In Python, __dict__ is a built-in attribute of an object or class that returns a dictionary containing the attributes and their values of the object or class.

For an object, __dict__ represents the instance's namespace, i.e., a dictionary containing all the instance variable names as keys and their corresponding values as values.

For a class, __dict__ represents the class's namespace, i.e., a dictionary containing all the class variable names as keys and their corresponding values as values.

Here is an example to demonstrate the usage of __dict__:

class Person:  
    def __init__(self, name, age):  
        self.name = name  
        self.age = age  
person = Person("John Doe", 25)  
print(person.__dict__)  # Output: {'name': 'John Doe', 'age': 25}  
person.__dict__["gender"] = "Male"  
print(person.__dict__)  # Output: {'name': 'John Doe', 'age': 25, 'gender': 'Male'}  
print(Person.__dict__)  # Output: {'__module__': '__main__', '__init__': <function Person.__init__ at 0x7f91900e4950>, '__dict__': <attribute '__dict__' of 'Person' objects>, '__weakref__': <attribute '__weakref__' of 'Person' objects>, '__doc__': None}  

In the above example, person.__dict__ returns a dictionary containing the attributes and values of the person object. Similarly, Person.__dict__ returns a dictionary containing the attributes and values of the Person class.