Python Language Reference Manual by Guido van Rossum and Fred L. Drake, Jr. Paperback (6"x9"), 120 pages ISBN 0954161785 RRP £12.95 ($19.95) Sales of this book support the Python Software Foundation! Get a printed copy>>> 
3.4.7 Emulating numeric types
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 nonintegral numbers) should be left undefined.
__add__(self, other)
__sub__(self, other)
__mul__(self, other)
__floordiv__(self, other)
__mod__(self, other)
__divmod__(self, other)
__pow__(self, other[, modulo])
__lshift__(self, other)
__rshift__(self, other)
__and__(self, other)
__xor__(self, other)
__or__(self, other)

These methods are
called to implement the binary arithmetic operations (
+
,
,*
,//
,%
,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. The__divmod__()
method should be the equivalent to using__floordiv__()
and__mod__()
; it should not be related to__truediv__()
(described below). Note that__pow__()
should be defined to accept an optional third argument if the ternary version of the builtinpow()
function is to be supported. If one of those methods does not support the operation with the supplied arguments, it should returnNotImplemented
.
__div__(self, other)
__truediv__(self, other)

The division operator (
/
) is implemented by these methods. The__truediv__()
method is used when__future__.division
is in effect, otherwise__div__()
is used. If only one of these two methods is defined, the object will not support division in the alternate context;TypeError
will be raised instead.
__radd__(self, other)
__rsub__(self, other)
__rmul__(self, other)
__rdiv__(self, other)
__rtruediv__(self, other)
__rfloordiv__(self, other)
__rmod__(self, other)
__rdivmod__(self, other)
__rpow__(self, other)
__rlshift__(self, other)
__rrshift__(self, other)
__rand__(self, other)
__rxor__(self, other)
__ror__(self, other)

These methods are
called to implement the binary arithmetic operations (
+
,
,*
,/
,%
,divmod()
,pow()
,**
,<<
,>>
,&
,^
,
) with reflected (swapped) operands. These functions are only called if the left operand does not support the corresponding operation and the operands are of different types.^{(8)} 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 ifx.__sub__(y)
returns NotImplemented. Note that ternarypow()
will not try calling__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 left operand's nonreflected method. This behavior allows subclasses to override their ancestors' operations.
__iadd__(self, other)
__isub__(self, other)
__imul__(self, other)
__idiv__(self, other)
__itruediv__(self, other)
__ifloordiv__(self, other)
__imod__(self, other)
__ipow__(self, other[, modulo])
__ilshift__(self, other)
__irshift__(self, other)
__iand__(self, other)
__ixor__(self, other)
__ior__(self, other)

These methods are called to implement the augmented arithmetic
operations (
+=
,=
,*=
,/=
,%=
,**=
,<<=
,>>=
,&=
,^=
,=
). These methods should attempt to do the operation inplace (modifying self) and return the result (which could be, but does not have to be, self). If a specific method is not defined, the augmented operation falls back to the normal methods. For instance, to evaluate the expression x+=
y, where x is an instance of a class that has an__iadd__()
method,x.__iadd__(y)
is called. If x is an instance of a class that does not define a__iadd__()
method,x.__add__(y)
andy.__radd__(x)
are considered, as with the evaluation of x+
y.
__neg__(self)
__pos__(self)
__abs__(self)
__invert__(self)

Called to implement the unary arithmetic operations (

,+
,abs()
and~
).
__complex__(self)
__int__(self)
__long__(self)
__float__(self)

Called to implement the builtin functions
complex()
,int()
,long()
, andfloat()
. Should return a value of the appropriate type.
__oct__(self)
__hex__(self)

Called to implement the builtin functions
oct()
andhex()
. Should return a string value.
__index__(self)

Called to implement
operator.index()
. Also called whenever Python needs an integer object (such as in slicing). Must return an integer (int or long). (Added in Python version 2.5)
__coerce__(self, other)

Called to implement "mixedmode" numeric arithmetic. Should either
return a 2tuple containing self and other converted to
a common numeric type, or
None
if conversion is impossible. When the common type would be the type ofother
, it is sufficient to returnNone
, 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). A return value ofNotImplemented
is equivalent to returningNone
.
ISBN 0954161785  Python Language Reference Manual  See the print edition 