A more realistic example of PEP 420 Implicit Namespace Packages. Demonstrating a possible layout(s) for a 'core' application that supports (a) plugin(s) 'plug'.
Switch to the root folder (ex1-same-root or ex2-diff-root/core-src). Run the core application using python -m ns_example_pkg. Since no __init__.py is allowed within the namespace folder, the __main__.py has to be placed within the namespace package ns_example_pkg if you want the actual package to be executable.
In this example submodules are within the same root and namespace folder. Since all submodules - plug in this example - share the same root folder, they'll be available as well when started using python -m ns_example_pkg. However since pylint as well as other tools don't yet fully support the way PEP 420 allows to structure a project they won't be able to resolve the submodules and report some errors which can be ignored. If you want to work around those issues just install the application in editable mode using pip install -e ..
In this example submodules are in different root folders. Altought the namespace folder has - and in fact must have - the same name, they are different locations on the local filesystem. This is a typical layout as it would look like if a plugin plug is developed by a 3rd party. There are different way how to run this in development. One way would be to simply install the core application using plain old pip install <name-of-core> to have all dependencies available. But what if you want to work on the core source as well while you develop the plugin and make upstream changes? Or what if you're the core developer and want to provide a set of default plugins that are loosely coupled?
In that case the structure shown in this example is typical but you won't be able to simply use python -m ns_example_pkg because the plugin won't be included in the module search path. For this to work, install all subpackages (plugins) you want to work on using development aka editable mode. Simply switch to the submodules root folder ex2-diff-root/plug-src and run pip install -e .. This will make the path of the submodule plug available in the system's module path. Any changes on the source code of plug will be included, no re-install required. Now you can run python -m ns_example_pkg from ex2-diff-root/core-src just like in example 1.
Simply install the application (core) and any submodule (plug) required using pip install .. Once installed the core application can be started using python -m ns_example_pkg again. When not run from the example root folder, the installed version from system path will be used.
A word of caution: this is still a example to illustrate the use of namespace packages. For real world use you should never make your top-level package a namespace package. Read the article about namespace packages for more details.
When done with testing, you may uninstall the example package using pip uninstall ns-example. If you want to uninstall the plugin 'plug', then you have to run pip uninstall ns-example-plug. This will work even if ns-example-plug was installed in editable aka development mode. Another way to uninstall editable packages without having to specify the name of the package is to run python setup.py develop -u within the folder of the corresponding setup.py.