Closures & nonlocal

A closure is a function that captures variables from its enclosing scope.

Why nonlocal?

By default, assigning to a name inside a function creates a new local. nonlocal says "no, write to the outer variable."

When closures are useful

• Function factories (make_counter, make_adder)
• Lightweight state without a class
• Decorators (every decorator is a closure)

Gotcha

Classic loop pitfall: [lambda: i for i in range(3)] — all three lambdas see i == 2. Capture explicitly with a default argument: lambda i=i: i.

Try it

1. Write make_adder(n) that returns a function adding n to its argument.
2. What does [f() for f in [lambda: i for i in range(3)]] produce?