inconsistency with += between different types ?
Andreas.Leitgeb at siemens.at
Wed Aug 7 05:43:47 EDT 2002
Huaiyu Zhu <huaiyu at gauss.almadan.ibm.com> wrote:
> Andreas Leitgeb <Andreas.Leitgeb at siemens.at> wrote:
>>Huaiyu Zhu <huaiyu at gauss.almadan.ibm.com> wrote:
>>> That's what it should be. Yet it is possible for whoever writes __iadd__ to
>>> define it the other way.
>>Is there something I've missed, or [...]
> The thing missing is that the return statement plays an unnecessarily big
> role here.
> ... if you define __iadd__, by
> returning an object other than self.
> By the way, if you forget to return self, you rebind the name to None.
Thanks, now (I think) I finally got it:
__iadd__ is SUPPOSED to modify self's attributes and then "return self"
If __iadd__ does return anything else than self, then it actually
breaks the intended semantics for __iadd__, but not the syntax -
it's still legal python.
This of course explains, why it causes that much trouble.
A possible path to change this situation might be as follows:
(Although I feel too new at Python to actually propose a change :-)
Automatically treat a None returnvalue from an __ixxx__-method as self
Issue a Warning, if __ixxx__ returns anything else than self or None
Ignore the return-value altogether.
Each of these steps might have to wait for a major-version-step in
This would make sure that clean implementations would stay ok, while
those implementations, that obfuscate the design of <op>= will hopefully
> class A:
> def __iadd__(self,x): print x
Good demonstration of a nasty pitfall.
Newsflash: Sproingy made it to the ground !
read more ... <http://avl.enemy.org/sproingy/>
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