Python __rfloordiv__() Magic Method

Syntax

object.__rfloordiv__(self, other)

The Python __rfloordiv__() method implements the reverse floor (integer) division operation with reflected, swapped operands. So, when you call x // y, Python attempts to call x.__floordiv__(y). If the method is not implemented, Python attempts to call __rfloordiv__ 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 Floor Division

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

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

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

class Data:
        
    def __floordiv__(self, other):
        return '... my result of floordiv...'


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

print(c)
# ... my result of floordiv...

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

Python Division Deep Dive

What’s the Difference Between __floordiv__() and __rfloordiv__()?

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

print(x // y)

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

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

So, the difference between x.__floordiv__(y) and x.__rfloordiv__(y) is that the former calculates x // y whereas the latter calculates y // x — both calling the respective floor 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:
    pass

class Data_2:
    def __rfloordiv__(self, other):
        return 'called reverse floor division'


x = Data_1()
y = Data_2()

print(x // y)
# called reverse floor division

References:

• https://docs.python.org/3/reference/datamodel.html

Where to Go From Here?

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