confusing doc: mutable and hashable

[John O'Hagan]

On Sat, 28 Apr 2012 11:35:12 -0700
Chris Rebert <clp2 at rebertia.com> wrote:

[...]
> 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)."
[...]

How do function objects fit into this scheme? They have __hash__, __eq__, seem
to work as dict keys and are mutable. Is it because their hash value doesn't
change? Under what circumstances does an object's hash value change?
--
John