Possibly dumb question about dicts and __hash__()

Joel Hedlund joel.hedlund at gmail.com
Wed May 3 21:38:05 CEST 2006


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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