How To Use the __str__() and __repr__() Methods in Python

Introduction

In this article, you’ll learn about the special methods __str__() and __repr__() that are defined in the Python data model. The __str__() and __repr__() methods can be helpful in debugging Python code by logging or printing useful information about an object.

Python special methods begin and end with a double underscore and are informally known as dunder methods. Dunder methods are the underlying methods for Python’s built-in operators and functions. You should avoid calling dunder methods directly, and instead implement the dunder methods in your class and then use the built-in functions that call them, such as str() and repr().

What’s the difference between __str__() and __repr__()?

The __str__() method returns a human-readable, or informal, string representation of an object. This method is called by the built-in print(), str(), and format() functions. If you don’t define a __str__() method for a class, then the built-in object implementation calls the __repr__() method instead.

The __repr__() method returns a more information-rich, or official, string representation of an object. This method is called by the built-in repr() function. If possible, the string returned should be a valid Python expression that can be used to recreate the object. In all cases, the string should be informative and unambiguous.

In general, the __str__() string is intended for users and the __repr__() string is intended for developers.

__str__() and __repr__() Examples Using a Built-In Class

The examples in this section call the __str__() and __repr__() methods directly for demonstration purposes.

The datetime.datetime class is a built-in Python class which has a default implementation of the __str__() and __repr__() methods.

The following example code shows the strings returned by the default implementation of the __str__() and __repr__() methods for a datetime.datetime object:

import datetime

mydate = datetime.datetime.now()

print("__str__() string: ", mydate.__str__())
print("str() string: ", str(mydate))

print("__repr__() string: ", mydate.__repr__())
print("repr() string: ", repr(mydate))

The output is:

Output
__str__() string:  2023-01-27 09:50:37.429078
str() string:  2023-01-27 09:50:37.429078
__repr__() string:  datetime.datetime(2023, 1, 27, 9, 50, 37, 429078)
repr() string:  datetime.datetime(2023, 1, 27, 9, 50, 37, 429078)

The output shows that the str() function calls __str__() and returns a human-friendly string, while the repr() function calls __repr__() and returns a more information-rich string that can be used to recreate the object. In fact, you can use the repr() function with the eval() function to create a new object from the string:

import datetime

mydate1 = datetime.datetime.now()
mydate2 = eval(repr(mydate1))

print("mydate1 repr() string: ", repr(mydate1))
print("mydate2 repr() string: ", repr(mydate2))

print("the values of the objects are equal: ", mydate1==mydate2)

The output is:

Output
mydate1 repr() string:  datetime.datetime(2023, 1, 26, 9, 43, 24, 479635)
mydate2 repr() string:  datetime.datetime(2023, 1, 26, 9, 43, 24, 479635)
the values of the objects are equal:  True

The preceding example code creates the mydate2 object from the repr() string for mydate1, and then verifies that the values of both objects are equal.

__str__() and __repr__() Examples Using a New Class

When you create a class, you should implement at least the ___repr__() method so that useful information is returned when built-in functions use __repr__().

The following class doesn’t implement the __str__() or __repr()__ methods:

class Ocean:

    def __init__(self, sea_creature_name, sea_creature_age):
        self.name = sea_creature_name
        self.age = sea_creature_age

c = Ocean('Jellyfish', 5)

print(str(c))
print(repr(c))

The output when you use str() and repr() is:

Output
<__main__.Ocean object at 0x102892860>
<__main__.Ocean object at 0x102892860>

The preceding example demonstrates that the default implementation of __repr()__ for the object returns a string with only the class and the object id in hexadecimal format, which is not very useful. Note that str() and repr() return the same value, because str() calls __repr__() when __str__() isn’t implemented.

Update the Ocean class with implementations of the __str__() and __repr__() methods:

class Ocean:

    def __init__(self, sea_creature_name, sea_creature_age):
        self.name = sea_creature_name
        self.age = sea_creature_age
    
    def __str__(self):
        return f'The creature type is {self.name} and the age is {self.age}'

    def __repr__(self):
        return f'Ocean(\'{self.name}\', {self.age})'

c = Ocean('Jellyfish', 5)

print(str(c))
print(repr(c))

The output is:

Output
The creature type is Jellyfish and the age is 5
Ocean('Jellyfish', 5)

The implementation of __str__() in the preceding example returns an easy-to-read string that provides the relevant details of the object for a user. The implementation of __repr__() returns a string that’s a valid Python expression which could be used to recreate the object: Ocean('Jellyfish', 5). The example uses f-string formatting for the strings, but you can format the strings using any format supported by Python.

Conclusion

In this article, you explored the differences between the __str__() and the __repr__() methods and implemented these special methods in a class, so that you didn’t need to call them directly. Learn more about working with strings in Python through our Python string tutorials.