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<section id="advanced-concepts">
<h1>Advanced concepts<a class="headerlink" href="#advanced-concepts" title="Permalink to this heading">¶</a></h1>
<section id="namespace-packages">
<h2>Namespace packages<a class="headerlink" href="#namespace-packages" title="Permalink to this heading">¶</a></h2>
<section id="hierarchical-package-structure">
<h3>Hierarchical package structure<a class="headerlink" href="#hierarchical-package-structure" title="Permalink to this heading">¶</a></h3>
<p>Traditionally, Python packages were organized into a hierarchical
structure with modules and subpackages being located inside the parent
package directory. When submodules are imported, they are represented
as attributes on the parent module. Consider the following session:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sphinx.addnodes</span>
<span class="gp">>>> </span><span class="n">sphinx</span>
<span class="go"><module 'sphinx' from '/usr/lib/python3.8/site-packages/sphinx/__init__.py'></span>
<span class="gp">>>> </span><span class="n">sphinx</span><span class="o">.</span><span class="n">addnodes</span>
<span class="go"><module 'sphinx.addnodes' from '/usr/lib/python3.8/site-packages/sphinx/addnodes.py'></span>
</pre></div>
</div>
<p>This works fine most of the time. However, it start being problematic
when multiple Gentoo packages install parts of the same top-level
package. This may happen e.g. with some plugin layouts where plugins
are installed inside the package. More commonly, it happens when
upstream wishes all their packages to start with a common component.</p>
<p>This is the case with Zope framework. Different Zope packages share
common <code class="docutils literal notranslate"><span class="pre">zope</span></code> top-level package. <code class="docutils literal notranslate"><span class="pre">dev-python/zope-interface</span></code>
installs into <code class="docutils literal notranslate"><span class="pre">zope.interface</span></code>, <code class="docutils literal notranslate"><span class="pre">dev-python/zope-event</span></code>
into <code class="docutils literal notranslate"><span class="pre">zope.event</span></code>. For this to work using the hierarchical layout,
a common package has to install <code class="docutils literal notranslate"><span class="pre">zope/__init__.py</span></code>, then other Zope
packages have to depend on it and install sub-packages inside that
directory. As far as installed packages are concerned, this is entirely
doable.</p>
<p>The real problem happens when we wish to test a freshly built package
that depends on an installed package. In that case, Python imports
<code class="docutils literal notranslate"><span class="pre">zope</span></code> from build directory that contains only <code class="docutils literal notranslate"><span class="pre">zope.interface</span></code>.
It will not be able to import <code class="docutils literal notranslate"><span class="pre">zope.event</span></code> that is installed in system
package directory:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">zope.interface</span>
<span class="gp">>>> </span><span class="n">zope</span>
<span class="go"><module 'zope' from '/tmp/portage/dev-python/zope-interface-4.7.1/work/zope.interface-4.7.1-python3_8/lib/zope/__init__.py'></span>
<span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">interface</span>
<span class="go"><module 'zope.interface' from '/tmp/portage/dev-python/zope-interface-4.7.1/work/zope.interface-4.7.1-python3_8/lib/zope/interface/__init__.py'></span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">zope.event</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">ModuleNotFoundError</span>: <span class="n">No module named 'zope.event'</span>
</pre></div>
</div>
<p>Now, this could be worked around by copying all other subpackages back
to the build directory. However, there is a better solution.</p>
</section>
<section id="namespace-package-structure">
<h3>Namespace package structure<a class="headerlink" href="#namespace-package-structure" title="Permalink to this heading">¶</a></h3>
<p>Unlike traditional packages, namespace packages act as a kind of proxy.
They are not strictly bound to the containing directory, and instead
permit loading subpackages from all directories found in module search
path. If we make <code class="docutils literal notranslate"><span class="pre">zope</span></code> a namespace package, we can import both
the locally built <code class="docutils literal notranslate"><span class="pre">zope.interface</span></code> and system <code class="docutils literal notranslate"><span class="pre">zope.event</span></code>
packages:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">zope.interface</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">zope.event</span>
<span class="gp">>>> </span><span class="n">zope</span>
<span class="go"><module 'zope' (namespace)></span>
<span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">interface</span>
<span class="go"><module 'zope.interface' from '/tmp/portage/dev-python/zope-interface-4.7.1/work/zope.interface-4.7.1-python3_8/lib/zope/interface/__init__.py'></span>
<span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">event</span>
<span class="go"><module 'zope.event' from '/usr/lib/python3.8/site-packages/zope/event/__init__.py'></span>
</pre></div>
</div>
<p>There are three common methods of creating namespace packages:</p>
<ol class="arabic simple">
<li><p><a class="reference external" href="https://www.python.org/dev/peps/pep-0420/">PEP 420</a> namespaces implemented in Python 3.3 and newer,</p></li>
<li><p>Using <a class="reference external" href="https://docs.python.org/3/library/pkgutil.html">pkgutil</a> standard library module,</p></li>
<li><p>Using <a class="reference external" href="https://setuptools.readthedocs.io/en/latest/setuptools.html#namespace-packages">namespace package support in setuptools</a> (discouraged).</p></li>
</ol>
<p>PEP 420 namespaces are created implicitly when a package directory
does not contain <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> file. While earlier versions
of Python (including Python 2.7) ignored such directories and did not
permit importing Python modules within them, Python 3.3 imports such
directories as namespace packages.</p>
<p>pkgutil namespaces use <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> with the following content:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">__path__</span> <span class="o">=</span> <span class="nb">__import__</span><span class="p">(</span><span class="s1">'pkgutil'</span><span class="p">)</span><span class="o">.</span><span class="n">extend_path</span><span class="p">(</span><span class="n">__path__</span><span class="p">,</span> <span class="vm">__name__</span><span class="p">)</span>
</pre></div>
</div>
<p>setuptools namespace can use <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> with the following
content:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nb">__import__</span><span class="p">(</span><span class="s1">'pkg_resources'</span><span class="p">)</span><span class="o">.</span><span class="n">declare_namespace</span><span class="p">(</span><span class="vm">__name__</span><span class="p">)</span>
</pre></div>
</div>
<p>Alternatively, setuptools normally installs a <code class="docutils literal notranslate"><span class="pre">.pth</span></code> file that is
automatically loaded by Python and implicitly injects the namespace
into Python.</p>
<p>Both pkgutil and setuptools namespaces are portable to all versions
of Python.</p>
<p>More general information on the topic can be found under <a class="reference external" href="https://packaging.python.org/guides/packaging-namespace-packages/">packaging
namespace packages</a> in Python Packaging User Guide.</p>
</section>
<section id="determining-whether-namespaces-are-used">
<h3>Determining whether namespaces are used<a class="headerlink" href="#determining-whether-namespaces-are-used" title="Permalink to this heading">¶</a></h3>
<p>The exact method of detecting namespace packages depends on the type
of namespace used.</p>
<p>PEP 420 namespaces can generally be recognized by the lack
of <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> in an installed package directory. However, since
they do not require any specific action, distinguishing them is not very
important.</p>
<p>pkgutil namespaces can be recognized through the content of their
<code class="docutils literal notranslate"><span class="pre">__init__.py</span></code>. Generally, you should find it suspicious if is
the only file in a top-level package directory, and if the name of this
directory is less specific than the package name (e.g. <code class="docutils literal notranslate"><span class="pre">zope</span></code> for
<code class="docutils literal notranslate"><span class="pre">zope.interface</span></code>, <code class="docutils literal notranslate"><span class="pre">ruamel</span></code> for <code class="docutils literal notranslate"><span class="pre">ruamel.yaml</span></code>). If you miss this,
then you will learn about the namespace from package collisions
on the respective <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code>.</p>
<p>setuptools namespaces usually do not install <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> but
do install a <code class="docutils literal notranslate"><span class="pre">.pth</span></code> file instead. The distutils-r1 eclass detects
this automatically and prints a warning. Prior to installation,
they can also be recognized by <code class="docutils literal notranslate"><span class="pre">namespace_packages</span></code> option
in <code class="docutils literal notranslate"><span class="pre">setup.py</span></code> or <code class="docutils literal notranslate"><span class="pre">setup.cfg</span></code>.</p>
</section>
<section id="adding-new-namespace-packages-to-gentoo">
<h3>Adding new namespace packages to Gentoo<a class="headerlink" href="#adding-new-namespace-packages-to-gentoo" title="Permalink to this heading">¶</a></h3>
<p>If the package uses PEP 420 namespaces, no special action is required.
Per PEP 420 layout, the package must not install <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> files
for namespaces.</p>
<p>If the package uses one of the other layouts, their respective files
must be removed from the install tree.</p>
<p>For pkgutil namespace, its <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> should be removed after
the PEP 517 build phase:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>python_compile<span class="o">()</span><span class="w"> </span><span class="o">{</span>
<span class="w"> </span>distutils-r1_python_compile
<span class="w"> </span>rm<span class="w"> </span><span class="s2">"</span><span class="si">${</span><span class="nv">BUILD_DIR</span><span class="si">}</span><span class="s2">/install</span><span class="k">$(</span>python_get_sitedir<span class="k">)</span><span class="s2">"</span>/jaraco/__init__.py<span class="w"> </span><span class="o">||</span><span class="w"> </span>die
<span class="o">}</span>
</pre></div>
</div>
<p>The equivalent code for the legacy eclass mode is:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>python_install<span class="o">()</span><span class="w"> </span><span class="o">{</span>
<span class="w"> </span>rm<span class="w"> </span><span class="s2">"</span><span class="si">${</span><span class="nv">BUILD_DIR</span><span class="si">}</span><span class="s2">"</span>/lib/jaraco/__init__.py<span class="w"> </span><span class="o">||</span><span class="w"> </span>die
<span class="w"> </span>distutils-r1_python_install
<span class="o">}</span>
</pre></div>
</div>
<p>For setuptools namespace, the <code class="docutils literal notranslate"><span class="pre">.pth</span></code> file should be removed instead:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>python_compile<span class="o">()</span><span class="w"> </span><span class="o">{</span>
<span class="w"> </span>distutils-r1_python_compile
<span class="w"> </span>find<span class="w"> </span><span class="s2">"</span><span class="si">${</span><span class="nv">BUILD_DIR</span><span class="si">}</span><span class="s2">"</span><span class="w"> </span>-name<span class="w"> </span><span class="s1">'*.pth'</span><span class="w"> </span>-delete<span class="w"> </span><span class="o">||</span><span class="w"> </span>die
<span class="o">}</span>
</pre></div>
</div>
<p>The setuptools code for the legacy mode is:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>python_install_all<span class="o">()</span><span class="w"> </span><span class="o">{</span>
<span class="w"> </span>distutils-r1_python_install_all
<span class="w"> </span>find<span class="w"> </span><span class="s2">"</span><span class="si">${</span><span class="nv">D</span><span class="si">}</span><span class="s2">"</span><span class="w"> </span>-name<span class="w"> </span><span class="s1">'*.pth'</span><span class="w"> </span>-delete<span class="w"> </span><span class="o">||</span><span class="w"> </span>die
<span class="o">}</span>
</pre></div>
</div>
<p>Some packages include an explicit <code class="docutils literal notranslate"><span class="pre">setuptools</span></code> runtime dependency
(<code class="docutils literal notranslate"><span class="pre">install_requires</span></code>) when using namespaces. If this is the only
use of <code class="docutils literal notranslate"><span class="pre">pkg_resources</span></code> and <code class="docutils literal notranslate"><span class="pre">setuptools</span></code> in installed package
sources, this dependency needs to be stripped, e.g.:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>src_prepare<span class="o">()</span><span class="w"> </span><span class="o">{</span>
<span class="w"> </span><span class="c1"># strip rdep specific to namespaces</span>
<span class="w"> </span>sed<span class="w"> </span>-i<span class="w"> </span>-e<span class="w"> </span><span class="s2">"/'setuptools'/d"</span><span class="w"> </span>setup.py<span class="w"> </span><span class="o">||</span><span class="w"> </span>die
<span class="w"> </span>distutils-r1_src_prepare
<span class="o">}</span>
</pre></div>
</div>
</section>
<section id="legacy-namespace-packages-in-gentoo">
<h3>Legacy namespace packages in Gentoo<a class="headerlink" href="#legacy-namespace-packages-in-gentoo" title="Permalink to this heading">¶</a></h3>
<p>Historically, Gentoo has used <code class="docutils literal notranslate"><span class="pre">dev-python/namespace-*</span></code> packages
to support namespaces. This method is deprecated and it is in process
of being retired.</p>
</section>
</section>
</section>
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