Possibly dumb question about dicts and __hash__()

Joel Hedlund joel.hedlund at gmail.com
Wed May 3 22:18:12 CEST 2006


Hi!

Thanks for the quick response!

 > Although this is a bit illegal, because repr is not supposed to be used
 > this way.

How illegal is it? If I document it and put it in an opensource project, will 
people throw tomatoes?

/Joel

johnzenger at gmail.com wrote:
> Use __repr__.  Behold:
> 
> 
>>>>class NamedThing(object):
> 
>      def __init__(self, name):
>          self.name = name
>      def __repr__(self):
>          return self.name
> 
> 
>>>>a = NamedThing("Delaware")
>>>>b = NamedThing("Hawaii")
>>>>d = {}
>>>>d[a] = 1
>>>>d[b] = 50
>>>>print d
> 
> {Delaware: 1, Hawaii: 50}
> 
>>>>d[a]
> 
> 1
> 
>>>>d[b]
> 
> 50
> 
> Although this is a bit illegal, because repr is not supposed to be used
> this way.
> 
> Joel Hedlund wrote:
> 
>>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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