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Bring the notes on the relationship between __cmp__(), __eq__(), and

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__hash__() up to date (re: use of objects which define these methods
as dictionary keys).

This closes SF bug #427698.
This commit is contained in:
Fred Drake 2001-05-29 16:02:35 +00:00
parent b9879e1020
commit 597bc1d46f
1 changed files with 15 additions and 10 deletions
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25
Doc/ref/ref3.tex
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@ -1010,10 +1010,14 @@ implement the operation for a given pair of arguments.
\begin{methoddesc}[object]{__cmp__}{self, other}
Called by comparison operations if rich comparison (see above) is not
defined. Should return a negative integer if
\code{self < other}, zero if \code{self == other}, a positive integer if
\code{self > other}. If no \method{__cmp__()} operation is defined, class
instances are compared by object identity (``address'').
defined. Should return a negative integer if \code{self < other},
zero if \code{self == other}, a positive integer if \code{self >
other}. If no \method{__cmp__()}, \method{__eq__()} or
\method{__ne__()} operation is defined, class instances are compared
by object identity (``address''). See also the description of
\method{__hash__()} for some important notes on creating objects which
support custom comparison operations and are usable as dictionary
keys.
(Note: the restriction that exceptions are not propagated by
\method{__cmp__()} has been removed in Python 1.5.)
\bifuncindex{cmp}
@ -1035,12 +1039,13 @@ mix together (e.g., using exclusive or) the hash values for the
components of the object that also play a part in comparison of
objects. If a class does not define a \method{__cmp__()} method it should
not define a \method{__hash__()} operation either; if it defines
\method{__cmp__()} but not \method{__hash__()} its instances will not be
usable as dictionary keys. If a class defines mutable objects and
implements a \method{__cmp__()} method it should not implement
\method{__hash__()}, since the dictionary implementation requires that
a key's hash value is immutable (if the object's hash value changes, it
will be in the wrong hash bucket).
\method{__cmp__()} or \method{__eq__()} but not \method{__hash__()},
its instances will not be usable as dictionary keys. If a class
defines mutable objects and implements a \method{__cmp__()} or
\method{__eq__()} method, it should not implement \method{__hash__()},
since the dictionary implementation requires that a key's hash value
is immutable (if the object's hash value changes, it will be in the
wrong hash bucket).
\withsubitem{(object method)}{\ttindex{__cmp__()}}
\end{methoddesc}