The following methods can be defined to emulate numeric objects. Methods corresponding to operations that are not supported by the particular kind of number implemented (e.g., bitwise operations for non-integral numbers) should be left undefined.
+,
-, *, /, %,
divmod(),
pow(), **, <<, >>,
&, ^, |). For instance, to evaluate the
expression x+y, where x is an instance of a
class that has an __add__() method,
x.__add__(y) is called. Note that
__pow__() should be defined to accept an optional third
argument if the ternary version of the built-in
pow() function is to be supported.
+,
-, *, /, %,
divmod(),
pow(), **, <<, >>,
&, ^, |) with reversed operands. These
functions are only called if the left operand does not support the
corresponding operation. For instance, to evaluate the expression
x-y, where y is an instance of a class that
has an __rsub__() method, y.__rsub__(x) is
called. Note that ternary pow() will not
try calling __rpow__() (the coercion rules would become too
complicated).
-, +,
abs() and ).
None if conversion is impossible. When
the common type would be the type of other, it is sufficient to
return None, since the interpreter will also ask the other
object to attempt a coercion (but sometimes, if the implementation of
the other type cannot be changed, it is useful to do the conversion to
the other type here).
Coercion rules: to evaluate x op y, the
following steps are taken (where __op__() and
__rop__() are the method names corresponding to op,
e.g., if varop is `+', __add__() and
__radd__() are used). If an exception occurs at any point,
the evaluation is abandoned and exception handling takes over.
x.__coerce__(y); skip to step 2 if the
coercion returns None.
x.__op__(y); otherwise, restore x and
y to their value before step 1a.
y.__coerce__(x); skip to step 3 if the
coercion returns None.
y.__rop__(x); otherwise, restore x
and y to their value before step 2a.
+' and x is a sequence,
sequence concatenation is invoked.
*' and one operand is a sequence
and the other an integer, sequence repetition is invoked.