Is __init__.py not required for packages in Python 3.3+

Question

I am using Python 3.5.1. I read the document and the package section here: https://docs.python.org/3/tutorial/modules.html#packages

Now, I have the following structure:

/home/wujek/Playground/a/b/module.py

module.py:

class Foo:
    def __init__(self):
        print('initializing Foo')

Now, while in /home/wujek/Playground:

~/Playground $ python3
>>> import a.b.module
>>> a.b.module.Foo()
initializing Foo
<a.b.module.Foo object at 0x100a8f0b8>

Similarly, now in home, superfolder of Playground:

~ $ PYTHONPATH=Playground python3
>>> import a.b.module
>>> a.b.module.Foo()
initializing Foo
<a.b.module.Foo object at 0x10a5fee10>

Actually, I can do all kinds of stuff:

~ $ PYTHONPATH=Playground python3
>>> import a
>>> import a.b
>>> import Playground.a.b

Why does this work? I though there needed to be __init__.py files (empty ones would work) in both a and b for module.py to be importable when the Python path points to the Playground folder?

This seems to have changed from Python 2.7:

~ $ PYTHONPATH=Playground python
>>> import a
ImportError: No module named a
>>> import a.b
ImportError: No module named a.b
>>> import a.b.module
ImportError: No module named a.b.module

With __init__.py in both ~/Playground/a and ~/Playground/a/b it works fine.

Answer 1

Python 3.3+ has Implicit Namespace Packages that allow it to create a packages without an __init__.py file.

Allowing implicit namespace packages means that the requirement to provide an __init__.py file can be dropped completely, and affected ... .

The old way with __init__.py files still works as in Python 2.