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Python __str__ and __repr__

__repr__ and __str__ are dunder methods that turn an object into text.

A repr for developers

A repr should show exactly what the object holds. Paste it back into Python, and you rebuild the object.

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

    def __repr__(self):
        return f"Book({self.title!r}, {self.pages})"

book = Book("Dune", 412)
print(repr(book))

A str for readers

print() and str() use __str__ when you define it. The text can read like a normal sentence.

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

    def __repr__(self):
        return f"Book({self.title!r}, {self.pages})"

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

book = Book("Dune", 412)
print(book)
print(str(book))

Force the repr in an f-string

Add !r just inside the braces. The f-string then asks for the repr, not the str.

print(f"Logged: {book!r}")
print(f"Shown: {book}")

Lists show the repr

Containers never ask items for their str. A list shows the repr of each item inside it.

print([book])
print((book,))

Example

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

    def __repr__(self):
        return f"Book({self.title!r}, {self.author!r}, {self.pages})"

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

book = Book("Dune", "Frank Herbert", 412)
print(book)
print(f"For the log: {book!r}")
print([book, Book("Emma", "Jane Austen", 474)])

Key points

  • Dunder: a special name with double underscores on both sides, like __repr__
  • Use __repr__ to return an exact string that shows what the object holds
  • Use __str__ to return a friendly string for people to read
  • Use !r in an f-string to choose the repr for one value
  • Containers display their items with repr, never with str

In this example, Book stores a title, an author, and a page count. __repr__ builds an exact string with !r around the text fields. __str__ builds a friendly line with the title and author. print(book) uses __str__, so you see the friendly line. The f-string uses !r, so you see the exact repr. The list shows the repr of both books inside it.

Note

Write __repr__ first if you write only one, because print() and str() both fall back to it.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Python Playground

Exercise

What does print([book]) show for each book in the list?