Understanding the ** Unpacking Operator in Python

The ** operator in Python is known as the dictionary unpacking operator. It allows you to expand a dictionary into named arguments when calling a function or creating an object. It's especially useful when you're working with functions or constructors that take many keyword arguments.

Basic Example

Suppose you have a function that takes several keyword arguments:

def greet(name, age, city):
    print(f"Hello, my name is {name}, I'm {age} years old and I live in {city}.")

Now you can call it like this:

data = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

greet(**data)

Behind the Scenes

The call greet(**data) is equivalent to:

greet(name="Alice", age=30, city="New York")

Using ** with Classes

You can also use ** to pass dictionary data to a class constructor:

class User:
    def __init__(self, id, name, email, role):
        self.id = id
        self.name = name
        self.email = email
        self.role = role

data = {
    "id": "123",
    "name": "Juan Pérez",
    "email": "juan@example.com",
    "role": "READ"
}

user = User(**data)

Real-World Usage: FastAPI or Pydantic Models

With libraries like Pydantic, it's common to use ** to construct new models based on the output of methods like .model_dump():

class UserOut(BaseModel):
    id: str
    name: str
    email: str
    role: str
    PK: str

class User(BaseModel):
    id: str
    name: str
    email: str
    role: str

    def to_db_model(self):
        return UserOut(PK=f"user#{self.id}", **self.model_dump())

Why Use **?

• Cleaner code: Avoids manually passing each attribute
• Maintainability: Adding new fields automatically updates the constructor call
• Flexibility: Easily integrate dynamic data structures