What is __path__ useful for?

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This is usually used with pkgutil to let a package be laid out across the disk. E.g., zope.interface and zope.schema are separate distributions (zope is a “namespace package”). You might have zope.interface installed in /usr/lib/python2.6/site-packages/zope/interface/, while you are using zope.schema more locally in /home/me/src/myproject/lib/python2.6/site-packages/zope/schema.

If you put pkgutil.extend_path(__path__, __name__) in /usr/lib/python2.6/site-packages/zope/__init__.py then both zope.interface and zope.schema will be importable because pkgutil will have change __path__ to ['/usr/lib/python2.6/site-packages/zope', '/home/me/src/myproject/lib/python2.6/site-packages/zope'].

pkg_resources.declare_namespace (part of Setuptools) is like pkgutil.extend_path but is more aware of zips on the path.

Manually changing __path__ is uncommon and probably not necessary, though it is useful to look at the variable when debugging import problems with namespace packages.

You can also use __path__ for monkeypatching, e.g., I have monkeypatched distutils at times by creating a file distutils/__init__.py that is early on sys.path:

import os
stdlib_dir = os.path.dirname(os.__file__)
real_distutils_path = os.path.join(stdlib_dir, 'distutils')
__path__.append(real_distutils_path)
execfile(os.path.join(real_distutils_path, '__init__.py'))
# and then apply some monkeypatching here...

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