## 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:**

## Where to Go From Here?

Enough theory. Let’s get some practice!

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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.

To help students reach higher levels of Python success, he founded the programming education website Finxter.com that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.

His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.