dictionary keys, __hash__, __cmp__

Jan-Erik Meyer-Lütgens python at meyer-luetgens.de
Wed Nov 5 16:10:40 CET 2003

Michael Hudson wrote:
> Jan-Erik Meyer-Lütgens  <python at meyer-luetgens.de> writes:
>>Michael Hudson wrote:
>>>Jan-Erik Meyer-Lütgens  <python at meyer-luetgens.de> writes:
>>>>   3. "If a class does not define a __cmp__() method it
>>>>       should not define a __hash__() operation either."
>>Ok, let me ask the following question: What is the reason
>>for that rule?
> Well, the idea is that dictionary lookup is done by equality.  If you
> don't define __cmp__ then equality is in fact identity (well, that's
> very nearly true...) so the default implementation of __hash__ (based

...and the full truth is? :-)

> on the pointer address) is as good as it can get (you have hash
> equality iff you have object equality).
> I think.

So, this rule is a hint, only. It could break performance,
not functionality, if I define my own hash function, right?

To make things totally clear, I repeat my question:

The only thing to be aware of when working with keys (besides
of object immutability) seems the following:

Keys are equivalent (in the sense of: a key inserted under
key1 can be retrieved with key2), if this is valid:

    hash(key1) == hash(key2) and key1 == key2

Is this guaranteed? In future implementations?


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