[Python-ideas] Why does += trigger UnboundLocalError?
ethan at stoneleaf.us
Wed Jun 1 13:24:30 CEST 2011
Carl M. Johnson wrote:
> On Tue, May 31, 2011 at 7:48 PM, Georg Brandl wrote:
>> 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.
> But the difference is that you can only use += if the LHS name already
> exists and is defined. So, it couldn't possibly be referring to a local
> name if it's the only assignment-like statement within a function body.
> How could it refer to a local if it has to refer to something that
> already exists?
Two problems. Firstly, what error should be raised here?
--> def accum():
... x = 0
... def inner():
... x1 += 1
... return x
... return inner
Secondly, the += operator may or may not be a mutating operator
depending on the object it's used on: if the object does not have a
__iadd__ method, it's not mutating; even if it does have an __iadd__
method, it may not be mutating -- it's up to the object to decide.
--> class ex_int(int):
... def __iadd__(self, other):
... return self + other
--> x = ex_int(7)
--> x = [1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3, 4]
-1 on changing the semantics.
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