[Python-Dev] inplace operators and setitem (original) (raw)

James Y Knight foom at fuhm.net
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

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