Python __floordiv__() Magic Method

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Syntax

object.__floordiv__(self, other)

The Python __floordiv__() method implements 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). If the method is not implemented, Python first 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.

Example

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 true 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...

If you hadn’t defined the __truediv__() method, Python would’ve raised a TypeError.

How to Resolve TypeError: unsupported operand type(s) for //

Consider the following code snippet where you try to divide two custom objects without defining the dunder method __truediv__():

class Data:
    pass


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

print(c)

Running this leads to the following error message on my computer:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 7, in <module>
    c = a // b
TypeError: unsupported operand type(s) for //: 'Data' and 'Data'

The reason for this error is that the __floordiv__() dunder method has never been defined—and it is not defined for a custom object by default. So, to resolve the TypeError: unsupported operand type(s) for //, you need to provide the __floordiv__(self, other) method in your class definition as shown previously:

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

Python __floordiv__ vs __div__ vs __truediv__

  • The Python __floordiv__() dunder method is called to implement the integer division operation in Python 3.
  • The Python __div__() dunder method is called to implement the normal “true division” operation in Python 2. It doesn’t work in Python 3 anymore.
  • The Python __truediv__() dunder method is called to implement the normal “true division” operation in Python 3.

Python __floordiv__ vs __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 division 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 floor division on the left operand x—but as it’s not implemented, Python simply calls the reverse floor division operation on the right operand y.

class Data_1:
    pass

class Data_2:
    def __rfloordiv__(self, other):
        return 'called rfloordiv'


x = Data_1()
y = Data_2()

print(x//y)
# called rfloordiv

References:

Explainer Video Division Operators

You can also check out my explainer video where I’ll give you a deep dive on the integer and true division operators and how to use them for various data types. Click to watch:

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