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Why does list.__getitem_ _ return a list instance for subclasses ofthe list type?

>>class ListyThing(list ): pass
....
>>assert isinstance(List yThing()[:], ListyThing) # I expect True!
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError
>>type(ListyThi ng()[:]) # I expect ListyThing!
<type 'list'>

I don't find this intuitive. Is this intentional? I believe this could
be avoided if list.__getitem_ _ used "self.__class__ ()" to make a new
instance, instead of "list()", but I don't know how that works under
the hood in C.

I believe this happens a lot of other cases too. Actually, I wrote up
some test cases at http://brodierao.com/etc/listslice/ but I haven't
taken a look at it in quite a while. I believe there's some other
funky stuff going on there as well.

Also, this happens with dict too:
>>class DictyThing(dict ): pass
....
>>assert isinstance(Dict yThing().copy() , DictyThing) # I expect True!
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError
>>type(DictyThi ng().copy()) # I expect DictyThing!
<type 'dict'>

Any thoughts?
Feb 6 '07 #1
0 1039

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