Python __format__() Magic Method

Rate this post

Syntax

object.__format__(self, spec)

The Python __format__() method implements the built-in format() function as well as the string.format() method. So, when you call format(x, spec) or string.format(spec), Python attempts to call x.__format__(spec). The return value is a string.

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 format()

Python’s built-in format(value, spec) function transforms input of one format into output of another format defined by you.

Specifically, it applies the format specifier spec to the argument value and returns a formatted representation of value.

For example, format(42, 'f') returns the string representation '42.000000'.

Example Custom __format__()

In the following example, you create a custom class Data and overwrite the __format__() magic method so that it returns a dummy string 'hello world'.

class Data:
    def __format__(self, spec):
        return 'hello ' + spec


x = Data()
print(format(x, 'world'))

This way, you can create your own little formatting language for a custom object.

class Data:
    def __format__(self, spec):
        if spec == '42':
            return 'finxter'
        return 'hello ' + spec
    

x = Data()

print(format(x, 'world'))
# hello world


print(format(x, '42'))
# finxter

In the first print() statement, you use the format specifier 'world' which has no special meaning in your custom implementation of the __format__() method. Thus, the output is 'hello world'—the result of the string concatenation 'hello' and the format specifier 'world'.

In the second print() statement, you use the special (according to your own definition) format specifier '42'. The __format__() method now returns a special string: ‘finxter’!

References:

Related Video: Python String Methods

The following video may help your understanding of some crucial string methods — such as the str.format() method that relies on the __format__() magic method!

Where to Go From Here?

Enough theory. Let’s get some practice!

Coders get paid six figures and more because they can solve problems more effectively using machine intelligence and automation.

To become more successful in coding, solve more real problems for real people. That’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?

You build high-value coding skills by working on practical coding projects!

Do you want to stop learning with toy projects and focus on practical code projects that earn you money and solve real problems for people?

🚀 If your answer is YES!, consider becoming a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.

If you just want to learn about the freelancing opportunity, feel free to watch my free webinar “How to Build Your High-Income Skill Python” and learn how I grew my coding business online and how you can, too—from the comfort of your own home.

Join the free webinar now!