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<!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>27.6. warnings — Warning control — Python 2.7.5 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: '2.7.5', 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 2.7.5 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 2.7.5 documentation" href="../index.html" /> <link rel="up" title="27. Python Runtime Services" href="python.html" /> <link rel="next" title="27.7. contextlib — Utilities for with-statement contexts" href="contextlib.html" /> <link rel="prev" title="27.5. __main__ — Top-level script environment" href="__main__.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </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="27.7. contextlib — Utilities for with-statement contexts" accesskey="N">next</a> |</li> <li class="right" > <a href="__main__.html" title="27.5. __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="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 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>27.6. <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 class="versionadded"> <span class="versionmodified">New in version 2.1.</span></p> <p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/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>27.6.1. 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 class="row-odd"><th class="head">Class</th> <th class="head">Description</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><a class="reference internal" href="exceptions.html#exceptions.Warning" title="exceptions.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#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a>.</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="exceptions.html#exceptions.UserWarning" title="exceptions.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 class="row-even"><td><a class="reference internal" href="exceptions.html#exceptions.DeprecationWarning" title="exceptions.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 class="row-odd"><td><a class="reference internal" href="exceptions.html#exceptions.SyntaxWarning" title="exceptions.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 class="row-even"><td><a class="reference internal" href="exceptions.html#exceptions.RuntimeWarning" title="exceptions.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 class="row-odd"><td><a class="reference internal" href="exceptions.html#exceptions.FutureWarning" title="exceptions.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 class="row-even"><td><a class="reference internal" href="exceptions.html#exceptions.PendingDeprecationWarning" title="exceptions.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 class="row-odd"><td><a class="reference internal" href="exceptions.html#exceptions.ImportWarning" title="exceptions.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 class="row-even"><td><a class="reference internal" href="exceptions.html#exceptions.UnicodeWarning" title="exceptions.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> </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#exceptions.Warning" title="exceptions.Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a> class.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.7: </span><a class="reference internal" href="exceptions.html#exceptions.DeprecationWarning" title="exceptions.DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a> is ignored by default.</p> </div> <div class="section" id="the-warnings-filter"> <span id="warning-filter"></span><h2>27.6.2. 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 class="row-odd"><th class="head">Value</th> <th class="head">Disposition</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">"error"</span></tt></td> <td>turn matching warnings into exceptions</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">"ignore"</span></tt></td> <td>never print matching warnings</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">"always"</span></tt></td> <td>always print matching warnings</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">"default"</span></tt></td> <td>print the first occurrence of matching warnings for each location where the warning is issued</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">"module"</span></tt></td> <td>print the first occurrence of matching warnings for each module where the warning is issued</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">"once"</span></tt></td> <td>print only the first occurrence of matching warnings, regardless of location</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#exceptions.Warning" title="exceptions.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#exceptions.Warning" title="exceptions.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#exceptions.Exception" title="exceptions.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>27.6.2.1. 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#exceptions.DeprecationWarning" title="exceptions.DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a> and <a class="reference internal" href="exceptions.html#exceptions.PendingDeprecationWarning" title="exceptions.PendingDeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">PendingDeprecationWarning</span></tt></a>, and <a class="reference internal" href="exceptions.html#exceptions.ImportWarning" title="exceptions.ImportWarning"><tt class="xref py py-exc docutils literal"><span class="pre">ImportWarning</span></tt></a> are ignored.</li> <li><tt class="xref py py-exc docutils literal"><span class="pre">BytesWarning</span></tt> is ignored unless the <em class="xref std std-option">-b</em> 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> </ul> </div> </div> <div class="section" id="temporarily-suppressing-warnings"> <span id="warning-suppress"></span><h2>27.6.3. 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-python"><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>27.6.4. 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-python"><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="bp">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"> <h2>27.6.5. 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-python"><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#exceptions.DeprecationWarning" title="exceptions.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> </div> <div class="section" id="available-functions"> <span id="warning-functions"></span><h2>27.6.6. 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><span class="optional">[</span>, <em>category</em><span class="optional">[</span>, <em>stacklevel</em><span class="optional">]</span><span class="optional">]</span><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#exceptions.UserWarning" title="exceptions.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#exceptions.Warning" title="exceptions.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-python"><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><span class="optional">[</span>, <em>module</em><span class="optional">[</span>, <em>registry</em><span class="optional">[</span>, <em>module_globals</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><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#exceptions.Warning" title="exceptions.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#exceptions.Warning" title="exceptions.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> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>Added the <em>module_globals</em> parameter.</p> </dd></dl> <dl class="function"> <dt id="warnings.warnpy3k"> <tt class="descclassname">warnings.</tt><tt class="descname">warnpy3k</tt><big>(</big><em>message</em><span class="optional">[</span>, <em>category</em><span class="optional">[</span>, <em>stacklevel</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#warnings.warnpy3k" title="Permalink to this definition">¶</a></dt> <dd><p>Issue a warning related to Python 3.x deprecation. Warnings are only shown when Python is started with the -3 option. Like <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> <em>message</em> must be a string and <em>category</em> a subclass of <a class="reference internal" href="exceptions.html#exceptions.Warning" title="exceptions.Warning"><tt class="xref py py-exc docutils literal"><span class="pre">Warning</span></tt></a>. <a class="reference internal" href="#warnings.warnpy3k" title="warnings.warnpy3k"><tt class="xref py py-func docutils literal"><span class="pre">warnpy3k()</span></tt></a> is using <a class="reference internal" href="exceptions.html#exceptions.DeprecationWarning" title="exceptions.DeprecationWarning"><tt class="xref py py-exc docutils literal"><span class="pre">DeprecationWarning</span></tt></a> as default warning class.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.6.</span></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><span class="optional">[</span>, <em>file</em><span class="optional">[</span>, <em>line</em><span class="optional">]</span><span class="optional">]</span><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> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.7: </span>The <em>line</em> argument is required to be supported.</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><span class="optional">[</span>, <em>line</em><span class="optional">]</span><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> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.6: </span>Added the <em>line</em> argument.</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><span class="optional">[</span>, <em>message</em><span class="optional">[</span>, <em>category</em><span class="optional">[</span>, <em>module</em><span class="optional">[</span>, <em>lineno</em><span class="optional">[</span>, <em>append</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><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><span class="optional">[</span>, <em>category</em><span class="optional">[</span>, <em>lineno</em><span class="optional">[</span>, <em>append</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><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>27.6.7. 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><span class="optional">[</span><em>*</em>, <em>record=False</em>, <em>module=None</em><span class="optional">]</span><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 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 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 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> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">In Python 3, the arguments to the constructor for <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> are keyword-only arguments.</p> </div> <p class="versionadded"> <span class="versionmodified">New in version 2.6.</span></p> </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="#">27.6. <tt class="docutils literal"><span class="pre">warnings</span></tt> — Warning control</a><ul> <li><a class="reference internal" href="#warning-categories">27.6.1. Warning Categories</a></li> <li><a class="reference internal" href="#the-warnings-filter">27.6.2. The Warnings Filter</a><ul> <li><a class="reference internal" href="#default-warning-filters">27.6.2.1. Default Warning Filters</a></li> </ul> </li> <li><a class="reference internal" href="#temporarily-suppressing-warnings">27.6.3. Temporarily Suppressing Warnings</a></li> <li><a class="reference internal" href="#testing-warnings">27.6.4. Testing Warnings</a></li> <li><a class="reference internal" href="#updating-code-for-new-versions-of-python">27.6.5. Updating Code For New Versions of Python</a></li> <li><a class="reference internal" href="#available-functions">27.6.6. Available Functions</a></li> <li><a class="reference internal" href="#available-context-managers">27.6.7. Available Context Managers</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="__main__.html" title="previous chapter">27.5. <tt class="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">27.7. <tt class="docutils literal"><span class="pre">contextlib</span></tt> — Utilities for <tt class="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" /> <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="27.7. contextlib — Utilities for with-statement contexts" >next</a> |</li> <li class="right" > <a href="__main__.html" title="27.5. __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="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 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> 1990-2019, 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 Jul 03, 2019. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.1.3. </div> </body> </html>
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