Common Python Class Attribute Mistakes and How to Fix Them

Sure! Let’s consider another example to illustrate the misuse of class attributes and the correct approach. This time, we’ll create a Library class where each library keeps track of its own collection of books.

Mistaken Use of Class Attributes

Incorrect Implementation

In this incorrect implementation, we mistakenly use a class attribute to store the books:

class Library:
    # Class attribute (incorrect use for storing books)
    books = []

    def __init__(self, name):
        self.name = name

    def add_book(self, book):
        self.books.append(book)

    def get_books(self):
        return self.books

# Create instances of the Library class
library1 = Library("City Library")
library2 = Library("Community Library")

# Add books to each library
library1.add_book("1984")
library2.add_book("To Kill a Mockingbird")

# Print books in each library
print(f"{library1.name} books: {library1.get_books()}")
print(f"{library2.name} books: {library2.get_books()}")

Output

City Library books: ['1984', 'To Kill a Mockingbird']
Community Library books: ['1984', 'To Kill a Mockingbird']

Explanation

In this incorrect implementation, books is a class attribute. When add_book is called, it modifies the class attribute, which is shared among all instances of the Library class. As a result, both library1 and library2 end up sharing the same list of books.

Correct Implementation

To fix this, we should use an instance attribute for storing books:

class Library:
    def __init__(self, name):
        self.name = name
        self.books = []  # Instance attribute

    def add_book(self, book):
        self.books.append(book)

    def get_books(self):
        return self.books

# Create instances of the Library class
library1 = Library("City Library")
library2 = Library("Community Library")

# Add books to each library
library1.add_book("1984")
library2.add_book("To Kill a Mockingbird")

# Print books in each library
print(f"{library1.name} books: {library1.get_books()}")
print(f"{library2.name} books: {library2.get_books()}")

Output

City Library books: ['1984']
Community Library books: ['To Kill a Mockingbird']

Explanation

In the correct implementation, books is an instance attribute, initialized within the __init__ method. This ensures that each instance of Library has its own separate collection of books. When add_book is called, it modifies the instance’s own list, not a shared list.

Summary

The example shows how using class attributes for instance-specific data can lead to unintended sharing of state between instances. By initializing books inside the __init__ method, each Library instance maintains its own collection of books, avoiding the issue of shared state.