confusing doc: mutable and hashable

[laymanzheng at gmail.com]

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?

Thanks,
Andy

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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.
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