On 01.06.2011 06:52, Carl M. Johnson wrote:
We all know that the following code won't work because of UnboundLocalError and that to get around it, one needs to use nonlocal:
def accum(): ... x = 0 ... def inner(): ... x += 1 ... return x ... return inner ... inc = accum() inc() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 4, in inner UnboundLocalError: local variable 'x' referenced before assignment
But why does this happen? Let's think about this a little more closely: += is not the same as =. A += can only happen if the left-hand term was already defined. So, why does the compiler treat this as though there were an assignment inside the function?
Because x += y is equivalent to x = x.__iadd__(y) and therefore an assignment is going on here. Therefore, it's only logical to treat it as such when determining scopes. Georg