String Representation of Python Objects: Differences and Customization of __str__ and __repr__

When you define a custom class in Python and print its instance using the print() function, by default, internal information (memory address) like <__main__.ClassName object at 0x...> is displayed, which is hard for humans to understand. To display the contents of an object (like attribute values) in an easy-to-understand way during debugging or logging, you need to define the special methods __str__ and __repr__.

This article explains the differences in roles between these two methods and how to implement them.

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Default Behavior (When Methods are Undefined)

First, let’s check the behavior when these methods are not defined. As an example, we create a Book class to manage book data.

class Book:
    def __init__(self, title, author, price):
        self.title = title
        self.author = author
        self.price = price

# Instantiation
my_book = Book("Intro to Python", "Taro Yamada", 2800)

# Output with print()
print(f"print output: {my_book}")

Output:

print output: <__main__.Book object at 0x000001A2B3C4D5E6>

As you can see, only the object type and memory address are displayed, making it impossible to tell which book it is.

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1. __str__ Method: Representation for Users

The __str__ method is used to define a “readable string representation for users” of the object. It is mainly called in the following situations:

• Passed to the print() function
• Converted to a string using the str() function
• Embedded in a string using f-strings or the format() method

class Book:
    def __init__(self, title, author, price):
        self.title = title
        self.author = author
        self.price = price

    def __str__(self):
        # Return a format easy for users to read (e.g., Title and Author)
        return f"『{self.title}』 Author:{self.author} ({self.price} yen)"

my_book = Book("Intro to Python", "Taro Yamada", 2800)

print(f"__str__ output: {my_book}")

Output:

Plaintext

__str__ output: 『Intro to Python』 Author:Taro Yamada (2800 yen)

The result of print() has changed to a format that is easy for humans to understand.

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2. __repr__ Method: Representation for Developers

The __repr__ method is used to define a “strict and unambiguous string representation for developers” of the object. Ideally, it is recommended to return a format (string representation as code) that can restore the original object if executed with the eval() function.

It is mainly called in the following situations:

• Passed to the repr() function
• Typing the variable name and pressing Enter in the interactive shell
• When a list or dictionary containing objects is displayed
• As a backup when __str__ is not defined

class Book:
    def __init__(self, title, author, price):
        self.title = title
        self.author = author
        self.price = price

    def __repr__(self):
        # For developers: Return a format that can recreate the object
        return f"Book(title='{self.title}', author='{self.author}', price={self.price})"

my_book = Book("Intro to Python", "Taro Yamada", 2800)

# Call using the repr() function
print(f"__repr__ output: {repr(my_book)}")

Output:

__repr__ output: Book(title='Intro to Python', author='Taro Yamada', price=2800)

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Using __str__ and __repr__ Together

Let’s check the behavior when both methods are defined and how they are displayed inside a list.

class Book:
    def __init__(self, title, author, price):
        self.title = title
        self.author = author
        self.price = price

    def __str__(self):
        return f"Book: {self.title}"

    def __repr__(self):
        return f"Book('{self.title}', '{self.author}', {self.price})"

book1 = Book("Future of AI", "Jiro Sato", 3500)
book2 = Book("Basics of Statistics", "Hanako Suzuki", 2200)

print("1. print(book1) (__str__ takes precedence):")
print(book1)

print("\n2. print(repr(book1)) (Specify __repr__):")
print(repr(book1))

print("\n3. Inside a list (Elements in a list are displayed using __repr__):")
book_list = [book1, book2]
print(book_list)

Output:

1. print(book1) (__str__ takes precedence):
Book: Future of AI

2. print(repr(book1)) (Specify __repr__):
Book('Future of AI', 'Jiro Sato', 3500)

3. Inside a list (Elements in a list are displayed using __repr__):
[Book('Future of AI', 'Jiro Sato', 3500), Book('Basics of Statistics', 'Hanako Suzuki', 2200)]

Important Point: When you store objects in a collection like a list or dictionary and print it, the element objects are displayed using __repr__, not __str__. This is to accurately grasp the contents during debugging.

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Summary

• __str__: For end-users. A readable string displayed by print().
• __repr__: For developers (debugging). A strict string displayed by repr(). Also used when displaying list contents.

As an implementation guideline, defining at least __repr__ is recommended first, as it functions as a substitute when __str__ is missing.