Python __rsub__() Magic Method

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Syntax

object.__rsub__(self, other)

The Python __rsub__() method implements the reverse subtraction operation that is subtraction with reflected, swapped operands. So, when you call x - y, Python attempts to call x.__sub__(y). Only if the method is not implemented on the left operand, Python attempts to call __rsub__ on the right operand and if this isn’t implemented either, it raises a TypeError.

We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Python __sub__ vs __rsub__

Say, you want to calculate the - operation on two custom objects x and y:

print(x - y)

Python first tries to call the left object’s __sub__() method x.__sub__(y). But this may fail for two reasons:

  1. The method x.__sub__() is not implemented in the first place, or
  2. The method x.__sub__() is implemented but returns a NotImplemented value indicating that the data types are incompatible.

If this fails, Python tries to fix it by calling the y.__rsub__() for reverse subtraction on the right operand y.

If the reverse subtraction method is implemented, Python knows that it doesn’t run into a potential problem of a non-commutative operation. If it would just execute y.__sub__(x) instead of x.__sub__(y), the result would be wrong because the operation may be non-commutative when defined as a custom operation. That’s why y.__rsub__(x) is needed.

So, the difference between x.__sub__(y) and x.__rsub__(y) is that the former calculates x - y whereas the latter calculates y - x — both calling the respective method defined on the object x.

You can see this in effect here where we attempt to call the operation on the left operand x—but as it’s not implemented, Python simply calls the reverse operation on the right operand y.

class Data_1:
    pass

class Data_2:
    def __rsub__(self, other):
        return 'called reverse -'


x = Data_1()
y = Data_2()

print(x - y)
# called reverse -

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