Class Variables vs Instance Variables in Python

Description
Class variables and instance variables are two important types of variables in object-oriented programming. Class variables belong to the class itself and are shared by all instances; instance variables belong to individual instances, with each instance having its own copy. Understanding their differences, access methods, and precedence relationships is crucial for writing correct object-oriented code.

Problem-Solving Process

1. Basic Concept Differentiation

   • Class Variable: Defined inside the class but outside any method, directly belonging to the class. All instances share the same class variable.
   • Instance Variable: Typically defined inside methods (e.g., __init__), using the self. prefix. Each instance has an independent copy.

2. Example Definition

class Dog:
    species = "Canis familiaris"  # Class variable (all dogs belong to the same species)
    
    def __init__(self, name, age):
        self.name = name          # Instance variable (each dog has its own name)
        self.age = age            # Instance variable (each dog has its own age)

3. Access Priority Rules

   • When accessing attributes through an instance, Python searches in the following order:
     1. The instance's namespace
     2. The class's namespace
     3. The parent class's namespace (following MRO order)
   • This means that if an instance variable and a class variable have the same name, the instance variable will "shadow" the class variable.

4. Specific Operation Demonstration

# Create two instances
buddy = Dog("Buddy", 5)
miles = Dog("Miles", 3)

# Access instance variables (independent)
print(buddy.name)    # "Buddy"
print(miles.name)    # "Miles"

# Access class variables (shared)
print(buddy.species)  # "Canis familiaris"
print(miles.species)  # "Canis familiaris"

# Modify instance variables (does not affect other instances)
buddy.age = 6
print(buddy.age)     # 6
print(miles.age)     # 3 (remains unchanged)

# Modify class variables (affects all instances)
Dog.species = "Canis lupus"
print(buddy.species)  # "Canis lupus"
print(miles.species)  # "Canis lupus"

5. Common Pitfall: Modifying Class Variables via Instances

# Misconception: Thinking this modifies the class variable
buddy.species = "New species"

# Actually, Python creates a new instance variable
print(buddy.species)    # "New species" (instance variable)
print(miles.species)    # "Canis lupus" (class variable, unchanged)
print(Dog.species)      # "Canis lupus" (class variable, unchanged)

# Verification: Check each __dict__
print(buddy.__dict__)   # {'name': 'Buddy', 'age': 6, 'species': 'New species'}
print(miles.__dict__)   # {'name': 'Miles', 'age': 3}

6. Correct Method for Modifying Class Variables
   • Class variables should be modified through the class itself, not through instances.
   • This ensures all instances see the change.

# Correct way
Dog.species = "Updated species"
print(buddy.species)    # "New species" (still shadowed by instance variable)
print(miles.species)    # "Updated species" (class variable updated)

# After deleting the instance variable, the class variable is visible again
del buddy.species
print(buddy.species)    # "Updated species" (now showing class variable)

7. Best Practice Recommendations
   • Use meaningful naming conventions: Class variables often use all caps or more descriptive names.
   • Avoid using the same name for instance and class variables.
   • Always modify class variables via the class name to ensure consistent behavior.
   • Use instance variables for instance-specific data and class variables for class-level shared data.