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<article class="bd-article">
<section id="coding-guidelines">
<span id="id1"></span><h1>Coding guidelines<a class="headerlink" href="#coding-guidelines" title="Link to this heading">#</a></h1>
<p>We appreciate these guidelines being followed because it improves the readability,
consistency, and maintainability of the code base.</p>
<div class="seealso admonition">
<p class="admonition-title">API guidelines</p>
<p>If adding new features, changing behavior or function signatures, or removing
public interfaces, please consult the <a class="reference internal" href="api_changes.html#api-changes"><span class="std std-ref">API guidelines</span></a>.</p>
</div>
<section id="pep8-as-enforced-by-flake8">
<span id="code-style"></span><h2>PEP8, as enforced by flake8<a class="headerlink" href="#pep8-as-enforced-by-flake8" title="Link to this heading">#</a></h2>
<p>Formatting should follow the recommendations of <a class="reference external" href="https://www.python.org/dev/peps/pep-0008/">PEP8</a>, as enforced by <a class="reference external" href="https://flake8.pycqa.org/">flake8</a>.
Matplotlib modifies PEP8 to extend the maximum line length to 88
characters. You can check flake8 compliance from the command line with</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="o">-</span><span class="n">m</span> <span class="n">pip</span> <span class="n">install</span> <span class="n">flake8</span>
<span class="n">flake8</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">module</span><span class="o">.</span><span class="n">py</span>
</pre></div>
</div>
<p>or your editor may provide integration with it. Note that Matplotlib intentionally
does not use the <a class="reference external" href="https://black.readthedocs.io/">black</a> auto-formatter (<a class="reference external" href="https://github.com/matplotlib/matplotlib/issues/18796">1</a>), in particular due to its inability
to understand the semantics of mathematical expressions (<a class="reference external" href="https://github.com/psf/black/issues/148">2</a>, <a class="reference external" href="https://github.com/psf/black/issues/1984">3</a>).</p>
</section>
<section id="package-imports">
<h2>Package imports<a class="headerlink" href="#package-imports" title="Link to this heading">#</a></h2>
<p>Import the following modules using the standard scipy conventions:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">numpy.ma</span> <span class="k">as</span> <span class="nn">ma</span>
<span class="kn">import</span> <span class="nn">matplotlib</span> <span class="k">as</span> <span class="nn">mpl</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">import</span> <span class="nn">matplotlib.cbook</span> <span class="k">as</span> <span class="nn">cbook</span>
<span class="kn">import</span> <span class="nn">matplotlib.patches</span> <span class="k">as</span> <span class="nn">mpatches</span>
</pre></div>
</div>
<p>In general, Matplotlib modules should <strong>not</strong> import <a class="reference internal" href="../api/matplotlib_configuration_api.html#matplotlib.rcParams" title="matplotlib.rcParams"><code class="xref py py-obj docutils literal notranslate"><span class="pre">rcParams</span></code></a> using <code class="docutils literal notranslate"><span class="pre">from</span>
<span class="pre">matplotlib</span> <span class="pre">import</span> <span class="pre">rcParams</span></code>, but rather access it as <code class="docutils literal notranslate"><span class="pre">mpl.rcParams</span></code>. This
is because some modules are imported very early, before the <a class="reference internal" href="../api/matplotlib_configuration_api.html#matplotlib.rcParams" title="matplotlib.rcParams"><code class="xref py py-obj docutils literal notranslate"><span class="pre">rcParams</span></code></a>
singleton is constructed.</p>
</section>
<section id="variable-names">
<h2>Variable names<a class="headerlink" href="#variable-names" title="Link to this heading">#</a></h2>
<p>When feasible, please use our internal variable naming convention for objects
of a given class and objects of any child class:</p>
<table class="table">
<thead>
<tr class="row-odd"><th class="head"><p>base class</p></th>
<th class="head"><p>variable</p></th>
<th class="head"><p>multiples</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><a class="reference internal" href="../api/figure_api.html#matplotlib.figure.FigureBase" title="matplotlib.figure.FigureBase"><code class="xref py py-obj docutils literal notranslate"><span class="pre">FigureBase</span></code></a></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">fig</span></code></p></td>
<td></td>
</tr>
<tr class="row-odd"><td><p><a class="reference internal" href="../api/_as_gen/matplotlib.axes.Axes.html#matplotlib.axes.Axes" title="matplotlib.axes.Axes"><code class="xref py py-obj docutils literal notranslate"><span class="pre">Axes</span></code></a></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">ax</span></code></p></td>
<td></td>
</tr>
<tr class="row-even"><td rowspan="2"><p><a class="reference internal" href="../api/transformations.html#matplotlib.transforms.Transform" title="matplotlib.transforms.Transform"><code class="xref py py-obj docutils literal notranslate"><span class="pre">Transform</span></code></a></p></td>
<td rowspan="2"><p><code class="docutils literal notranslate"><span class="pre">trans</span></code></p></td>
<td rowspan="2"><p><code class="docutils literal notranslate"><span class="pre">trans_<source>_<target></span></code></p>
<p><code class="docutils literal notranslate"><span class="pre">trans_<source></span></code> when target is screen</p>
</td>
</tr>
<tr class="row-odd"></tr>
</tbody>
</table>
<p>Generally, denote more than one instance of the same class by adding suffixes to
the variable names. If a format isn't specified in the table, use numbers or
letters as appropriate.</p>
</section>
<section id="type-hints">
<span id="id5"></span><h2>Type hints<a class="headerlink" href="#type-hints" title="Link to this heading">#</a></h2>
<p>If you add new public API or change public API, update or add the
corresponding <a class="reference external" href="https://mypy.readthedocs.io/en/latest/">mypy</a> type hints.
We generally use <a class="reference external" href="https://typing.readthedocs.io/en/latest/source/stubs.html#type-stubs">stub files</a>
(<code class="docutils literal notranslate"><span class="pre">*.pyi</span></code>) to store the type information; for example <code class="docutils literal notranslate"><span class="pre">colors.pyi</span></code> contains
the type information for <code class="docutils literal notranslate"><span class="pre">colors.py</span></code>. A notable exception is <code class="docutils literal notranslate"><span class="pre">pyplot.py</span></code>,
which is type hinted inline.</p>
<p>Type hints are checked by the mypy <a class="reference internal" href="development_setup.html#pre-commit-hooks"><span class="std std-ref">pre-commit hook</span></a>,
can often be verified by running <code class="docutils literal notranslate"><span class="pre">tox</span> <span class="pre">-e</span> <span class="pre">stubtest</span></code>.</p>
</section>
<section id="new-modules-and-files-installation">
<h2>New modules and files: installation<a class="headerlink" href="#new-modules-and-files-installation" title="Link to this heading">#</a></h2>
<ul class="simple">
<li><p>If you have added new files or directories, or reorganized existing ones, make sure the
new files are included in the <code class="file docutils literal notranslate"><span class="pre">meson.build</span></code> in the corresponding directories.</p></li>
<li><p>New modules <em>may</em> be typed inline or using parallel stub file like existing modules.</p></li>
</ul>
</section>
<section id="c-c-extensions">
<h2>C/C++ extensions<a class="headerlink" href="#c-c-extensions" title="Link to this heading">#</a></h2>
<ul class="simple">
<li><p>Extensions may be written in C or C++.</p></li>
<li><p>Code style should conform to PEP7 (understanding that PEP7 doesn't
address C++, but most of its admonitions still apply).</p></li>
<li><p>Python/C interface code should be kept separate from the core C/C++
code. The interface code should be named <code class="file docutils literal notranslate"><span class="pre">FOO_wrap.cpp</span></code> or
<code class="file docutils literal notranslate"><span class="pre">FOO_wrapper.cpp</span></code>.</p></li>
<li><p>Header file documentation (aka docstrings) should be in Numpydoc
format. We don't plan on using automated tools for these
docstrings, and the Numpydoc format is well understood in the
scientific Python community.</p></li>
<li><p>C/C++ code in the <code class="file docutils literal notranslate"><span class="pre">extern/</span></code> directory is vendored, and should be kept
close to upstream whenever possible. It can be modified to fix bugs or
implement new features only if the required changes cannot be made elsewhere
in the codebase. In particular, avoid making style fixes to it.</p></li>
</ul>
</section>
<section id="keyword-argument-processing">
<span id="id6"></span><h2>Keyword argument processing<a class="headerlink" href="#keyword-argument-processing" title="Link to this heading">#</a></h2>
<p>Matplotlib makes extensive use of <code class="docutils literal notranslate"><span class="pre">**kwargs</span></code> for pass-through customizations
from one function to another. A typical example is
<a class="reference internal" href="../api/_as_gen/matplotlib.axes.Axes.text.html#matplotlib.axes.Axes.text" title="matplotlib.axes.Axes.text"><code class="xref py py-obj docutils literal notranslate"><span class="pre">text</span></code></a>. The definition of <a class="reference internal" href="../api/_as_gen/matplotlib.pyplot.text.html#matplotlib.pyplot.text" title="matplotlib.pyplot.text"><code class="xref py py-obj docutils literal notranslate"><span class="pre">matplotlib.pyplot.text</span></code></a> is a
simple pass-through to <a class="reference internal" href="../api/_as_gen/matplotlib.axes.Axes.text.html#matplotlib.axes.Axes.text" title="matplotlib.axes.Axes.text"><code class="xref py py-obj docutils literal notranslate"><span class="pre">matplotlib.axes.Axes.text</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># in pyplot.py</span>
<span class="k">def</span> <span class="nf">text</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">fontdict</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">return</span> <span class="n">gca</span><span class="p">()</span><span class="o">.</span><span class="n">text</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">fontdict</span><span class="o">=</span><span class="n">fontdict</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p><a class="reference internal" href="../api/_as_gen/matplotlib.axes.Axes.text.html#matplotlib.axes.Axes.text" title="matplotlib.axes.Axes.text"><code class="xref py py-obj docutils literal notranslate"><span class="pre">matplotlib.axes.Axes.text</span></code></a> (simplified for illustration) just
passes all <code class="docutils literal notranslate"><span class="pre">args</span></code> and <code class="docutils literal notranslate"><span class="pre">kwargs</span></code> on to <code class="docutils literal notranslate"><span class="pre">matplotlib.text.Text.__init__</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># in axes/_axes.py</span>
<span class="k">def</span> <span class="nf">text</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">fontdict</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">Text</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="n">y</span><span class="p">,</span> <span class="n">text</span><span class="o">=</span><span class="n">s</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>and <code class="docutils literal notranslate"><span class="pre">matplotlib.text.Text.__init__</span></code> (again, simplified)
just passes them on to the <a class="reference internal" href="../api/_as_gen/matplotlib.artist.Artist.update.html#matplotlib.artist.Artist.update" title="matplotlib.artist.Artist.update"><code class="xref py py-obj docutils literal notranslate"><span class="pre">matplotlib.artist.Artist.update</span></code></a> method:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># in text.py</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">text</span><span class="o">=</span><span class="s1">''</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">update</span></code> does the work looking for methods named like
<code class="docutils literal notranslate"><span class="pre">set_property</span></code> if <code class="docutils literal notranslate"><span class="pre">property</span></code> is a keyword argument. i.e., no one
looks at the keywords, they just get passed through the API to the
artist constructor which looks for suitably named methods and calls
them with the value.</p>
<p>As a general rule, the use of <code class="docutils literal notranslate"><span class="pre">**kwargs</span></code> should be reserved for
pass-through keyword arguments, as in the example above. If all the
keyword args are to be used in the function, and not passed
on, use the key/value keyword args in the function definition rather
than the <code class="docutils literal notranslate"><span class="pre">**kwargs</span></code> idiom.</p>
<p>In some cases, you may want to consume some keys in the local
function, and let others pass through. Instead of popping arguments to
use off <code class="docutils literal notranslate"><span class="pre">**kwargs</span></code>, specify them as keyword-only arguments to the local
function. This makes it obvious at a glance which arguments will be
consumed in the function. For example, in
<a class="reference internal" href="../api/_as_gen/matplotlib.axes.Axes.plot.html#matplotlib.axes.Axes.plot" title="matplotlib.axes.Axes.plot"><code class="xref py py-meth docutils literal notranslate"><span class="pre">plot()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">scalex</span></code> and <code class="docutils literal notranslate"><span class="pre">scaley</span></code> are
local arguments and the rest are passed on as
<a class="reference internal" href="../api/_as_gen/matplotlib.lines.Line2D.html#matplotlib.lines.Line2D" title="matplotlib.lines.Line2D"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Line2D()</span></code></a> keyword arguments:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># in axes/_axes.py</span>
<span class="k">def</span> <span class="nf">plot</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="n">scalex</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">scaley</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="n">lines</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">_get_lines</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">add_line</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
<span class="n">lines</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="using-logging-for-debug-messages">
<span id="using-logging"></span><h2>Using logging for debug messages<a class="headerlink" href="#using-logging-for-debug-messages" title="Link to this heading">#</a></h2>
<p>Matplotlib uses the standard Python <a class="reference external" href="https://docs.python.org/3/library/logging.html#module-logging" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging</span></code></a> library to write verbose
warnings, information, and debug messages. Please use it! In all those places
you write <a class="reference external" href="https://docs.python.org/3/library/functions.html#print" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">print</span></code></a> calls to do your debugging, try using <a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.debug" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.debug</span></code></a>
instead!</p>
<p>To include <a class="reference external" href="https://docs.python.org/3/library/logging.html#module-logging" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging</span></code></a> in your module, at the top of the module, you need to
<code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">logging</span></code>. Then calls in your code like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">_log</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="vm">__name__</span><span class="p">)</span> <span class="c1"># right after the imports</span>
<span class="c1"># code</span>
<span class="c1"># more code</span>
<span class="n">_log</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Here is some information'</span><span class="p">)</span>
<span class="n">_log</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Here is some more detailed information'</span><span class="p">)</span>
</pre></div>
</div>
<p>will log to a logger named <code class="docutils literal notranslate"><span class="pre">matplotlib.yourmodulename</span></code>.</p>
<p>If an end-user of Matplotlib sets up <a class="reference external" href="https://docs.python.org/3/library/logging.html#module-logging" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging</span></code></a> to display at levels more
verbose than <code class="docutils literal notranslate"><span class="pre">logging.WARNING</span></code> in their code with the Matplotlib-provided
helper:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">plt</span><span class="o">.</span><span class="n">set_loglevel</span><span class="p">(</span><span class="s2">"debug"</span><span class="p">)</span>
</pre></div>
</div>
<p>or manually with</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></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">DEBUG</span><span class="p">)</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
</pre></div>
</div>
<p>Then they will receive messages like</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>DEBUG:matplotlib.backends:backend MacOSX version unknown
DEBUG:matplotlib.yourmodulename:Here is some information
DEBUG:matplotlib.yourmodulename:Here is some more detailed information
</pre></div>
</div>
<p>Avoid using pre-computed strings (<code class="docutils literal notranslate"><span class="pre">f-strings</span></code>, <code class="docutils literal notranslate"><span class="pre">str.format</span></code>,etc.) for logging because
of security and performance issues, and because they interfere with style handlers. For
example, use <code class="docutils literal notranslate"><span class="pre">_log.error('hello</span> <span class="pre">%s',</span> <span class="pre">'world')</span></code> rather than <code class="docutils literal notranslate"><span class="pre">_log.error('hello</span>
<span class="pre">{}'.format('world'))</span></code> or <code class="docutils literal notranslate"><span class="pre">_log.error(f'hello</span> <span class="pre">{s}')</span></code>.</p>
<section id="which-logging-level-to-use">
<h3>Which logging level to use?<a class="headerlink" href="#which-logging-level-to-use" title="Link to this heading">#</a></h3>
<p>There are five levels at which you can emit messages.</p>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.critical" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.critical</span></code></a> and <a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.error" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.error</span></code></a> are really only there for errors that
will end the use of the library but not kill the interpreter.</p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.warning" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.warning</span></code></a> and <a class="reference internal" href="../api/_api_api.html#matplotlib._api.warn_external" title="matplotlib._api.warn_external"><code class="xref py py-obj docutils literal notranslate"><span class="pre">_api.warn_external</span></code></a> are used to warn the user,
see below.</p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.info" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.info</span></code></a> is for information that the user may want to know if the
program behaves oddly. They are not displayed by default. For instance, if
an object isn't drawn because its position is <code class="docutils literal notranslate"><span class="pre">NaN</span></code>, that can usually
be ignored, but a mystified user could call
<code class="docutils literal notranslate"><span class="pre">logging.basicConfig(level=logging.INFO)</span></code> and get an error message that
says why.</p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.debug" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.debug</span></code></a> is the least likely to be displayed, and hence can be the
most verbose. "Expected" code paths (e.g., reporting normal intermediate
steps of layouting or rendering) should only log at this level.</p></li>
</ul>
<p>By default, <a class="reference external" href="https://docs.python.org/3/library/logging.html#module-logging" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging</span></code></a> displays all log messages at levels higher than
<code class="docutils literal notranslate"><span class="pre">logging.WARNING</span></code> to <a class="reference external" href="https://docs.python.org/3/library/sys.html#sys.stderr" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">sys.stderr</span></code></a>.</p>
<p>The <a class="reference external" href="https://docs.python.org/3/howto/logging.html#logging-basic-tutorial">logging tutorial</a> suggests that the difference between <a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.warning" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.warning</span></code></a>
and <a class="reference internal" href="../api/_api_api.html#matplotlib._api.warn_external" title="matplotlib._api.warn_external"><code class="xref py py-obj docutils literal notranslate"><span class="pre">_api.warn_external</span></code></a> (which uses <a class="reference external" href="https://docs.python.org/3/library/warnings.html#warnings.warn" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">warnings.warn</span></code></a>) is that
<a class="reference internal" href="../api/_api_api.html#matplotlib._api.warn_external" title="matplotlib._api.warn_external"><code class="xref py py-obj docutils literal notranslate"><span class="pre">_api.warn_external</span></code></a> should be used for things the user must change to stop
the warning (typically in the source), whereas <a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.warning" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.warning</span></code></a> can be more
persistent. Moreover, note that <a class="reference internal" href="../api/_api_api.html#matplotlib._api.warn_external" title="matplotlib._api.warn_external"><code class="xref py py-obj docutils literal notranslate"><span class="pre">_api.warn_external</span></code></a> will by default only
emit a given warning <em>once</em> for each line of user code, whereas
<a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.warning" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">logging.warning</span></code></a> will display the message every time it is called.</p>
<p>By default, <a class="reference external" href="https://docs.python.org/3/library/warnings.html#warnings.warn" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">warnings.warn</span></code></a> displays the line of code that has the <code class="docutils literal notranslate"><span class="pre">warn</span></code>
call. This usually isn't more informative than the warning message itself.
Therefore, Matplotlib uses <a class="reference internal" href="../api/_api_api.html#matplotlib._api.warn_external" title="matplotlib._api.warn_external"><code class="xref py py-obj docutils literal notranslate"><span class="pre">_api.warn_external</span></code></a> which uses <a class="reference external" href="https://docs.python.org/3/library/warnings.html#warnings.warn" title="(in Python v3.12)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">warnings.warn</span></code></a>,
but goes up the stack and displays the first line of code outside of
Matplotlib. For example, for the module:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># in my_matplotlib_module.py</span>
<span class="kn">import</span> <span class="nn">warnings</span>
<span class="k">def</span> <span class="nf">set_range</span><span class="p">(</span><span class="n">bottom</span><span class="p">,</span> <span class="n">top</span><span class="p">):</span>
<span class="k">if</span> <span class="n">bottom</span> <span class="o">==</span> <span class="n">top</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="s1">'Attempting to set identical bottom==top'</span><span class="p">)</span>
</pre></div>
</div>
<p>running the script:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">matplotlib</span> <span class="kn">import</span> <span class="n">my_matplotlib_module</span>
<span class="n">my_matplotlib_module</span><span class="o">.</span><span class="n">set_range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span> <span class="c1"># set range</span>
</pre></div>
</div>
<p>will display</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>UserWarning: Attempting to set identical bottom==top
warnings.warn('Attempting to set identical bottom==top')
</pre></div>
</div>
<p>Modifying the module to use <a class="reference internal" href="../api/_api_api.html#matplotlib._api.warn_external" title="matplotlib._api.warn_external"><code class="xref py py-obj docutils literal notranslate"><span class="pre">_api.warn_external</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">matplotlib</span> <span class="kn">import</span> <span class="n">_api</span>
<span class="k">def</span> <span class="nf">set_range</span><span class="p">(</span><span class="n">bottom</span><span class="p">,</span> <span class="n">top</span><span class="p">):</span>
<span class="k">if</span> <span class="n">bottom</span> <span class="o">==</span> <span class="n">top</span><span class="p">:</span>
<span class="n">_api</span><span class="o">.</span><span class="n">warn_external</span><span class="p">(</span><span class="s1">'Attempting to set identical bottom==top'</span><span class="p">)</span>
</pre></div>
</div>
<p>and running the same script will display</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>UserWarning: Attempting to set identical bottom==top
my_matplotlib_module.set_range(0, 0) # set range
</pre></div>
</div>
</section>
</section>
<section id="licenses-for-contributed-code">
<span id="licence-coding-guide"></span><h2>Licenses for contributed code<a class="headerlink" href="#licenses-for-contributed-code" title="Link to this heading">#</a></h2>
<p>Matplotlib only uses BSD compatible code. If you bring in code from
another project make sure it has a PSF, BSD, MIT or compatible license
(see the Open Source Initiative <a class="reference external" href="https://opensource.org/licenses">licenses page</a> for details on individual
licenses). If it doesn't, you may consider contacting the author and
asking them to relicense it. GPL and LGPL code are not acceptable in
the main code base, though we are considering an alternative way of
distributing L/GPL code through an separate channel, possibly a
toolkit. If you include code, make sure you include a copy of that
code's license in the license directory if the code's license requires
you to distribute the license with it. Non-BSD compatible licenses
are acceptable in Matplotlib toolkits (e.g., basemap), but make sure you
clearly state the licenses you are using.</p>
<section id="why-bsd-compatible">
<h3>Why BSD compatible?<a class="headerlink" href="#why-bsd-compatible" title="Link to this heading">#</a></h3>
<p>The two dominant license variants in the wild are GPL-style and
BSD-style. There are countless other licenses that place specific
restrictions on code reuse, but there is an important difference to be
considered in the GPL and BSD variants. The best known and perhaps
most widely used license is the GPL, which in addition to granting you
full rights to the source code including redistribution, carries with
it an extra obligation. If you use GPL code in your own code, or link
with it, your product must be released under a GPL compatible
license. i.e., you are required to give the source code to other
people and give them the right to redistribute it as well. Many of the
most famous and widely used open source projects are released under
the GPL, including linux, gcc, emacs and sage.</p>
<p>The second major class are the BSD-style licenses (which includes MIT
and the python PSF license). These basically allow you to do whatever
you want with the code: ignore it, include it in your own open source
project, include it in your proprietary product, sell it,
whatever. python itself is released under a BSD compatible license, in
the sense that, quoting from the PSF license page:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">There</span> <span class="ow">is</span> <span class="n">no</span> <span class="n">GPL</span><span class="o">-</span><span class="n">like</span> <span class="s2">"copyleft"</span> <span class="n">restriction</span><span class="o">.</span> <span class="n">Distributing</span>
<span class="n">binary</span><span class="o">-</span><span class="n">only</span> <span class="n">versions</span> <span class="n">of</span> <span class="n">Python</span><span class="p">,</span> <span class="n">modified</span> <span class="ow">or</span> <span class="ow">not</span><span class="p">,</span> <span class="ow">is</span> <span class="n">allowed</span><span class="o">.</span> <span class="n">There</span>
<span class="ow">is</span> <span class="n">no</span> <span class="n">requirement</span> <span class="n">to</span> <span class="n">release</span> <span class="nb">any</span> <span class="n">of</span> <span class="n">your</span> <span class="n">source</span> <span class="n">code</span><span class="o">.</span> <span class="n">You</span> <span class="n">can</span> <span class="n">also</span>
<span class="n">write</span> <span class="n">extension</span> <span class="n">modules</span> <span class="k">for</span> <span class="n">Python</span> <span class="ow">and</span> <span class="n">provide</span> <span class="n">them</span> <span class="n">only</span> <span class="ow">in</span> <span class="n">binary</span>
<span class="n">form</span><span class="o">.</span>
</pre></div>
</div>
<p>Famous projects released under a BSD-style license in the permissive
sense of the last paragraph are the BSD operating system, python and
TeX.</p>
<p>There are several reasons why early Matplotlib developers selected a
BSD compatible license. Matplotlib is a python extension, and we
choose a license that was based on the python license (BSD
compatible). Also, we wanted to attract as many users and developers
as possible, and many software companies will not use GPL code in
software they plan to distribute, even those that are highly committed
to open source development, such as <a class="reference external" href="https://www.enthought.com">enthought</a>, out of legitimate concern that use of the
GPL will "infect" their code base by its viral nature. In effect, they
want to retain the right to release some proprietary code. Companies
and institutions who use Matplotlib often make significant
contributions, because they have the resources to get a job done, even
a boring one. Two of the Matplotlib backends (FLTK and WX) were
contributed by private companies. The final reason behind the
licensing choice is compatibility with the other python extensions for
scientific computing: ipython, numpy, scipy, the enthought tool suite
and python itself are all distributed under BSD compatible licenses.</p>
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