Mutable state in a decorator closure

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Accepted Answer

The trick here is calls = [0] rather than calls = 0. If you wrote calls = 0 and then calls += 1 inside wrapper, Python would treat calls as a local variable in wrapper and raise UnboundLocalError - the same problem that requires 'nonlocal'. Using a list sidesteps that: calls[0] += 1 mutates the existing list object rather than rebinding the name, so Python doesn't need a 'nonlocal' declaration. This is a classic Python closure workaround that predates the 'nonlocal' keyword (added in Python 3). The state persists across calls because calls is captured by reference in wrapper's closure - it's the same list object every time. The modern way is 'nonlocal calls' with a plain integer, but you'll see the list trick in older codebases.