Python __floor__() Magic Method

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Syntax and Definition

object.__floor__(self)

The Python __floor__() method implements the behavior of the math.floor() function. For example, if you attempt to call math.floor(x), Python will run the x.__floor__() method to obtain the return value.

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

The following code snippet overrides the __floor__() dunder method to return the “rounded down” age of a Person when you pass an object of type Person into the math.floor() function:

import math


class Person:
    def __init__(self, age):
        self.age = age

    def __floor__(self):
        floor_value = int(self.age)
        return floor_value



alice = Person(42.99999)
print(math.floor(alice))
# 42

bob = Person(42.0)
print(math.floor(bob))
# 42

How to fix “TypeError: must be real number, not XXX”?

Note that without defining the __floor__() method, Python would’ve raised a TypeError:

import math


class Person:
    def __init__(self, age):
        self.age = age


alice = Person(42.99999)
print(math.floor(alice))

Output:

Traceback (most recent call last):
  File "C:\Users\...\code.py", line 10, in <module>
    print(math.floor(alice))
TypeError: must be real number, not Person

To fix this TypeError, simply define the __floor__() method as outlined in the first code snippet in this article.

Background

Let’s first recall what the floor function ⌊⋅⌋ does in mathematical terms. For a real number x, its floor function ⌊x⌋ is just x “rounded down”, i.e. the largest integer not exceeding x. In particular, if x is an integer, then its floor is just itself.

For instance, if x=9.1, then the floor of x is just 9. On the other hand, if x=−9.1, then the largest integer not exceeding x is −10 (rather than −9), so ⌊x⌋=−10.

If we rephrase this in terms of the integer part n of x, we get

You can read more in our full guide:

References:

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