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Document implicit namespace packages for python

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Mike Romberg 2016-04-03 21:53:19 -06:00
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@ -5865,6 +5865,77 @@ class Bar(pkg3.foo.Foo): pass
effect (note, that the Python 2 case also needs the <tt>-relativeimport</tt>
workaround).</p>
<H3>Implicit Namespace Packages</H3>
<p> Python 3.3 intoduced
<a href="https://www.python.org/dev/peps/pep-0420/">PEP 0420</a> which
implements implicit namespace packages. In a nutshell, implicit namespace
packages remove the requirement of an __init__.py file and allow packages
to be split across multiple PATH elements. For example:
</p>
<div class="diagram">
<pre>
/fragment1/pkg1/mod1.py
/fragment2/pkg1/mod2.py
/fragment3/pkg1/mod3.py
</pre>
</div>
<p>If PYTHONPATH is set to "/fragment1:/fragment2:/fragment3", then mod1, mod2
and mod3 will be part of pkg1. This allows for splitting of packages into
separate pieces. This can be useful for swig generated wrappers in the
following way.
</p>
<p> Suppose you create a swig wrapper for a module called robin. The swig
generated code consists of two files robin.py and _robin.so. You wish to
make these modules part of a subpackage (brave.sir). With implicit namespace
packages you can place these files in the following configurations:
</p>
<p>Using PYTHONPATH="/some/path"</p>
<div class="diagram">
<pre>
/some/path/brave/sir/robin.py
/some/path/brave/sir/_robin.so
</pre>
</div>
<p>Using PYTHONPATH="/some/path:/some/other/path"
<div class="diagram">
<pre>
/some/path/brave/sir/robin.py
/some/other/path/brave/sir/_robin.so
</pre>
</div>
<p> Finally suppose that your pure python code is stored in a .zip file or
some other way (database, web service connection, etc). Python can load the
robin.py module using a custom importer. But the _robin.so module will need
to be located on a file system. Implicit namespace packages make this
possible. For example, using PYTHONPATH="/some/path/foo.zip:/some/other/path"
<p> Contents of foo.zip</p>
<div class="diagram">
<pre>
brave/
brave/sir/
brave/sir/robin.py
</pre>
</div>
<p> File system contents</p>
<div class="diagram">
<pre>
/some/other/path/brave/sir/_robin.so
</pre>
</div>
<p>Support for implicit namespace packages was added to python-3.3. The
zipimporter requires python-3.5.1 or newer to work with subpackages.
</p>
<H2><a name="Python_python3support">36.12 Python 3 Support</a></H2>