How to change the string representation of class instances in python

· 1 min read April 25, 2022


We often find our selves working with classes in python . And while debugging , if we print the instances , we will get a crazy output like

<main.{{class_name}} at 0x7f7a7bc76760>

In this small post we will learn to change the output produced by printing or viewing instances to something more sensible.

Lets say have a class Cat which currently does nothing .

class Cat :

If we create an instance of it and print that . We will see an output like this .

persian = Cat()


# >> <__main__.Cat at 0x7f7a7bc76760>

The output prints the name of the class and the memory address of the instance

What is the good string representation of a class instance

An instance by default returns the memory address as its string representation . We don’t like it as it doesn’t help us with debugging in any sort , so what should it return instead ?

The best practice is that the string representation should be as same as the object initialization syntax .suppose we have a class Fraction which takes two arguments numerator and denominator .Then we can create an fraction like :

half = Fraction(1,2)

So the string representation of the class instance should be like Fraction(1,2) .

So , How can we achieve this .

To change the string representation of an instance, define the repr() .

For example:

class Fraction:
    def __init__(self,numerator,demominator) :
        self.numerator = numerator
        self.demominator = demominator

    def __repr__(self) :
        return f"Fraction({self.numerator},{self.demominator})"

   def __str__(self):
        return f"{self.numerator}/{self.denominator}"

half = Fraction(1,2)

print(repr(half))  #repr() returns the string representation

# >> Fraction(1,2)

# str() calls the __str__ method of the object to convert the object to str

# >> '1/2'


# >> '1/2'

The _ _ repr _ _ () method returns the code representation of an instance, The built-in repr() function returns this text, as does the interactive interpreter when inspecting values. The _ _ str _ _ () method converts the instance to a string, and is the output produced by the str() and print() functions

Thanks for reading . Happy coding ..

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