Mike Meyer wrote:
> The mutability - or immutability - of an object is irrelevant to
> the question of whether or not they can be a dictionary key/set
> element. The critical property is that they are hashable.
> *Most* immutable builtin types are hashable, and no builtin
> mutable types are hashable, which deserves a mention.
I think the problem we are facing is that you must distinguish between
hashable/mutable *types* and hashable/mutable objects. You can have
*types* like tuple which are hashable/immutable themselves, but can have
*instances* that are mutable and not hashable, such as ([]).
For the built-in types I think objects are hashable if and only if they
are immutable (not 100% sure, but can you give a counterexample?).
That's why I got back to talk about mutability first. But I agree one
should also explain the problem of non-built-in types.
Maybe one can split the explanation and talk about the built-in
datatypes first and then explain the situation for user-defined types.
>>>And the footnote is:
>>>Instances of Python classes are hashable if they define a __hash__
>>>method, or if they don't define either __cmp__ or __eq__.
>>
>>I think that's not exact enough. For instance, ordinary lists also
>>define a __hash__ method but are not hashable since that method just
>>raises an exception.
>
> Ordinary lists aren't Python classes.
In your first sentence, it was unclear whether "they" refered to the
class or the instance. But anyway, what about the following class:
class mylist(list):
pass
This class definitely has a __hash__ method. But instances of this class
are not hashable.
-- Christoph
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