How Python Works Underneath

Tiers 0–2 taught you to use Python. Tier 3 explains why it behaves as it does — the handful of underlying models that turn surprising behaviour into predictable behaviour. The thesis of the tier: almost every Python "gotcha" is a consequence of one of a few mechanisms — names bind to objects, iteration is a protocol, functions are objects, exceptions propagate up the stack — and once you hold those, you can derive the behaviour instead of memorising it.

Six chapters, in order:

1. The Object Model — everything is an object; names are labels. is vs ==, mutability, aliasing, copying, argument passing.
2. Iterators, Iterables & Generators — how for really works, and yield for lazy streams.
3. Functions in Depth — functions as values: *args/**kwargs, closures, decorators, the mutable-default and late-binding traps.
4. Errors & Exceptions — propagation, try/except/else/finally, custom exceptions, EAFP.
5. Modules, Packages & Imports — organising code across files; the import-once-and-cache model.
6. Files & Context Managers — with and guaranteed cleanup via __enter__/__exit__.

Two of these are deep passes that revisit gentle Tier 0–1 topics with full rigour: The Object Model deepens Variables & Basic Types, and Functions in Depth deepens Functions, the Basics. They reference the earlier passes rather than repeating them.