[docs] [issue13707] Clarify hash() constancy period

[Martin v. Löwis]

Martin v. Löwis <martin at v.loewis.de> added the comment:

> Martin, I do not understand. The default hash is based on id (as is
> default equality comparison), not value.

In the default implementation, the id *is* the object's value (i.e.
objects, by default, only compare equal if they are identical). So
the default implementation is just a special case of the more general
rule that hashes need to be consistent with equality.

> Are you OK with hash values changing if the 'value' changes?

An object that can change its value (i.e. a mutable object) should
fail to hash.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13707>
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