What Is Python Packaging, Anyway?

1. You have some code that you import in a Python project.
2. You want other projects to also be able to import them.

Python packaging helps you to go from 1. to 2.

How does import work?

To make code importable, the module needs to be in a searchable location in sys.path:

>>> import os, sys
>>> for p in sys.path: print(os.path.normpath(p))
.
C:\Users\uranusjr\AppData\Local\Programs\Python\Python39\python39.zip
C:\Users\uranusjr\AppData\Local\Programs\Python\Python39\DLLs
C:\Users\uranusjr\AppData\Local\Programs\Python\Python39\lib
C:\Users\uranusjr\AppData\Local\Programs\Python\Python39
C:\Users\uranusjr\AppData\Local\Programs\Python\Python39\lib\site-packages

The first . (dot) means the current working directory, which is why you can directly import other files within a project. Otherwise, The next four paths contain standard libraries.

The last one, site-packages, is the one third-party libraries are generally put into.

Prepare the demo project to distribute

example-project/
    my_package/
        __init__.py

my_package/__init__.py

def hello():
    print("Greetings from my package!")

$ cd /path/to/example-project
$ py
>>> import my_package
>>> my_package.hello()
Greeting from my package!

Use my_package in another project

other-project/
    main.py

main.py

import my_package

my_package.hello()

Now try running main.py:

$ cd /path/to/other-project
$ py main.py
Traceback (most recent call last):
  File "main.py", line 1, in <module>
    import my_package
ModuleNotFoundError: No module named 'my_package'

It does not work. This is expected, since my_package is not available anywhere in other-project and its empty virtual environment.

The “fix” can be simple: just copy the my_package directory into one of the directories in sys.path.

Generally, third-party packages (i.e. packages that are not a part of the standard library, and does not belong specifically to a project) are copied into the site-packages directory.

We can correctly import the package once it’s copied into the environment:

$ cd /path/to/other-project
$ py main.py
Greetings from my package!