[Python-Dev] bool does not want to be subclassed? (original) (raw)
Phillip J. Eby pje at telecommunity.com
Mon Feb 16 15:38:10 EST 2004
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At 03:15 PM 2/16/04 -0500, François Pinard wrote:
So, instead of explaining how we should cleverly use
_hash_' to_ _describe immutability, we might describe whathash' ought to be, and then merely notice that hashable objects could be useful to represent immutable objects (whether user objects or builtin objects).
Actually, it might be even better to start with equality. Hashing is only meaningful for objects that can be tested for equality. That is:
By default, objects are equal only to themselves. But you can change this by overriding cmp or eq.
Hashing is a technique used to group objects according to possible equality. So, if you want to create a hashable object that can be "equal to" other objects, it must return the same hash value as the objects it compares equal to.
Hash values must remain constant over the lifetime of an object. Thus, if an object may compare equal to a different set of objects over its lifetime, it must not be hashable.
Therefore, if an object is to be hashable, any attributes of the object that are used in calculating the hash (or performing comparisons) may not be changed once the its hash method has been called. Usually, this is implemented by making an object completely unchangeable once created (e.g. tuples), but some user classes may implement this by "freezing" an object's contents once it is hashed (e.g. kjbuckets.kjSet).
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