Python modules with submodules and functions

Question

I had a question on how libraries like numpy work. When I import numpy, I'm given access to a host of built in classes, functions, and constants such as numpy.array, numpy.sqrt etc.

But within numpy there are additional submodules such as numpy.testing.

How is this done? In my limited experience, modules with submodules are simply folders with a __init__.py file, while modules with functions/classes are actual python files. How does one create a module "folder" that also has functions/classes?

Answer 1

A folder with .py files and a __init__.py is called a package. One of those files containing classes and functions is a module. Folder nesting can give you subpackages.

So for example if I had the following structure:

  mypackage      __init__.py      module_a.py      module_b.py         mysubpackage              __init__.py              module_c.py              module_d.py 

I could import mypackage.module_a or mypackage.mysubpackage.module_c and so on.

You could also add functions to mypackage (like the numpy functions you mentioned) by placing that code in the __init__.py. Though this is usually considered to be ugly.

If you look at numpy's __init__.py you will see a lot of code in there - a lot of this is defining these top-level classes and functions. The __init__.py code is the first thing executed when the package is loaded.