Classes, modules & files. Where Python starts to scale.

Defining your own types. The class keyword, instances, when to write a class instead of using a dict, and when not to bother.

How instances get their state. self, instance vs class attributes, and the mutable class-attribute trap that bites every beginner.

Functions that live on an instance. Mutating vs returning methods, internal helpers, and the dunder methods that make your objects play nice with print(), ==, and len().

Sharing behavior between related classes. The subclass syntax, super(), method overriding, and the case for using composition instead most of the time.

Where a name is visible. The LEGB rule, the global keyword, nonlocal for nested functions, and why globals are almost always a code smell.

A module is a .py file. The three import forms, what the standard library gives you for free, third-party packages with pip, and when to split your code across files.

Reading and writing the disk. The four file modes, the with statement, line-by-line streaming, JSON and CSV in the standard library, and the encoding pitfall.