Editing PYTHONPATH (or "Where's my module?!")

Python not finding your modules with import MyModule for some reason? It's probably your PYTHONPATH. Here's how you edit it so that Python sees all your modules.

A Word About Directories

Firstly, if you are the kind of person who keeps files scattered about your computer, dumps everything into the root directory, or is afraid to use those nice little blue folders, then perhaps programming and computers in general are not for you.

Logical and neat directory structure will make your own, your computer's and your collaborators' lives much easier.

My recommendation: Create a master code directory (called "Modules" or something) in your Documents folder. This will be the new home of all your future Python projects.

Now every time you create a .py file, it should go into a project folder inside your Modules directory, i.e. /Documents/Modules/NewProject/MyModule.py. Note that you should have no actual modules inside your Modules directory! Keep those puppies in their warm, snuggly project subdirectories.

This way you can also initialize that project folder (i.e. /NewProject) as a Git repository and just push and pull the contents to keep it up-to-date!

Editing PYTHONPATH

Python won't just search your computer for the MyModule.py file you're trying to import. You have to tell it explicitly each time where to get it. The PYTHONPATH is a list of directories for your computer to check whenever you type import MyModule into the interpreter.

To add a path, launch ipython and type:

import sys
sys.path.append("path/to/Modules")
print sys.path

Note: You only have to update your PYTHONPATH once, not every time you use Python!

So now your path is updated but this is only the path to your master Python folder.

In order to have Python see the modules inside each subdirectory, add a blank file called __init__.py to each subdirectory (with two underscores on each side).

Now to import the module and use a function called foo() do:

from NewProject import MyModule as m
m.foo()

That is, it's checking the main python directory you added to your PYTHONPATH, then looking within the NewProject subdirectory via the __init__.py file for the module you're trying to import.