PEP420 makes __init__.py files optional: https://docs.python.org/3/whatsnew/3.3.html#pep-420-implicit-namespace-packages

Though it seems without them, pkgutil.walk_packages does not function as desired: https://docs.python.org/3/library/pkgutil.html#pkgutil.walk_packages

Consider the following example:

```
$ tree foo
foo
├── bar
│   ├── baz.py
│   └── __init__.py
├── __init__.py
└── womp.py
```

And a test script

# test.py
```
import pkgutil

import foo


for _, mod, _ in pkgutil.walk_packages(foo.__path__, foo.__name__ + '.'):
    print(mod)
```

In both python2 and python3 I get the following output:

```
$ python2.7 test.py
foo.bar
foo.bar.baz
foo.womp
$ python3.5 test.py
foo.bar
foo.bar.baz
foo.womp
```

Removing the __init__.py files and only using python3, I get this:

```
$ find -name '__init__.*' -delete
$ python3.5 test.py
foo.bar
```

The modules are definitely importable:

```
$ python3.5 -c 'import foo.bar.baz'
$
```

Originally asked as a question on stackoverflow: http://stackoverflow.com/questions/41203765/init-py-required-for-pkgutil-walk-packages-in-python3

While it is rather trivial to implement the proposed functionality - all that's required here is to eliminate the check for __init__.py from pkgutil._iter_file_finder_modules - this would have undesired impacts on, e.g., pydoc.apropos:
This function would then recursively report *any* directory/subdirectory on sys.path, which is quite surely not what people want.

I think this is a fundamental problem with namespace packages: they are nice and flexible for specific imports, but they make it impossible to know whether a directory found on the filesystem is *intended* as a Python package or not.

> all that's required here is to eliminate the check for __init__.py from pkgutil._iter_file_finder_modules

Ok, I was exaggerating here. To do it right would require a more complex change, but that's all that's needed to get an estimate of the effect the real thing would have.

> PEP420 makes __init__.py files optional

This is almost wrong.  PEP 420 added a new way for "namespace pacakge."
PEP 420 doesn't make empty __init__.py file in regular package.

(See https://dev.to/methane/don-t-omit-init-py-3hga)

Then, should pkgutil.walk_packages walk into all directories (e.g.  node_modules) ?  I don't think so.

If the resolution here is that this is behaving as intended (which personally I disagree with), I think this issue should remain open as a documentation task - the docs should clearly state that this does not apply to PEP420 namespace packages.

I am totally agree with Wolfgang:

> they make it impossible to know whether a directory found on the filesystem is *intended* as a Python package or not.

I think we shouldn't treat normal directory as namespace package until some portion in the directory is imported, or it is specified explicitly.

So walk_packages() should be used like:

  walk_packages("/path/to/namespace", "namespace")

I already rejected similar issue: #29642.

If you can not agree with me, please make an thread in python-dev ML or discuss.python.org.