🐍 python global vs nonlocal keyword — when to use each?

A variable can be modified inside a nested function without being passed as an argument — if Python’s scope resolution rules allow it through global or nonlocal.

Can I use both global and nonlocal in the same function?

Why do I get UnboundLocalError when I didn’t use global or nonlocal?

Does nonlocal work with nested classes or only functions?

Python resolves names using the LEGB rule : Local → Enclosing → Global → Built-in. This governs read operations: when you reference x, Python checks these scopes in order. At function definition time, the compiler scans all assignments. If any statement assigns to a name (e.g., x = 1, x += 1), that name is classified as local to the function unless declared otherwise with global or nonlocal. This means assignment changes the scope interpretation of a name—even if the assignment comes after a read.