How do closures and late binding work in Python?

Explains lexical scope, captured names, late lookup, loop callback failures, and deliberate state ownership in closures.

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TL;DR

A closure is a function that uses names from an enclosing function scope after that enclosing call has returned. Python resolves those captured names when the inner function runs, so the closure keeps access to the binding rather than automatically copying its value at creation. That late binding explains why callbacks created in a loop can all observe the loop variable's final value.

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A correct scope model and the ability to diagnose callbacks that all observe the final loop value.

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Closures are useful for factories and decorators, but they capture bindings rather than frozen snapshots of every value.
A closure is a function that uses names from an enclosing function scope after that enclosing call has returned. Python resolves those captured names when the inner function runs, so the closure keeps access to the binding rather than automatically copying its value at creation. That late binding explains why callbacks created in a loop can all observe the loop variable's final value.
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A common fix is to bind the current value as a default argument in the inner function or call a helper that receives the current value and creates a new enclosing scope. nonlocal allows an inner function to rebind a name in the nearest enclosing function scope; global targets module scope. I use these deliberately because hidden mutable closure state can be harder to test and reason about than an explicit object.

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Saying a closure always copies captured values.
Expecting each loop callback to freeze the loop variable automatically.
Using global when the state is in an enclosing function.
Hiding large or mutable service state inside an opaque closure.

How do closures and late binding work in Python?

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