Python __ceil__() Magic Method

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Syntax and Description

object.__ceil__(self)

The Python __ceil__() method implements the behavior of the math.ceil() function. For example, if you attempt to call math.ceil(x), Python will run the x.__ceil__() 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 __ceil__() dunder method to return the “rounded up” age of a Person when you pass an object of type Person into the math.ceil() function:

import math


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

    def __ceil__(self):
        floor_value = int(self.age)
        if floor_value < self.age:
            return floor_value + 1
        return floor_value



alice = Person(42.42424242)
print(math.ceil(alice))
# 43

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

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

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

import math


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




alice = Person(42.42424242)
print(math.ceil(alice))

Output:

Traceback (most recent call last):
  File "C:\Users\...\code.py", line 12, in <module>
    print(math.ceil(alice))
TypeError: must be real number, not Person

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

Background

Next we will look at the ceil function ⌈⋅⌉. Just as the floor function is the real number x rounded down, ⌈x⌉ is just x “rounded up”, i.e. the smallest integer greater than x.

For instance, if x=9.1, then the ceil of x is just 10. On the other hand, if x=−9.1, then the smallest integer greater than x is −9, so ⌈x⌉=−9. If x is an integer, then its ceil is just itself.

If we phrase this in terms of the integer and fractional part from before, we get

You can see from the above discussion that if x is not an integer, then ⌈x⌉=⌊x⌋+1

You can read more in our full guide:

References:

Where to Go From Here?

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