why is self not passed to id()?
tjreedy at udel.edu
Thu Sep 4 23:35:18 CEST 2008
Maric Michaud wrote:
> Le Thursday 04 September 2008 22:26:53 Ruediger, vous avez écrit :
>> class foo(list):
>> __hash__ = lambda x: id(x)
> Wow ! You are really going on trouble with this, believe me there is a real
> good reason for list not to be hashable. A dictionnary or set containing some
> of your foo is virtually inconsistent, read carefully the manual about
> prerequesites for dict keys, they *need* to be immutable.
No, the id comparison needs to be immutable -- which it is by default
for object()s, being based on id. Mutable instances of classes derived
from object work fine as keys as long as they keep default __eq__ and
__hash__. List over-rides the default, so foo needs to reverse that
def __eq__(self, other):
return id(self) == id(other)
This means, of course, that foo loses value-based equality comparison.
More information about the Python-list