What are the differences between a class and a mixin in object-oriented programming?

A class is the fundamental building block of object-oriented programming that describes the structure and behavior of objects (attributes and methods). It can be instantiated, meaning objects are created from the class.

A mixin is a special class or set of methods intended to add additional functionality to other classes through multiple inheritance, but not for standalone object creation. Mixins usually do not have state and serve to extend the capabilities of a class without creating a deep inheritance hierarchy.

Main differences:

• Purpose: a class is a standalone type with state and behavior; a mixin provides additional functionality to other classes.
• Instantiation: classes can be instantiated, but mixins usually cannot.
• Inheritance: mixins are used for multiple inheritance to avoid code duplication.

Example of a mixin in Python:

class JsonSerializableMixin:
    def to_json(self):
        import json
        return json.dumps(self.__dict__)

class User(JsonSerializableMixin):
    def __init__(self, name, age):
        self.name = name
        self.age = age

user = User('Alice', 30)
print(user.to_json())  # {"name": "Alice", "age": 30}