Lexical scope and closures

The idea

A nested function can read names from its enclosing function. Assigning to a name normally creates a local binding in the assigning function, even when an outer name has the same spelling. nonlocal explicitly targets an existing enclosing-function binding. Mutating a captured list through its methods does not rebind the list’s variable and therefore follows a different rule.

Closures retain access to bindings rather than freezing every value when a function is created. Functions built in a loop can all read the loop variable’s final value when called later. A default argument can deliberately capture the current value for each function, because defaults are evaluated at definition time. Prefer a small factory when that intent would otherwise be obscure.

def counter():
    value = 0
    def step():
        nonlocal value
        value += 1
        return value
    return step
a, b = counter(), counter()
print(a(), a(), b())

1 2 1

Each factory call creates a separate enclosing value; calls through a share one of them.