Closures and nonlocal in Python

Inner functions that remember and mutate outer state.

def make_counter(start=0):
    count = start

    def bump(step=1):
        nonlocal count
        count += step
        return count
    return bump


clicks = make_counter()
clicks()
clicks()
third = clicks()

score = make_counter(100)
bonus = score(50)

How it works

1. bump closes over count from make_counter.
2. nonlocal lets it rebind that captured variable.
3. Each counter keeps its own independent state.

Keywords and builtins used here

• bump
• def
• make_counter
• nonlocal
• return

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Step 1 of 5 in Decorators & closures, step 5 of 53 in Pythonic Python.

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