[Python-ideas] Why does += trigger UnboundLocalError?

[Georg Brandl]

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