python如何从不同的位置导入不同的分包？

Question

I have an old system that I'm trying to replicate. It is based on an old Debian Wheezy (7.1) system and uses Apache 2.2.22 and libapache2-mod-wsgi 3.3, in daemon mode, and Python 2.7.3. The WSGI application uses packages from a virtualenv, created with access to the global site-packages modules. It uses two subpackages from the same namespace package, that are installed by the system in different places:

• repoze.who , installed in /usr/lib/pymodules/python2.7/
• repoze.lru , installed in /usr/lib/python2.7/dist-packages/

My problem is that it does not work on the replicated system: the application fails to import repoze.who . On the other hand, on the original system, it works file and successfully imports both subpackages.

I understand that it's not supposed to work since, in normal configuration, Python does not expect a package to be split in several places.

I read the python import different subpackages with the same root packge name and different locations question and its answer. If I modify both repoze/__init__.py files as suggested, it works fine on the replicated system.

But I checked both files on the original system, and they are empty.

I added a few print instructions in the WSGI application, to investigate the original system. sys.path looks fine and similar on both systems, with /usr/lib/python2.7/dist-packages before /usr/lib/pymodules/python2.7 on both. Surprisingly, the repoze package is imported from /usr/lib/pymodules/python2.7 on the original system, while it is imported from /usr/lib/python2.7/dist-packages on the replicated system. Moreover, repoze.__path__ contains both locations on the original system, while it only contains /usr/lib/python2.7/dist-packages/repoze on the replicated system.

Last but not least, if I run the virtualenv's python interpreter on the original system, it fails to import the repoze.who package while it imports repoze.lru file, which looks like a normal behaviour. Hence, there must be something special with mod-wsgi or the application.

I guess something “clever” was done to allow the WSGI application to load both packages from different locations, but I have no idea what. Any suggestion is welcome.

Answer 1

repoze is using namespace packages .

From its setup.py :

      namespace_packages=['repoze', 'repoze.who', 'repoze.who.plugins'],

• See PEP 420 for the modern way to build namespace packages.
• See pkg_resources for the old way (the approach I have personal experience with, back in the mid-2000s).
• See the pkgutil approach when you need compatibility with existing code already using that approach.