Gregory Ewing wrote: > Ethan Furman wrote: >> Ulrich Eckhardt wrote: >> >>> If two equal objects have different hashes, they >>> will be stored in different places in the hash map. Looking for >>> object1 will then not turn up with object2, even though they are equal. >> >> In this case this is the behavior I want. > > You can't rely on it, though. The hash value gets reduced > modulo the size of the dict, so even if two objects have > different hashes, in some cases they will land on the same > dict slot anyway. > > So an object such as you're postulating would behave > unpredictably when used as a dict key. Sometimes a lookup > using a different but equal object would find it, and > sometimes not, seemingly at random.
I think for every potential match the current dict implementation checks identity, then hashes -- and equality only if the hash values are equal. The relevant function is lookdict() in dictobject.c. While that means that the problem you describe cannot occur a simple approach that avoids relying on an implementation detail (?) would be to use (hash(obj), obj) tuples instead of just obj as the key. -- http://mail.python.org/mailman/listinfo/python-list
