__repr__ and __str__ methods in python classes

Kenny Chou · August 26, 2024

Python   Tutorials

Understanding these closely related methods can help us write better classes. The devs who use your code will thank you!

General concepts:

__repr__: Should return a string that is a valid Python expression that could be used to recreate the object (ideally). It’s mainly used for debugging and development. The goal is to provide an “official” string representation of the object.

__str__: Should return a human-readable string representation of the object. This is what gets printed by default when you print an object.

From the user perspective

When calling a variable interactively, its __repr__ method is called.

>>> a = 'hello'
>>> a
'hello'

When calling the variable inside of a print() statement, its __str__ method is called:

>>> print(a)
hello

From the developer perspective

Let’s write a class with custom repr and str methods:

class MyClass:
    def __init__(self, value):
        self.value = value

    def __repr__(self):
        return f"MyClass(value={self.value!r})"

    def __str__(self):
        return f"MyClass with value: {self.value}"

From the user perspective, it will look like:

>>> obj = MyClass(42)
>>> repr(obj) # the __repr__ method returns a string
'MyClass(value=42)'
>>> print(obj) # the print() function calls the __str__ method of "obj".
MyClass with value: 42
>>> obj # when called interactively, we're implicitly calling print(repr(obj))
MyClass(value=42)

You can call the __str__ representation of an object by calling str(obj), which implicitly calls the object’s __str__ method:

>>> str(obj) # this calls the __repr__ of the string object, so you get the single quotes. See the a = 'hello' example above.
'MyClass with value: 42'
>>> print(str(obj)) # and this calls the __str__ of the string object
MyClass with value: 42

f-string specifiers

We can specify f-strings to show the __repr__ or __str__ of an object.

>>> print(f'{a!r}') # the __repr__ method is called
'hello'
>>> print(f'{a!s}') # the __str__ method is called
hello

Example code for you to play around with:

class MyClass:
    def __init__(self, value):
        self.value = value

    def __repr__(self):
        return f"MyClass(value={self.value!r})"

    def __str__(self):
        return f"MyClass with value: {self.value}"

def main():
    # Example usage
    obj = MyClass(42)
    print(repr(obj))  # Output: MyClass(value=42)
    print(str(obj))   # Output: MyClass with value: 42

if __name__=='__main__':
    main()