nonlocal counter

Question

nonlocal counter

Accepted Answer

nonlocal n tells Python that n inside counter refers to the n in make_counter's enclosing scope, not a new local variable. Without nonlocal, n += 1 would create a local n and raise UnboundLocalError on the read of n before assignment. Each call to c() increments the same n and returns the new value, so you get 1, 2, 3. This is the standard pattern for stateful closures - the counter state lives in the closure's enclosing scope rather than in a class or global variable.