Hi!
There's one thing about dictionaries and __hash__() methods that puzzle me. I
have a class with several data members, one of which is 'name' (a str). I would
like to store several of these objects in a dict for quick access
({name:object} style). Now, I was thinking that given a list of objects I might
do something like
d = {}
for o in objects:
d[o] = o
and still be able to retrieve the data like so:
d[name]
if I just defined a __hash__ method like so:
def __hash__(self):
return self.name.__hash__()
but this fails miserably. Feel free to laugh if you feel like it. I cooked up a
little example with sample output below if you care to take the time.
Code:
---------------------------------------------------------------
class NamedThing(object):
def __init__(self, name):
self.name = name
def __hash__(self):
return self.name.__hash__()
def __repr__(self):
return '<foo>'
name = 'moo'
o = NamedThing(name)
print "This output puzzles me:"
d = {}
d[o] = o
d[name] = o
print d
print
print "If I wrap all keys in hash() calls I'm fine:"
d = {}
d[hash(o)] = o
d[hash(name)] = o
print d
print
print "But how come the first method didn't work?"
---------------------------------------------------------------
Output:
---------------------------------------------------------------
This output puzzles me:
{'moo': <foo>, <foo>: <foo>}
If I wrap all keys in hash() calls I'm fine:
{2038943316: <foo>}
But how come the first method didn't work?
---------------------------------------------------------------
I'd be grateful if anyone can shed a litte light on this, or point me to some
docs I might have missed.
Also:
Am I in fact abusing the __hash__() method? If so - what's the intended use of
the __hash__() method?
Is there a better way of implementing this?
I realise I could just write
d[o.name] = o
but this problem seems to pop up every now and then and I'm curious if there's
some neat syntactic trick that I could legally apply here.
Thanks for your time!
/Joel Hedlund
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