Hello All,
I wrote the function to test hashability of arbitrary objects. My reason
is that the built-in python (2.5) hashing is too permissive for some
uses. A symptom of this permissiveness comes from the ability to
successfully hash() arbitrary objects:
py> class C(object): pass
...
py> {C():4}[C()]
------------------------------------------------------------
Traceback (most recent call last):
File "<ipython console>", line 1, in <module>
<type 'exceptions.KeyError'>: <__main__.C object at 0xe21610>
The basis for the exception is that the two instances do not have the
same hash() although conceptually they might seem equal to the
unitiated. Were I to re-design python, I'd throw an exception in this
case because of the ill-defined behavior one might expect if a C()
serves as a key for a dict.
To prevent users of one of my libraries from falling into this and
similar traps (which have potentially problematic consequences), I came
up with this test for hashability:
def hashable(k):
try:
hash(k)
except TypeError:
good = False
else:
good = (hasattr(k, '__hash__') and
(hasattr(k, '__eq__') or hasattr(k, '__cmp__')))
return good
It works as I would like for most of the cases I can invent:
py> all(map(hashable, [1,1.0,"",(1,2,3)]))
True
py> any(map(hashable, [None, [1,2], {}, C(), __import__('sys')]))
False
Can anyone think of boundary cases I might be missing with this approach?
James
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