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.mysubpacakge.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.