mirror of https://github.com/python/cpython
Bug #1153163: describe __add__ vs __radd__ behavior when adding
objects of same type/of subclasses of the other.
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@ -1918,13 +1918,28 @@ called to implement the binary arithmetic operations (\code{+},
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\function{pow()}\bifuncindex{pow}, \code{**}, \code{<<},
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\code{>>}, \code{\&}, \code{\^}, \code{|}) with reflected
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(swapped) operands. These functions are only called if the left
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operand does not support the corresponding operation. For instance,
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to evaluate the expression \var{x}\code{-}\var{y}, where \var{y} is an
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instance of a class that has an \method{__rsub__()} method,
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\code{\var{y}.__rsub__(\var{x})} is called. Note that ternary
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operand does not support the corresponding operation and the
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operands are of different types.\footnote{
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For operands of the same type, it is assumed that if the
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non-reflected method (such as \method{__add__()}) fails the
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operation is not supported, which is why the reflected method
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is not called.}
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For instance, to evaluate the expression \var{x}\code{-}\var{y},
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where \var{y} is an instance of a class that has an
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\method{__rsub__()} method, \code{\var{y}.__rsub__(\var{x})}
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is called if \code{\var{x}.__sub__(\var{y})} returns
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\var{NotImplemented}.
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Note that ternary
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\function{pow()}\bifuncindex{pow} will not try calling
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\method{__rpow__()} (the coercion rules would become too
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complicated).
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\note{If the right operand's type is a subclass of the left operand's
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type and that subclass provides the reflected method for the
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operation, this method will be called before the right operand's
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non-reflected method. This behavior allows subclasses to
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override their ancestors' operations.}
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\end{methoddesc}
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\begin{methoddesc}[numeric object]{__iadd__}{self, other}
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