow">Show Source</a></li>
</ul>
<div id="searchbox" style="display: none">
<h3>Quick search</h3>
<form class="search" action="../search.html" method="get">
<input type="text" name="q" size="18" />
<input type="submit" value="Go" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
<p class="searchtip" style="font-size: 90%">
Enter search terms or a module, class or function name.
</p>
</div>
<script type="text/javascript">$('#searchbox').show(0);</script>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="../genindex.html" title="General Index"
>index</a></li>
<li class="right" >
<a href="../py-modindex.html" title="Python Module Index"
>modules</a> |</li>
<li class="right" >
<a href="contextlib.html" title="27.6. contextlib — Utilities for with-statement contexts"
>next</a> |</li>
<li class="right" >
<a href="__main__.html" title="27.4. __main__ — Top-level script environment"
>previous</a> |</li>
<li><img src="../_static/py.png" alt=""
style="vertical-align: middle; margin-top: -1px"/></li>
<li><a href="../index.html">Python v3.2.2 documentation</a> »</li>
<li><a href="index.html" >The Python Standard Library</a> »</li>
<li><a href="python.html" >27. Python Runtime Services</a> »</li>
</ul>
</div>
<div class="footer">
© <a href="../copyright.html">Copyright</a> 1990-2011, Python Software Foundation.
<br />
The Python Software Foundation is a non-profit corporation.
<a href="http://www.python.org/psf/donations/">Please donate.</a>
<br />
Last updated on Sep 04, 2011.
<a href="../bugs.html">Found a bug</a>?
<br />
Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.0.7.
</div>
</body>
</html>
distrib
>
Mandriva
>
2010.1
>
x86_64
>
media
>
contrib-backports
>
by-pkgid
>
3ba3bd1608c672ba2129b098a48e9e4d
>
files
>
846
