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<section id="python-interpreters">
<h1>Python interpreters<a class="headerlink" href="#python-interpreters" title="Permalink to this heading">¶</a></h1>
<section id="versions-of-python">
<h2>Versions of Python<a class="headerlink" href="#versions-of-python" title="Permalink to this heading">¶</a></h2>
<p>By a <em>version of Python</em> we usually mean the variant of Python language
and standard library interface as used by a specific version
of <a class="reference external" href="https://www.python.org/">CPython</a>, the reference implementation of Python.</p>
<p>Python versions are determined from the two first version components.
The major version is incremented when major incompatible changes are
introduced in the language, as was the case in Python 3. Along with
minor version changes, the new releases introduce new features
and remove deprecated APIs. The Python documentation generally
indicates when a particular API was added or deprecated, and when it
is planned to be removed.</p>
<p>Practically speaking, this means that a program written purely
for Python 2 is unlikely to work on Python 3, and requires major changes
to achieve compatibility. On the other hand, a program written for
Python 3.7 is very likely to work with Python 3.8, and reasonably likely
to support Python 3.6 as well. If that is not the case, minor changes
are usually sufficient to fix that.</p>
<p>For example, Python 3.7 introduced a new <a class="reference external" href="https://docs.python.org/3.7/library/importlib.html#module-importlib.resources">importlib.resources</a> module.
If your program uses it, it will not work on Python 3.6 without
a backwards compatibility code.</p>
<p>Python 3.8 removed the deprecated <a class="reference external" href="https://docs.python.org/3.7/library/platform.html#platform.linux_distribution">platform.linux_distribution()</a>
function. If your program used it, it will not work on Python 3.8
without changes. However, it was deprecated since Python 3.5, so if you
were targetting 3.7, you should not have been using it in the first
place.</p>
<p>Gentoo supports building packages against Python 2.7 and a shifting
window of 3-4 versions of Python 3. They are provided as slots
of <code class="docutils literal notranslate"><span class="pre">dev-lang/python</span></code>.</p>
</section>
<section id="alternative-python-implementations">
<h2>Alternative Python implementations<a class="headerlink" href="#alternative-python-implementations" title="Permalink to this heading">¶</a></h2>
<p>CPython is the reference and most commonly used Python implementation.
However, there are other interpreters that aim to maintain reasonable
compatibility with it.</p>
<p><a class="reference external" href="https://www.pypy.org/">PyPy</a> is an implementation of Python built using in-house RPython
language, using a Just-in-Time compiler to achieve better performance
(generally in long-running programs running a lot of Python code).
It maintains quite good compatibility with CPython, except when programs
rely on its implementation details or GC behavior.</p>
<p>PyPy upstream provides PyPy variants compatible with Python 2.7
and one version of Python 3. Gentoo supports building packages against
PyPy3. PyPy2.7 is provided as <code class="docutils literal notranslate"><span class="pre">dev-python/pypy</span></code>, while PyPy3 is
provided as <code class="docutils literal notranslate"><span class="pre">dev-python/pypy3</span></code>.</p>
<p><a class="reference external" href="https://www.jython.org/">Jython</a> is an implementation of Python written in Java. Besides being
a stand-alone Python interpreter, it supports bidirectional interaction
between Python and Java libraries.</p>
<p>Jython development is very slow paced, and it is currently bound
to Python 2.7. Gentoo does not support building packages for Jython
anymore. The interpreter is still provided as <code class="docutils literal notranslate"><span class="pre">dev-java/jython</span></code>.</p>
<p><a class="reference external" href="https://ironpython.net/">IronPython</a> is an implementation of Python for the .NET framework.
Alike Jython, it supports bidirectional interaction between Python
and .NET Framework. It is currently bound to Python 2.7. It is not
packaged in Gentoo.</p>
<p><a class="reference external" href="https://www.brython.info/">Brython</a> is an implementation of Python 3 for client-side web
programming (in JavaScript). It provides a subset of Python 3 standard
library combined with access to DOM objects. It is packaged in Gentoo
as <code class="docutils literal notranslate"><span class="pre">dev-python/brython</span></code>.</p>
<p><a class="reference external" href="https://micropython.org/">MicroPython</a> is an implementation of Python 3 aimed for microcontrollers
and embedded environments. It aims to maintain some compatibility
with CPython while providing stripped down standard library
and additional modules to interface with hardware. It is packaged
as <code class="docutils literal notranslate"><span class="pre">dev-lang/micropython</span></code>.</p>
<p><a class="reference external" href="https://github.com/naftaliharris/tauthon">Tauthon</a> is a fork of Python 2.7 that aims to backport new language
features and standard library modules while preserving backwards
compatibility with existing code. It is not packaged in Gentoo.</p>
</section>
<section id="support-for-multiple-implementations">
<h2>Support for multiple implementations<a class="headerlink" href="#support-for-multiple-implementations" title="Permalink to this heading">¶</a></h2>
<p>The support for simultaneously using multiple Python implementations
is implemented primarily through USE flags. The packages installing
or using Python files define either <code class="docutils literal notranslate"><span class="pre">PYTHON_TARGETS</span></code>
or <code class="docutils literal notranslate"><span class="pre">PYTHON_SINGLE_TARGET</span></code> flags that permit user to choose which
implementations are used.</p>
<p>Modules and extensions are installed separately for each interpreter,
in its specific site-packages directory. This means that a package
can run using a specific target correctly only if all its dependencies
were also installed for the same implementation. This is enforced
via USE dependencies.</p>
<p>Additionally, <code class="docutils literal notranslate"><span class="pre">dev-lang/python-exec</span></code> provides a mechanism for
installing multiple variants of each Python script simultaneously. This
is necessary to support scripts that differ between Python versions
(particularly between Python 2 and Python 3) but it is also used
to prevent scripts from being called via unsupported interpreter
(i.e. one that does not have its accompanying modules or dependencies
installed).</p>
<p>This also implies that all installed Python scripts must have their
shebangs adjusted to use a specific Python interpreter (not <code class="docutils literal notranslate"><span class="pre">python</span></code>
nor <code class="docutils literal notranslate"><span class="pre">python3</span></code> but e.g. <code class="docutils literal notranslate"><span class="pre">python3.7</span></code>), and all other executables must
also be modified to call specific version of Python directly.</p>
</section>
<section id="backports">
<h2>Backports<a class="headerlink" href="#backports" title="Permalink to this heading">¶</a></h2>
<p>A common method of improving compatibility with older versions of Python
is to backport new standard library modules or features. Packages doing
that are generally called <em>backports</em>.</p>
<p>Ideally, backports copy the code from the standard library with minimal
changes, and provide a matching API. In some cases, new versions
of backports are released as the standard library changes, and their
usability extends from providing a missing module to extending older
version of the module. For example, the <code class="docutils literal notranslate"><span class="pre">dev-python/funcsigs</span></code> package
originally backported function signatures from Python 3.3 to older
versions, and afterwards was updated to backport new features from
Python 3.6, becoming useful to versions 3.3 through 3.5.</p>
<p>Sometimes, the opposite happens. <code class="docutils literal notranslate"><span class="pre">dev-python/mock</span></code> started
as a stand-alone package, and was integrated into the standard library
as <a class="reference external" href="https://docs.python.org/3.3/library/unittest.mock.html">unittest.mock</a> later on. Afterwards, the external package became
a backport of the standard library module.</p>
<p>In some cases backports effectively replace external packages. Once
<a class="reference external" href="https://docs.python.org/3.3/library/lzma.html">lzma</a> module has been added to the standard library, its backport
<code class="docutils literal notranslate"><span class="pre">dev-python/backports-lzma</span></code> has effectively replaced the competing
LZMA packages.</p>
<p>Individual backports differ by the level of compatibility with
the standard library provided, and therefore on the amount of additional
code needed in your program. The exact kind of dependencies used
depends on that.</p>
<p><code class="docutils literal notranslate"><span class="pre">dev-python/ipaddress</span></code> is a drop-in backport of the <a class="reference external" href="https://docs.python.org/3.3/library/ipaddress.html">ipaddress</a> module
from Python 3.3. It is using the same module name, so a code written
to use this module will work out-of-the-box on Python 2.7 if the package
is installed. As a side note, since Python always prefers built-in
modules over external packages, there is no point in enabling Python 3
in this package as the installed module would never be used.
Appropriately, you should depend on this package only for the Python
versions needing it.</p>
<p><code class="docutils literal notranslate"><span class="pre">dev-python/mock</span></code> is a compatible backport of the <a class="reference external" href="https://docs.python.org/3.3/library/unittest.mock.html">unittest.mock</a>
module. It can’t use the same name as the standard library module,
therefore the packages need to use it conditionally, e.g.:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">unittest.mock</span> <span class="kn">import</span> <span class="n">Mock</span>
<span class="k">except</span> <span class="ne">ImportError</span><span class="p">:</span> <span class="c1"># py<3.3</span>
<span class="kn">from</span> <span class="nn">mock</span> <span class="kn">import</span> <span class="n">Mock</span>
</pre></div>
</div>
<p>or:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">sys</span>
<span class="k">if</span> <span class="n">sys</span><span class="o">.</span><span class="n">hexversion</span> <span class="o">>=</span> <span class="mh">0x03030000</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">unittest.mock</span> <span class="kn">import</span> <span class="n">Mock</span>
<span class="k">else</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">mock</span> <span class="kn">import</span> <span class="n">Mock</span>
</pre></div>
</div>
<p>However, the actual API remains compatible, so the programs do not need
more compatibility code than that. In some cases, upstreams fail (or
even refuse) to use the external <code class="docutils literal notranslate"><span class="pre">mock</span></code> package conditionally —
in that case, you either need to depend on this package unconditionally,
or patch it.</p>
<p><code class="docutils literal notranslate"><span class="pre">dev-python/trollius</span></code> aimed to provide a backport of <a class="reference external" href="https://docs.python.org/3.4/library/asyncio.html">asyncio</a>
for Python 2. Since the asyncio framework relies on new Python syntax,
the backport cannot be API compatible and requires using a different
syntax than native asyncio code.</p>
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