Let Python understand your objects.

How does Python know what to display when we print an object or ask for its length?

01 Learn02 Try03 Predict

01 Learn

Special methods let objects work naturally with print() and len().

How does Python know what to display when we print an object or ask for its length?

class Book:
    def __init__(self, title, author): self.title, self.author = title, author
    def __str__(self): return f"{self.title} by {self.author}"
book = Book("Dune", "Frank Herbert")
print(book)

class BookCollection:
    def __init__(self): self.books = []
    def add_book(self, book): self.books.append(book)
    def __len__(self): return len(self.books)
collection = BookCollection()
for title in ("Dune", "The Hobbit", "Python Basics"): collection.add_book(title)
print(len(collection))

Without __str__(), print(book) usually shows an unreadable default object description. __str__() returns readable text for print(book) and str(book); it must use return, not print(). BookCollection stores books in a list, and len(collection) uses its __len__() method to return a non-negative integer. This prepares us for a Library of real Book objects in the next Composition lesson. __str__() returns a string, while __len__() returns a non-negative integer.

Dune by Frank Herbert
3

Try changing one value, then explain which part of the collection changed and why.

↳

Keep this in mind: Read the collection operation before predicting the result.

Try it

class Book:
    def __init__(self, title, author): self.title, self.author = title, author
    def __str__(self): return f"{self.title} by {self.author}"
book = Book("Dune", "Frank Herbert")
print(book)

class BookCollection:
    def __init__(self): self.books = []
    def add_book(self, book): self.books.append(book)
    def __len__(self): return len(self.books)
collection = BookCollection()
for title in ("Dune", "The Hobbit", "Python Basics"): collection.add_book(title)
print(len(collection))

Run the example, then change one value and observe the result.

02 Try

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03 Predict

What will this program print?

Read each operation before choosing an answer.

class Book:
    def __init__(self,title): self.title=title
    def __str__(self): return f"Book: {self.title}"
book=Book("Dune")
print(book)
print(str(book))

Practice independently

Mini Challenge — Describe an EBook

Override __str__() with super() and return a readable result.

Your goalOverride __str__() in EBook to return the title, author, and file size. Use super().__str__() for the basic book information, and return the final text instead of printing it.

ebook_str.py

Optional hints