On Sat, Apr 28, 2012 at 11:09 AM, <laymanzh...@gmail.com> wrote: > I'm just learning Python. The python doc about mutable and hashable is > confusing to me. > > In my understanding, there is no directly relation between mutable and > hashable in Python. Any class with __hash__ function is "hashable". > > According the wiki: http://en.wikipedia.org/wiki/Immutable_object > > In object-oriented and functional programming, an immutable object is an > object whose state cannot be modified after it is created.[1] This is in > contrast to a mutable object, which can be modified after it is created. > > We surely can define __hash__ function in user-define class and the instance > of that class can be changed thus mutable. > > But following statement seems correct in practice but not technically. Any > comments on this?
Correct. Pedantically, you can define __hash__() on mutable objects; it's just not very useful or sensible, so people generally don't. As http://docs.python.org/reference/datamodel.html#object.__hash__ states [emphasis added]: "If a class defines *mutable* objects and implements a __cmp__() or __eq__() method, it *should not* implement __hash__(), since hashable collection implementations require that a object’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket)." > ------------------------ > http://docs.python.org/py3k/library/stdtypes.html#set-types-set-frozenset: > Since it is mutable, it has no hash value and cannot be used as either a > dictionary key or as an element of another set. > ------------------------ Cheers, Chris -- http://mail.python.org/mailman/listinfo/python-list
