"Peter Maas" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
There was a huge and sometimes heated debate about tuples, lists and
dictionaries recently, and the mainstream opinion was that dictionary
keys must not be mutable, so lists are not allowed as dictionary keys.
BUT: objects are allowed as dictionary keys, aren't they? See the
interpreter session below:
...
This strikes me because if one can do this with instances of user
defined classes why not with lists? Trying to use lists as dict
keys yields "TypeError: list objects are unhashable". So why are
list objects unhashable and user defined objects hashable? For
user defined objects hash(x1) = id(x1), why not do the same
with lists?
I think it's because of the existence of list literals. If you
would use id() as a hash function for lists how should d[[1,2,3]]
be stored? For instances of user defined classes there is no
literal and accordingly no problem to use id() as a hash function
and instances as dictionary keys.
Is that correct?
No. The basic answer is that it's up to the object whether
it will allow itself to be used as a dictionary key. In other
words, if the designer of an object thinks it makes sense
for instances to be dictionary keys, then he can supply a
__hash__() method. If he doesn't, then he doesn't supply
such a method, and it won't work.
Each class gets to make its own decision. If a class
has a __hash__() method defined, then it can be
used as a dictionary key. If it doesn't, then it can't.
There are no other constraints on the class designer;
in particular, global logical considerations simply don't
apply. It's the designer's whim, and nothing more.
The designers of the list object decided not to allow it
to be used as a dictionary key. That was their choice,
and I pretty much agree with it.
John Roth
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