Python loop callbacks: bind the intended value before deferred execution

What it is

Python's programming FAQ explains that a closure accesses an outer variable when the function is called, not when it is defined. Its loop example demonstrates callbacks all using the final value, and shows binding a default argument to capture each iteration's value. The behavior applies to ordinary nested functions as well as lambdas. Python programming FAQ

Why it matters

A coding agent may test callbacks immediately inside the loop and miss the deferred case that the real event system uses. The design question is whether the callback should observe future changes or remember the registration-time value. Both can be intentional, but they require different tests.

How to apply

• Locate the callback registration and the later invocation. List names read from the enclosing scope and identify which can change before invocation.
• Write the intended capture policy for each name. For a per-item callback, bind the item through a default argument or a factory with its own local value, using a style consistent with the surrounding code.
• Propose a fixture that registers all callbacks first, then invokes them after the loop. Assert each callback's target independently rather than checking only the number of callbacks.
• Add a case where the outer name changes again before invocation. This distinguishes value capture from an intentionally live lookup.
• If the captured value is mutable, decide whether retaining the object or copying its relevant data matches the contract. Binding an object reference is not a deep snapshot.

Pitfalls

Do not mechanically rewrite every closure: some callbacks intentionally read current configuration. A default argument also becomes part of the callable's signature, which can matter to a framework passing arguments. Check the registration API before choosing the pattern. This is a diagnostic and review method; the example behavior comes from the cited documentation, and no new runtime experiment is claimed.