diff options
| -rw-r--r-- | Doc/ref/ref3.tex | 25 |
1 files changed, 15 insertions, 10 deletions
diff --git a/Doc/ref/ref3.tex b/Doc/ref/ref3.tex index 984f76d..a82ce8c 100644 --- a/Doc/ref/ref3.tex +++ b/Doc/ref/ref3.tex @@ -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} |