Richard D. Moores wrote:
I'm trying to write a general test for hashability. How can I test if
an object has both a __hash__() method and an __eq__() method?
Just because an object has a __hash__ method doesn't mean it is
guaranteed to be hashable. The method might (deliberately, or
accidentally) fail and raise an exception.
>>> t = (1, 2, 3)
>>> t.__hash__
<method-wrapper '__hash__' of tuple object at 0xb7f09fcc>
>>> hash(t)
-378539185
But:
>>> t = (1, 2, [3])
>>> t.__hash__
<method-wrapper '__hash__' of tuple object at 0xb7f0c6e4>
>>> hash(t)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: list objects are unhashable
The only effective way to find out if an object is hashable is to try it
and see.
Even more effective is to avoid trying to find out whether it is
hashable, and just use it, as required, and deal with the error if it
isn't. This is the "Easy to get forgiveness than to ask permission"
model of error handling, as opposed to "Look before you leap".
For various reasons, LBYL can be risky... like in Quantum Mechanics, the
very act of *looking* at an object might change its behaviour. (Methods
can have side-effects.) So, even if is_hashable(obj) returns True, you
can't *quite* be 100% certain that mydict[obj] will succeed until you
try it.
--
Steven
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