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Patch #1602128: clarify that richcmp methods can return NotImplemented 

and should return True or False otherwise.
This commit is contained in:
Georg Brandl 2007-03-06 18:29:58 +00:00
parent cff1ae3a2f
commit 1579265aac
1 changed files with 9 additions and 7 deletions
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16
Doc/ref/ref3.tex
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@ -1282,10 +1282,14 @@ follows:
\code{\var{x}.__ne__(\var{y})},
\code{\var{x}>\var{y}} calls \code{\var{x}.__gt__(\var{y})}, and
\code{\var{x}>=\var{y}} calls \code{\var{x}.__ge__(\var{y})}.
These methods can return any value, but if the comparison operator is
used in a Boolean context, the return value should be interpretable as
a Boolean value, else a \exception{TypeError} will be raised.
By convention, \code{False} is used for false and \code{True} for true.
A rich comparison method may return the singleton \code{NotImplemented} if it
does not implement the operation for a given pair of arguments.
By convention, \code{False} and \code{True} are returned for a successful
comparison. However, these methods can return any value, so if the
comparison operator is used in a Boolean context (e.g., in the condition
of an \code{if} statement), Python will call \function{bool()} on the
value to determine if the result is true or false.
There are no implied relationships among the comparison operators.
The truth of \code{\var{x}==\var{y}} does not imply that \code{\var{x}!=\var{y}}
@ -1299,9 +1303,7 @@ the right argument does); rather, \method{__lt__()} and
\method{__ge__()} are each other's reflection, and \method{__eq__()}
and \method{__ne__()} are their own reflection.
Arguments to rich comparison methods are never coerced. A rich
comparison method may return \code{NotImplemented} if it does not
implement the operation for a given pair of arguments.
Arguments to rich comparison methods are never coerced.
\end{methoddesc}
\begin{methoddesc}[object]{__cmp__}{self, other}