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Packages and modules in Python

Previously, and now currently, packages in Python have been a source of much woe to me. Plagued by hundred of “No module named xxx”, “Attempted relative import in non-package” etc, it is now time to write myself a little guide to refer to in future.

Packages and Modules

A package is a collection of modules with a __init__.py file inside the package directory and a module is simply a python file.

Say I have the layout:

* core:
    * __init__.py
    * queue.py
    * tests:
        * queue_tests.py

and I want the Queue class from core.queue. How do I import this in queue_tests.py?

You’d think

from ..queue import Queue

def blah():
    stuff()

would do it; and you’d be correct. But check how you’re running the file. If you’re running it directly like python tests/queue_tests.py, then the name of the file will be __main__ which breaks relative imports. You have to run it like so: python -m tests.queue_tests. This runs the module queue_tests as a script, but fixes the __name__ issue.