Python submodule imports using __init__.py

Question

I'm learning Python, and I can't figure out how imports in __init__.py work.

I understand from the Python tutorial that the __init__.py file initializes a package, and that I can import subpackages here.

I'm doing something wrong, though. Could you explain for me (and for future Python-learners) what I'm doing wrong?

Here's a simplified example of what I'm trying to do.

This is my file structure:

package
    __init__.py
    test.py
    subpackage
        __init__.py
        hello_world.py

The contents of hello_world.py:

def do_something():
    print "Hello, world!"

subpackage/__init__.py is empty.

package/__init__.py contains:

import test.submodule.do_something

And finally, test.py contains:

do_something()

This is how I attempt to run hello_world.py using OSX terminal and Python 3:

python test.py

Python then throws the following error:

NameError: name 'do_something' is not defined

Answer 1

First, you have to understand how import alone work:

import test.submodule.do_something

Would try to load do_something from submodule itself loaded from test.

You want to load something from subpackage, so start with that:

import subpackage

Fine, subpackage/__init__.py is loaded.

Now, you want the do_something() function which is in the file (a "module") hello_world.py. Easy:

from subpackage.hello_world import do_something

And you are done! Just read this line loud, it does exactly what it says: import do_something from the module hello_world which is in the subpackage package.

Try that in test.py

from subpackage.hello_world import do_something

do_something()

It should work just fine.

Now, the second issue:

__init__.py won't be called in package/ since you don't use package/ as a package. __init__.py will be used if you do an import of package/ or anything in it, for eg:

from package import test

Otherwise, it won't be loaded at all.

However, if you want to load do_something() on the import of subpackage, put from submodule.hello_word import do_something in subpackage/__init__.py, and then, in you test.py, do a import subpackage.

Answer 2

It's an absolute hard-and-fast rule in Python that a name must always be defined or imported within the module where you're using it. Here you never import anything inside test.py - so as the error says, do_something is not defined.

Even if your package/__init__.py file was executed (which, as others have pointed out, it isn't), your code still wouldn't work as it is, because the import of do_something has to be done inside test.py if you want to reference it in that file.