I use closures quite a lot in python, but the variable lifetime semantics of closures are undocumented as far as I can tell. For example, consider the following code: def f(): a=1 b=1 def g(): return b return g If we call ‘f’ and store the value, ‘b’ will be kept alive. Is ‘a’ kept alive? A naive implementation of closures might store a reference to the entire stack frame of ‘f’ inside the ‘g’ closure, causing ‘a’ to live even though it will never be used.