The fact is this works: >>> class B(object): ... __class__ = dict >>> b=B()
but this does not >>> class B(object): ... def __init__(self): ... self.__class__ = dict >>> b=B() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in __init__ TypeError: __class__ assignment: only for heap types On Aug 20, 1:39 pm, Ian Kelly <ian.g.ke...@gmail.com> wrote: > On Mon, Aug 20, 2012 at 12:01 PM, Massimo Di Pierro > > <massimo.dipie...@gmail.com> wrote: > > I discovered I can do this: > > > class A(object): pass > > class B(object): > > __class__ = A # <<<< magic > > > b = B() > > isinstance(b,A) # returns True (as if B derived from A) > > isinstance(b,B) # also returns True > > > I have some reasons I may want to do this (I an object with same > > methods as a dict but it is not derived from dict and I want > > isinstance(x,dict)==True to use it in place of dict in some other > > code). > > > What are the problems with the magic trick above? Why does it work? > > Normally with __class__ assignment, you would assign to the __class__ > attribute of the *instance*, not the class declaration. This actually > changes the class of the object, and so isinstance(b, B) would no > longer return True. > > I've never heard of assigning it in the class declaration, and as far > as I know, this behavior isn't documented anywhere. I expect that > what's happening here is that Python is not actually updating the > class of the instance, but that A is merely assigned to the > "__class__" attribute in the class dict, and that isinstance is > somehow (perhaps accidentally) finding this. So I think this is > probably a bug, and I would not rely on it to work correctly in all > cases. > > In any event, the use case that you're looking for is usually > accomplished using abstract base classes. Instead of "isinstance(x, > dict)", you should use "isinstance(x, collections.MutableMapping)", > and then inherit your class from or register it with the > MutableMapping ABC. -- http://mail.python.org/mailman/listinfo/python-list
