Why can `__init__.py` import a submodule but a test file with `from module import *` can't?

Question

I have the following folder structure.

project/
    sandbox.py
    module/
        __init__.py
        klass.py

__init__.py has the following:

from klass import Klass

if __name__ == '__main__':
    'do stuff'

This works fine. sandbox.py looks like this:

from module import *

if __name__ == '__main__":
    'do similar stuff'

On the command line, while in the project directory:

$ python module/__init__.py
# things work correctly
$ python sandbox.py
Traceback (most recent call last):
  File "sandbox.py", line 1, in <module>
    from module import *
  File "/Uses/chuck/.../project/module/__init__.py, line 1 in <module>
    from klass import Klass
ImportError: No module named 'klass'

I think this might have to do with relative imports, and tried changing from klass import Klass to from .klass import Klass , but that didn't work either.

My goal is, from files outside the module, to be able to use from module import * or from module import Klass . What do I need to do?

In case it matters, this is with Python 3.5.2 under macOS within a virtual environment.

Answer 1

Use absolute path and __main__.py file for this:

from module.klass import Klass

'do stuff'

In __init__.py :

from .klass import Klass

Now you can execute it as follows:

$ python sandbox.py
do similar stuff
$ python -m module
do somthing