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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
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23
Doc/ref/ref3.tex
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@ -1918,13 +1918,28 @@ called to implement the binary arithmetic operations (\code{+},
\function{pow()}\bifuncindex{pow}, \code{**}, \code{<<}, \function{pow()}\bifuncindex{pow}, \code{**}, \code{<<},
\code{>>}, \code{\&}, \code{\^}, \code{|}) with reflected \code{>>}, \code{\&}, \code{\^}, \code{|}) with reflected
(swapped) operands. These functions are only called if the left (swapped) operands. These functions are only called if the left
operand does not support the corresponding operation. For instance, operand does not support the corresponding operation and the
to evaluate the expression \var{x}\code{-}\var{y}, where \var{y} is an operands are of different types.\footnote{
instance of a class that has an \method{__rsub__()} method, For operands of the same type, it is assumed that if the
\code{\var{y}.__rsub__(\var{x})} is called. Note that ternary 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 \function{pow()}\bifuncindex{pow} will not try calling
\method{__rpow__()} (the coercion rules would become too \method{__rpow__()} (the coercion rules would become too
complicated). 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} \end{methoddesc}
\begin{methoddesc}[numeric object]{__iadd__}{self, other} \begin{methoddesc}[numeric object]{__iadd__}{self, other}