Originally reported by: hongqn (Bitbucket: hongqn, GitHub: hongqn)

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To reproduce this issue, do the following in a virtualenv:

1. prepare source code of two packages with same namespace package ns:

   mkdir -p pkg1/ns/pkg1 pkg2/ns/pkg2 echo "from setuptools import setup, find_packages; setup(name='pkg1', packages=find_packages(), namespace_packages=['ns'])" > pkg1/setup.py echo "__import__('pkg_resources').declare_namespace(__name__)" > pkg1/ns/__init__.py touch pkg1/ns/pkg1/__init__.py echo "from setuptools import setup, find_packages; setup(name='pkg2', packages=find_packages(), namespace_packages=['ns'])" > pkg2/setup.py echo "__import__('pkg_resources').declare_namespace(__name__)" > pkg2/ns/__init__.py touch pkg2/ns/pkg2/__init__.py 

2. install pkg1 using --single-version-externally-managed

   cd pkg1 python setup.py install --single-version-externally-managed --record record.txt cd .. 

3. install pkg2 using develop

   cd pkg2 python setup.py develop cd .. 

4. pkg1 is accessible

   python -c 'import ns.pkg1' 

5. pkg2 is unexpectedly inaccessible

   python -c 'import ns.pkg2' Traceback (most recent call last): File "<string>", line 1, in <module> ImportError: No module named pkg2 

This was discussed in pip's issue tracker pypa/pip#3 years ago (because pip uses install --single-version-externally-managed for pip install and use develop for pip install --editable), but they still have not gotten a resolution.

I think this issue should be resolved in setuptools instead of pip. install --single-version-externally-managed creates a {pkg}-nspkg.pth in site-packages directory. It will create a ns module and assign its __path__ to the ns directory in site-packages. So pkg_resources.declare_namespace() in pkg2's ns/init.py has no chance to run.

I have two proposals to fix it:

1. Fix install --single-version-exteranlly-managed. Modify the {pkg}-nspkg.pth file to append a line

   import pkg_resources; pkg_resources.declare_namespace('ns') 

   at the end of it. So it will scan sys.path to gather other directories into ns.__path__.

   A tricky point is that we must ensure the {pkg}-nspkg.pth filename is alphabetically larger than easy-install.pth, so that we have source code directory installed by develop command in sys.path when running declare_namespace. Is to rename to zzz-{pkg}-nspkg.pth a good idea?

2. Fix develop command. Create a {pkg}-nspkg.pth in site-packages directory when a namespace package is installed using develop command, like what install --single-version-externally-managed does. The content of the pth file could be:

   /path/to/sourcecode import pkg_resources; pkg_resources.declare_namespace('ns') 

   The /path/to/sourcecode line is duplicated with what is inserted in easy-install.pth. It is here to make sure sys.path contains it before declare_namespace runs. It could be omitted if we have a zzz- prefix for the pth filename.

What's the dev team's opinion? I prefer to the second proposal because it only affects develop command which is mostly used in develop environment so it will not affect production systems.

I'd like to contribute code fix if you agree with either one of the proposals.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/250