KITFORMA · EDITORIAL GUIDE
Why do Python callbacks created in a loop all use the last value?
KitForma editorial guide. A closure normally looks up the captured variable when called. The loop has already advanced by then.
Step-by-step guidance
A closure normally looks up the captured variable when called. The loop has already advanced by then.
For a fixed snapshot, bind the current value with
lambda value=value: value * 2, or use a factory receiving the value as an argument. Build callbacks for 0, 1 and 2, call them after the loop, and expect 0, 2 and 4. This binds the object reference rather than making a deep copy: if the value is a mutable dictionary, later dictionary changes remain visible. Choose late binding deliberately when a callback should read current configuration rather than its creation-time value.
Minimal example:
callbacks = [lambda value=value: value * 2 for value in range(3)]
assert [callback() for callback in callbacks] == [0, 2, 4]Sources and verification
Sources checked:
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