[EMAIL PROTECTED] writes:
> Mike> Tuples have the problem that they are immutable, except when
> Mike> they're not (or for proper values of immutable, your
> Mike> choice). They're hashable, except when they're not. Or
> Mike> equivalently, they can be used as dictionary keys - or set
> Mike> elements - except when they can't.
> For those of us not following this thread closely, can you identify cases
> where tuples are mutable, not hashable or can't be used as dictionary keys?
> I've never encountered any such cases.
Actually, that didn't come from this thread. But it happens if one of
the elements in the tuple is mutable. For instance:
>>> t = [],
>>> type(t)
<type 'tuple'>
>>> t == [],
True
>>> hash(t)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: list objects are unhashable
>>> t[0].append(1)
>>> t == [],
False
>>> {t: 23}
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: list objects are unhashable
>>>
For builtins, the three cases - hashable, immutable and usable as
dictionary keys - are all identical. Instances of Python classes with
proper __hash__ and __eq__ defintions will be hashable and usable as
dictionary keys. Immutable for them is a messy question.
<mike
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Mike Meyer <[EMAIL PROTECTED]> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
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