Modules and packages

As code grows, you split it across files:

A module is any .py file.
A package is a folder of modules with an __init__.py file.

code

lab/

  library/                ← package

    __init__.py           ← runs on `import library`; decides what the package exposes

    models.py             ← Book, Member

    library.py            ← Library, BookNotAvailableError

    storage.py            ← save(), load()

  tests/

    test_library.py

python

# library/__init__.py

from library.library import BookNotAvailableError, Library

from library.models import Book, Member

__all__ = ["Book", "BookNotAvailableError", "Library", "Member"]

Now users write from library import Library, Book without knowing which file each lives in. This is how every real library presents a clean public surface.

Import styles:

python

import json                        # use it as json.dumps(...)

from pathlib import Path           # use it as Path(...)

from library.models import Book    # an absolute import from your own package: prefer this

Circular imports. If models.py imports library.py and library.py imports models.py, Python can fail with ImportError: cannot import name .... Keep the dependencies flowing one way: models → logic → storage/CLI.

Where Python looks. If you get ModuleNotFoundError: No module named 'library', you're probably running from the wrong folder. Run commands from the folder that contains the library/ package (here, lab/).