Python Best Practices: Using default_factory for Mutable Defaults

When defining classes in Python, using mutable default values for instance variables can lead to unexpected behavior.

For example, if you use a list as a default value in a class’s __init__ method, all instances of the class will share the same list object:

class Book:
    def __init__(self, title, authors=[]):
        self.title = title
        self.authors = authors


book1 = Book("Book 1")
book1.authors.append("Author 1")

book2 = Book("Book 2")
print(book2.authors)

['Author 1']

In this example, book1 and book2 share the same list object, which is why modifying the list in book1 affects book2.

To avoid this issue, you can use the default_factory parameter in dataclasses, which creates a new object for each instance:

from dataclasses import dataclass, field


@dataclass
class Book:
    title: str
    authors: list = field(default_factory=list)


book1 = Book("Book 1")
book1.authors.append("Author 1")

book2 = Book("Book 2")
print(book2.authors)  # Output: []

[]

Now, each instance has its own separate list object, and modifying one instance’s list does not affect others.