str and repr

Learn representation with __str__ and __repr__.

The str method

__str__ is the method that turns an instance into a human readable string. print() and str() call it. It must return a str.

Example: class Book:\n def __str__(self):\n return f"Book: {self.title}". Now print(b) shows Book: ...; otherwise you would see an ugly <Book object at 0x...>. __str__ produces the short text shown to the user.

The repr method

__repr__ turns an instance into a developer representation. repr() and the interactive shell call it. Ideally it returns a string that can recreate the instance with eval.

Example: class Book:\n def __repr__(self):\n return f"Book(title={self.title!r})". Now repr(b) returns Book(title='...'). If __str__ is not defined, __repr__ is used as a fallback. While debugging, __repr__ is more informative.

Exercise Add str to the Book class

Define a class named Book. Add the method def __init__(self, title): and in the body write self.title = title. Also add the method def __str__(self): and write return f"Book: {self.title}". Then create an instance with b = Book("Les Mis") and print it with print(b).

Show example solution

# Define the Book class with __init__ and __str__
class Book:
    def __init__(self, title):
        self.title = title
    def __str__(self):
        return f"Book: {self.title}"

b = Book("Les Mis")
print(b)

Test yourself

Complete the __str__ signature

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

Show answer

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

Match the two methods

__str__

Human readable
__repr__

Developer repr
print(b)

Calls __str__
Which method does print(b) call?
- __str__
- __repr__
- __init__
- __len__
Show answer

__str__ — print() calls the __str__ method first.

Next lesson: Summary - Classes

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