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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">warnings – Non-fatal alerts</a><ul>
<li><a class="reference internal" href="#categories-and-filtering">Categories and Filtering</a></li>
<li><a class="reference internal" href="#generating-warnings">Generating Warnings</a></li>
<li><a class="reference internal" href="#filtering-with-patterns">Filtering with Patterns</a></li>
<li><a class="reference internal" href="#repeated-warnings">Repeated Warnings</a></li>
<li><a class="reference internal" href="#alternate-message-delivery-functions">Alternate Message Delivery Functions</a></li>
<li><a class="reference internal" href="#formatting">Formatting</a></li>
<li><a class="reference internal" href="#stack-level-in-warnings">Stack Level in Warnings</a></li>
</ul>
</li>
</ul>
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<div class="section" id="module-warnings">
<span id="warnings-non-fatal-alerts"></span><h1>warnings – Non-fatal alerts<a class="headerlink" href="#module-warnings" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">Deliver non-fatal alerts to the user about issues encountered when running a program.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">2.1 and later</td>
</tr>
</tbody>
</table>
<p>The <a class="reference internal" href="#module-warnings" title="warnings: Deliver non-fatal alerts to the user about issues encountered when running a program."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> module was introduced in <span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0230"><strong>PEP 230</strong></a> as a way to
warn programmers about changes in language or library features in
anticipation of backwards incompatible changes coming with Python
3.0. Since warnings are not fatal, a program may encounter the same
warn-able situation many times in the course of running. The
<a class="reference internal" href="#module-warnings" title="warnings: Deliver non-fatal alerts to the user about issues encountered when running a program."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> module suppresses repeated warnings from the same
source to cut down on the annoyance of seeing the same message over
and over. You can control the messages printed on a case-by-case
basis using the <em class="xref std std-option">-W</em> option to the interpreter or by calling
functions found in <a class="reference internal" href="#module-warnings" title="warnings: Deliver non-fatal alerts to the user about issues encountered when running a program."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> from your code.</p>
<div class="section" id="categories-and-filtering">
<h2>Categories and Filtering<a class="headerlink" href="#categories-and-filtering" title="Permalink to this headline">¶</a></h2>
<p>Warnings are categorized using subclasses of the built-in exception
class <tt class="xref py py-class docutils literal"><span class="pre">Warning</span></tt>. Several standard values are <a class="reference internal" href="../exceptions/index.html#exceptions-warning"><em>described in
the documentation</em></a>, and you can add your own by
subclassing from <tt class="xref py py-class docutils literal"><span class="pre">Warning</span></tt> to create a new class.</p>
<p>Messages are filtered using settings controlled through the
<em class="xref std std-option">-W</em> option to the interpreter. A filter consists of 5 parts,
the <em>action</em>, <em>message</em>, <em>category</em>, <em>module</em>, and <em>line number</em>. When
a warning is generated, it is compared against all of the registered
filters. The first filter that matches controls the action taken for
the warning. If no filter matches, the default action is taken.</p>
<p>The actions understood by the filtering mechanism are:</p>
<ul class="simple">
<li>error: Turn the warning into an exception.</li>
<li>ignore: Discard the warning.</li>
<li>always: Always emit a warning.</li>
<li>default: Print the warning the first time it is generated from each location.</li>
<li>module: Print the warning the first time it is generated from each module.</li>
<li>once: Print the warning the first time it is generated.</li>
</ul>
<p>The <em>message</em> portion of the filter is a regular expression that is
used to match the warning text.</p>
<p>The <em>category</em> is a name of an exception class, as described above.</p>
<p>The <em>module</em> contains a regular expression to be matched against the
module name generating the warning.</p>
<p>And the <em>line number</em> can be used to change the handling on specific
occurrences of a warning. Use <tt class="docutils literal"><span class="pre">0</span></tt> to have the filter apply to all
occurrences.</p>
</div>
<div class="section" id="generating-warnings">
<h2>Generating Warnings<a class="headerlink" href="#generating-warnings" title="Permalink to this headline">¶</a></h2>
<p>The simplest way to emit a warning from your own code is to just call
<tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt> with the message as an argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="k">print</span> <span class="s">'Before the warning'</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">'This is a warning message'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'After the warning'</span>
</pre></div>
</div>
<p>Then when your program runs, the message is printed:</p>
<div class="highlight-python"><pre>$ python warnings_warn.py
warnings_warn.py:13: UserWarning: This is a warning message
warnings.warn('This is a warning message')
Before the warning
After the warning</pre>
</div>
<p>Even though the warning is printed, the default behavior is to continue past
the warning and run the rest of the program. We can change that behavior with
a filter:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">simplefilter</span><span class="p">(</span><span class="s">'error'</span><span class="p">,</span> <span class="ne">UserWarning</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Before the warning'</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">'This is a warning message'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'After the warning'</span>
</pre></div>
</div>
<p>This filter tells the warnings module to raise an exception when the warning
is issued.</p>
<div class="highlight-python"><pre>$ python warnings_warn_raise.py
Before the warning
Traceback (most recent call last):
File "warnings_warn_raise.py", line 15, in <module>
warnings.warn('This is a warning message')
UserWarning: This is a warning message</pre>
</div>
<p>We can also control the filter behavior from the command line. For
example, if we go back to <tt class="docutils literal"><span class="pre">warnings_warn.py</span></tt> and set the filter to
raise an error on <tt class="xref py py-class docutils literal"><span class="pre">UserWarning</span></tt>, we see the exception:</p>
<div class="highlight-python"><pre>$ python -W "error::UserWarning::0" warnings_warn.py
Before the warning
Traceback (most recent call last):
File "warnings_warn.py", line 13, in <module>
warnings.warn('This is a warning message')
UserWarning: This is a warning message</pre>
</div>
<p>Since I left the fields for <em>message</em> and <em>module</em> blank, they were
interpreted as matching anything.</p>
</div>
<div class="section" id="filtering-with-patterns">
<h2>Filtering with Patterns<a class="headerlink" href="#filtering-with-patterns" title="Permalink to this headline">¶</a></h2>
<p>To filter on more complex rules programmatically, use
<tt class="xref py py-func docutils literal"><span class="pre">filterwarnings()</span></tt>. For example, to filter based on the content
of the message text:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">filterwarnings</span><span class="p">(</span><span class="s">'ignore'</span><span class="p">,</span> <span class="s">'.*do not.*'</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">'Show this 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="s">'Do not show this message'</span><span class="p">)</span>
</pre></div>
</div>
<p>The pattern contains “<tt class="docutils literal"><span class="pre">do</span> <span class="pre">not</span></tt>”, but the actual message uses “<tt class="docutils literal"><span class="pre">Do</span>
<span class="pre">not</span></tt>”. The pattern matches because the regular expression is always
compiled to look for case insensitive matches.</p>
<div class="highlight-python"><pre>$ python warnings_filterwarnings_message.py
warnings_filterwarnings_message.py:14: UserWarning: Show this message
warnings.warn('Show this message')</pre>
</div>
<p>Running this source from the command line:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">'Show this 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="s">'Do not show this message'</span><span class="p">)</span>
</pre></div>
</div>
<p>yields:</p>
<div class="highlight-python"><pre>$ python -W "ignore:do not:UserWarning::0" warnings_filtering.py
warnings_filtering.py:12: UserWarning: Show this message
warnings.warn('Show this message')</pre>
</div>
<p>The same pattern matching rules apply to the name of the source module
containing the warning call. To suppress all warnings from the
<tt class="docutils literal"><span class="pre">warnings_filtering</span></tt> module:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">filterwarnings</span><span class="p">(</span><span class="s">'ignore'</span><span class="p">,</span>
<span class="s">'.*'</span><span class="p">,</span>
<span class="ne">UserWarning</span><span class="p">,</span>
<span class="s">'warnings_filtering'</span><span class="p">,</span>
<span class="p">)</span>
<span class="kn">import</span> <span class="nn">warnings_filtering</span>
</pre></div>
</div>
<p>Since the filter is in place, no warnings are emitted when
<tt class="docutils literal"><span class="pre">warnings_filtering</span></tt> is imported:</p>
<div class="highlight-python"><pre>$ python warnings_filterwarnings_module.py</pre>
</div>
<p>To suppress only the warning on line 14 of <tt class="docutils literal"><span class="pre">warnings_filtering</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">filterwarnings</span><span class="p">(</span><span class="s">'ignore'</span><span class="p">,</span>
<span class="s">'.*'</span><span class="p">,</span>
<span class="ne">UserWarning</span><span class="p">,</span>
<span class="s">'warnings_filtering'</span><span class="p">,</span>
<span class="mi">14</span><span class="p">)</span>
<span class="kn">import</span> <span class="nn">warnings_filtering</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python warnings_filterwarnings_lineno.py
/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/warnings/warnings_filtering.py:12: UserWarning: Show this message
warnings.warn('Show this message')
/Users/dhellmann/Documents/PyMOTW/src/PyMOTW/warnings/warnings_filtering.py:13: UserWarning: Do not show this message
warnings.warn('Do not show this message')</pre>
</div>
</div>
<div class="section" id="repeated-warnings">
<h2>Repeated Warnings<a class="headerlink" href="#repeated-warnings" title="Permalink to this headline">¶</a></h2>
<p>By default, most types of warnings are only printed the first time they occur
in a given location, where location is defined as the combination of module
and line number.</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">function_with_warning</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">'This is a warning!'</span><span class="p">)</span>
<span class="n">function_with_warning</span><span class="p">()</span>
<span class="n">function_with_warning</span><span class="p">()</span>
<span class="n">function_with_warning</span><span class="p">()</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python warnings_repeated.py
warnings_repeated.py:13: UserWarning: This is a warning!
warnings.warn('This is a warning!')</pre>
</div>
<p>The “once” action can be used to suppress instances of the same message from
different locations.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">simplefilter</span><span class="p">(</span><span class="s">'once'</span><span class="p">,</span> <span class="ne">UserWarning</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">'This is a warning!'</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">'This is a warning!'</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">'This is a warning!'</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python warnings_once.py
warnings_once.py:14: UserWarning: This is a warning!
warnings.warn('This is a warning!')</pre>
</div>
<p>Similarly, “module” will suppress repeated messages from the same module, no
matter what line number.</p>
</div>
<div class="section" id="alternate-message-delivery-functions">
<h2>Alternate Message Delivery Functions<a class="headerlink" href="#alternate-message-delivery-functions" title="Permalink to this headline">¶</a></h2>
<p>Normally warnings are printed to <a class="reference internal" href="../sys/runtime.html#sys-input-output"><em>sys.stderr</em></a>. You can change that behavior by replacing the
<tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt> function inside the <a class="reference internal" href="#module-warnings" title="warnings: Deliver non-fatal alerts to the user about issues encountered when running a program."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> module. For
example, if you wanted warnings to go to a log file instead of stderr,
you could replace <tt class="xref py py-func docutils literal"><span class="pre">showwarning()</span></tt> with a function like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">warnings</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">send_warnings_to_log</span><span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="n">category</span><span class="p">,</span> <span class="n">filename</span><span class="p">,</span> <span class="n">lineno</span><span class="p">,</span> <span class="nb">file</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span>
<span class="s">'</span><span class="si">%s</span><span class="s">:</span><span class="si">%s</span><span class="s">: </span><span class="si">%s</span><span class="s">:</span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span>
<span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">lineno</span><span class="p">,</span> <span class="n">category</span><span class="o">.</span><span class="n">__name__</span><span class="p">,</span> <span class="n">message</span><span class="p">))</span>
<span class="k">return</span>
<span class="n">old_showwarning</span> <span class="o">=</span> <span class="n">warnings</span><span class="o">.</span><span class="n">showwarning</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">showwarning</span> <span class="o">=</span> <span class="n">send_warnings_to_log</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">'This is a warning message'</span><span class="p">)</span>
</pre></div>
</div>
<p>So that when <tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt> is called, the warnings are emitted with
the rest of the log messages.</p>
<div class="highlight-python"><pre>$ python warnings_showwarning.py
WARNING:root:warnings_showwarning.py:24: UserWarning:This is a warning message</pre>
</div>
</div>
<div class="section" id="formatting">
<h2>Formatting<a class="headerlink" href="#formatting" title="Permalink to this headline">¶</a></h2>
<p>If it is OK for warnings to go to stderr, but you don’t like the
formatting, you can replace <tt class="xref py py-func docutils literal"><span class="pre">formatwarning()</span></tt> instead.</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">warning_on_one_line</span><span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="n">category</span><span class="p">,</span> <span class="n">filename</span><span class="p">,</span> <span class="n">lineno</span><span class="p">,</span> <span class="nb">file</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">line</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="k">return</span> <span class="s">' </span><span class="si">%s</span><span class="s">:</span><span class="si">%s</span><span class="s">: </span><span class="si">%s</span><span class="s">:</span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">lineno</span><span class="p">,</span> <span class="n">category</span><span class="o">.</span><span class="n">__name__</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="s">'This is a warning message, before'</span><span class="p">)</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">formatwarning</span> <span class="o">=</span> <span class="n">warning_on_one_line</span>
<span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s">'This is a warning message, after'</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python warnings_formatwarning.py
warnings_formatwarning.py:15: UserWarning: This is a warning message, before
warnings.warn('This is a warning message, before')
warnings_formatwarning.py:17: UserWarning:This is a warning message, after</pre>
</div>
</div>
<div class="section" id="stack-level-in-warnings">
<h2>Stack Level in Warnings<a class="headerlink" href="#stack-level-in-warnings" title="Permalink to this headline">¶</a></h2>
<p>You’ll notice that by default the warning message includes the source
line that generated it, when available. It’s not all that useful to
see the line of code with the actual warning message, though. Instead,
you can tell <tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt> how far up the stack it has to go to find
the line the called the function containing the warning. That way
users of a deprecated function see where the function is called,
instead of the implementation of the function.</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">old_function</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">'old_function() is deprecated, use new_function() instead'</span><span class="p">,</span>
<span class="n">stacklevel</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">caller_of_old_function</span><span class="p">():</span>
<span class="n">old_function</span><span class="p">()</span>
<span class="n">caller_of_old_function</span><span class="p">()</span>
</pre></div>
</div>
<p>Notice that in this example <tt class="xref py py-func docutils literal"><span class="pre">warn()</span></tt> needs to go up the stack 2
levels, one for itself and one for <tt class="xref py py-func docutils literal"><span class="pre">old_function()</span></tt>.</p>
<div class="highlight-python"><pre>$ python warnings_warn_stacklevel.py
warnings_warn_stacklevel.py:18: UserWarning: old_function() is deprecated, use new_function() instead
old_function()</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://docs.python.org/lib/module-warnings.html">warnings</a></dt>
<dd>Standard library documentation for this module.</dd>
<dt><span class="target" id="index-1"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0230"><strong>PEP 230</strong></a></dt>
<dd>Warning Framework</dd>
<dt><a class="reference internal" href="../exceptions/index.html#module-exceptions" title="exceptions: Built-in error classes"><tt class="xref py py-mod docutils literal"><span class="pre">exceptions</span></tt></a></dt>
<dd>Base classes for exceptions and warnings.</dd>
</dl>
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