How to load implicit namespace package by adding the package egg to sys.path?

Since python 3.3, implicit namespace packages are supported, so that I can have two packages:

a
├── b
    ├── __init__.py

a
├── c
    ├── __init__.py

and import a.b and a.c without problems. But it seems that I can only do it with pip install; if I only create an egg of either package and add the egg path into sys.path, the module cannot be loaded:

import sys
sys.path.append('./a-b.egg')

import a.b # ModuleNotFoundError: No module named 'a.b'

There are three ways to create namespace packages:

1. native namespace packages
2. pkgutil-style namespace packages
3. pkg_resources-style namespace packages

The former two are not zip-safe, and that's why the egg doesn't work. The third one is zip-safe.

Basically, the __init__.py file for the namespace package needs to contain only the following:

__import__('pkg_resources').declare_namespace(__name__)

and every distribution must provide the namespace_packages argument to setup() in setup.py:

from setuptools import find_packages, setup

setup(
    name='mynamespace-subpackage-a',
    ...
    packages=find_packages()
    namespace_packages=['mynamespace']
)