Possibly dumb question about dicts and __hash__()

[Peter Otten]

Joel Hedlund wrote:

> 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__()

Just the hash is not enough. You need to define equality, too:

>>> class Named(object):
...     def __init__(self, name):
...             self.name = name
...     def __hash__(self):
...             return hash(self.name)
...     def __eq__(self, other):
...             try:
...                     other_name = other.name
...             except AttributeError:
...                     return self.name == other
...             return self.name == other_name
...     def __repr__(self):
...             return "Named(name=%r)" % self.name
...
>>> items = [Named(n) for n in "alpha beta gamma".split()]
>>> d = dict(zip(items, items))
>>> d["alpha"]
Named(name='alpha')

Peter