05_Functions — what happens when def runs

Level: 201 · for Python programmers, especially ones coming from C, Rust or ABAP

def is not a declaration. It is a statement that runs, in order with the statements around it, and when it runs it evaluates the default values, applies the decorators, stores the annotations and builds a function object that holds references into the scope it was made in. Every page in this chapter is a consequence of that sentence, and every surprise is a piece of work that happened earlier than the reader thought — a default, a decorator — or later — a closure's lookup, a type check that never comes.

#	Lesson	The question it answers	Status
1	A default is computed once	Why does my function remember the previous call's list — and why does None fix it?	written
2	A closure captures the variable, not the value	Why do three lambdas built in one loop all return the last value — and which of the three fixes should I use?	written
3	Assignment makes a name local	Why did adding total += 1 at the bottom of a function break the print(total) at the top?	stub
4	A decorator is a call, made when def runs	What does @deco actually do, and when — and why did every function I decorated get renamed wrapper?	stub
5	Annotations are not checked	Why did f('a') run when I wrote def f(x: int) — and what does check it?	stub

Where this sits relative to the other libraries

The Rust library's closures chapter ↗ is the sharp comparison for page 2: the compiler chooses how a closure captures and refuses the loop shape Python allows, and move ↗ is capture by value on demand. Rust has no default arguments, and Optional arguments ↗ is what it does instead. C has neither closures nor defaults, so a callback there is a function pointer and a void *, and ABAP has neither either: a method's DEFAULT must be a constant, and an object stands in for a closure. Both of those absences are said on the pages, in the bridge sections, rather than in a page of their own.