Bug #1153163: describe __add__ vs __radd__ behavior when adding

objects of same type/of subclasses of the other.
This commit is contained in:
Georg Brandl 2006-06-14 08:31:39 +00:00
parent bcae6222e5
commit c54173c234
1 changed files with 19 additions and 4 deletions

View File

@ -1918,13 +1918,28 @@ called to implement the binary arithmetic operations (\code{+},
\function{pow()}\bifuncindex{pow}, \code{**}, \code{<<},
\code{>>}, \code{\&}, \code{\^}, \code{|}) with reflected
(swapped) operands. These functions are only called if the left
operand does not support the corresponding operation. For instance,
to evaluate the expression \var{x}\code{-}\var{y}, where \var{y} is an
instance of a class that has an \method{__rsub__()} method,
\code{\var{y}.__rsub__(\var{x})} is called. Note that ternary
operand does not support the corresponding operation and the
operands are of different types.\footnote{
For operands of the same type, it is assumed that if the
non-reflected method (such as \method{__add__()}) fails the
operation is not supported, which is why the reflected method
is not called.}
For instance, to evaluate the expression \var{x}\code{-}\var{y},
where \var{y} is an instance of a class that has an
\method{__rsub__()} method, \code{\var{y}.__rsub__(\var{x})}
is called if \code{\var{x}.__sub__(\var{y})} returns
\var{NotImplemented}.
Note that ternary
\function{pow()}\bifuncindex{pow} will not try calling
\method{__rpow__()} (the coercion rules would become too
complicated).
\note{If the right operand's type is a subclass of the left operand's
type and that subclass provides the reflected method for the
operation, this method will be called before the right operand's
non-reflected method. This behavior allows subclasses to
override their ancestors' operations.}
\end{methoddesc}
\begin{methoddesc}[numeric object]{__iadd__}{self, other}