Python __format__() Magic Method

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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'.

Python format() Function: No-BS Guide by Example

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’!


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!

Python String Methods [Ultimate Guide]

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