Python __rtruediv__() Magic Method


object.__rtruediv__(self, other)

The Python __rtruediv__() method implements the reverse true division operation with reflected, swapped operands. So, when you call x / y, Python attempts to call x.__truediv__(y). If the method is not implemented, Python attempts to call __rtruediv__ 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.

Background True Division

The Python __truediv__() method is called to implement the normal division operation / called true division—as opposed to the floor division operation //.

For example to evaluate the expression x / y, Python attempts to call x.__truediv__(y).

In the following example, you create a custom class Data and overwrite the __truediv__() method so that it returns a dummy string when trying to divide two Data objects using the true division operation a / b.

class Data:
    def __truediv__(self, other):
        return '... my result of truediv...'

a = Data()
b = Data()
c = a / b

# ... my result of truediv...

To understand this operation in detail, feel free to read over our tutorial or watch the following video:

What’s the Difference Between __truediv__() and __rtruediv__()?

Say, you want to divide two objects x and y using true division:

print(x / y)

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

  1. The method x.__truediv__() is not implemented in the first place, or
  2. The method x.__truediv__() 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.__rtruediv__() for reverse true division on the right operator y. If this 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.__truediv__(x) instead of x.__truediv__(y), it could cause an error if the operation is non-commutative. That’s why y.__rtruediv__(x) is needed which indicates that true division is possible after all.

So, the difference between x.__truediv__(y) and x.__rtruediv__(y) is that the former calculates x / y whereas the latter calculates y / x — both calling the respective true division method defined on 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:

class Data_2:
    def __rtruediv__(self, other):
        return 'called reverse true division'

x = Data_1()
y = Data_2()

print(x / y)
# called reverse true division


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