pip install --editable does not work as expected for namespace packages

Environment

• pip version: pip 19.3.1
• Python version: Python 3.6.8
• OS: linux

Description This is not a duplicate of #3

I have a mainpackage that is a standard package with an __init__.py. It has a subpackage, mainpackage.ext, that is a namespace package according to PEP 420 (no __init__.py). All of this is located in a main_package folder with a setup.py.

Now I have a separate folder some_plugin with another setup.py. This folder contains a plugin for mainpackage. It therefore has a package mainpackage.ext.someplugin with an __init__.py in mainpackage/ext/someplugin/ but not in mainpackage/ or mainpackage/ext/.

Using a normal pip install, this works as expected. But when using pip install -e to install both main_package and some_plugin, the plugin cannot actually be imported.

Expected behavior Same behavior between pip install and pip install -e, i.e. the ability to import mainpackage.ext.someplugin without a ModuleNotFoundError.

How to Reproduce I’ve created a repository with a minimal example: https://github.com/jonasrauber/python_namespace_packages_editable_bug

Works

sudo pip3 install main_package/
sudo pip3 install some_plugin/

python3 -c "import mainpackage; print(mainpackage.a)"
# -> hello

python3 -c "import mainpackage.ext.someplugin; print(mainpackage.ext.someplugin.a)"
# -> plugin

Fails

sudo pip3 install -e main_package/
sudo pip3 install -e some_plugin/

python3 -c "import mainpackage; print(mainpackage.a)"
# -> hello

python3 -c "import mainpackage.ext.someplugin; print(mainpackage.ext.someplugin.a)"
# -> ERROR

# Traceback (most recent call last):
#   File "<string>", line 1, in <module>
# ModuleNotFoundError: No module named 'mainpackage.ext.someplugin'

[pfmoore]

Does the same behaviour happen if you use python setup.py develop rather than pip install -e? The editable install mechanism is implemented by setuptools, so I suspect this may be down to how setuptools handles PEP 420 namespace packages. Checking setup.py develop will help confirm this.

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