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Closure Review

The whole point of a closure is that it's stateful. It's a function that "remembers" the values from the enclosing scope even after the enclosing scope has finished executing.

It's as if you're saving the state of a function at a particular point in time, and then you can use and update that state later on.

from collections.abc import Callable


def concatter() -> Callable[[str], str]:
    doc: str = ""

    def inner_func(word: str) -> str:
        # "nonlocal" tells Python to use the doc
        # variable from the enclosing scope
        nonlocal doc
        if doc:
            doc += " "
        doc += word
        return doc

    return inner_func


harry_potter_aggregator: Callable[[str], str] = concatter()
harry_potter_aggregator("Mr.")
harry_potter_aggregator("and")
harry_potter_aggregator("Mrs.")
harry_potter_aggregator("Dursley")
harry_potter_aggregator("of")
harry_potter_aggregator("number")
harry_potter_aggregator("four,")
harry_potter_aggregator("Privet")

print(harry_potter_aggregator("Drive"))
# Mr. and Mrs. Dursley of number four, Privet Drive

That means that in many cases, closures are not pure functions. They can mutate state outside of their scope and have side effects.