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Raymond told me in comp.lang.python to submit
this bug, he is working on a fix.
>>> s = slice(None,5)
>>> s.__hash__()
136748064
>>> hash(s)
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: unhashable type

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Terry J. Reedy commented in comp.lang.python:
-------------------------------------------------------------------------
Ref Manual 5.3.3 Slicings says nothing about hashability. 
If not
hashable, in spite of being mutable, "Slices are not
hashable" could
be added at end.
Ref Manual 3.3.1 Basic customization, __hash__( self) entry says
"Called for the key object for dictionary operations, and by the
built-in function hash() "
This implies to me that if there is a __hash__() method,
then hash()
will call it. If not true, then a 'maybe' could be added.
Lif Maual 2.1 Built-in Functions, hash( object) says only
"Return the hash value of the object (if it has one)."
If having __hash__() does not mean 'has one', then what does.

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Fixed. Now, they both raise TypeError.
See Objects/sliceobject.c 2.21.16.1 and 2.22.
Thanks for the bug report.