[Python-Dev] inplace operators and setitem

Wed Sep 28 16:08:44 CEST 2005

On Sep 28, 2005, at 9:12 AM, Reinhold Birkenfeld wrote:
> Hi,
>> a general question. Consider:
>> class A(list):
> def __setitem__(self, index, item):
> # do something with index and item
> return list.__setitem__(self, index, item)
>> lst = A([1,set()])
>> lst[0] |= 1
>> lst[1] |= set([1])
>> Do we want lst.__setitem__ to be called in the second inplace 
> assignment?

Yes. Right now, you can roughly explain the behavior by stating that, 
after "x=a", "x |= y" is the same as "x = x | y", except that "a"'s 
value is undefined (it might have changed, or it might have not).
> A case where this matters is here: http://python.org/sf/1306777

This confusion between modification of immutable types and 
modification of mutable types is why I feel that it's often best to 
simply avoid the inplace operators in favor of their explicit 
equivalents. In this case, set.update().
James

[Python-Dev] inplace operators and __setitem__

[Python-Dev] inplace operators and setitem