Understanding '*args' and '**kwargs' in Python

🔹 Understanding *args and `kwargs` in Python**

In Python, *args and **kwargs are used to pass a variable number of arguments to a function.

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🔹 What is *args?

• *args allows a function to accept any number of positional arguments.
• These arguments are collected as a tuple.

Example: Using *args

def add_numbers(*args):
    return sum(args)

print(add_numbers(1, 2, 3))         # Output: 6
print(add_numbers(10, 20, 30, 40))  # Output: 100
print(add_numbers())                # Output: 0 (No arguments)

How It Works?

• *args collects 1, 2, 3 as a tuple → (1, 2, 3)
• The function sums all numbers and returns the result.

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🔹 What is `kwargs`?**

• **kwargs allows a function to accept any number of keyword arguments.
• These arguments are collected as a dictionary.

Example: Using `kwargs`**

def print_info(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

print_info(name="Alice", age=25, city="New York")

Output:

name: Alice
age: 25
city: New York

How It Works?

• **kwargs collects name="Alice", age=25, city="New York" as a dictionary:

  {'name': 'Alice', 'age': 25, 'city': 'New York'}

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🔹 Using *args and `kwargs` Together**

You can combine both in a function.

Example:

def user_info(user_id, *args, **kwargs):
    print(f"User ID: {user_id}")
    print("Additional Info:", args)
    print("Other Details:", kwargs)

user_info(101, "Developer", "Python", name="John", age=30)

Output:

User ID: 101
Additional Info: ('Developer', 'Python')
Other Details: {'name': 'John', 'age': 30}

How It Works?

• 101 → goes into user_id
• "Developer", "Python" → goes into *args as a tuple
• name="John", age=30 → goes into `kwargs`** as a dictionary

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🔹 Example in Django View

In Django ViewSets, *args, **kwargs are often used to pass additional arguments.

Example in Django ViewSet

from rest_framework.viewsets import ViewSet
from rest_framework.response import Response

class MyViewSet(ViewSet):
    def list(self, request, *args, **kwargs):
        print("Args:", args)      # Tuple of extra positional arguments
        print("Kwargs:", kwargs)  # Dictionary of extra keyword arguments
        return Response({"message": "Success"})

How It Works in Django?

• *args captures extra positional arguments.
• **kwargs captures additional keyword arguments (like pk in retrieve).

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🔹 Summary Table

Concept	Description	Stored As	Example Call	Example Inside Function
*args	Multiple positional arguments	tuple	func(1, 2, 3)	(1, 2, 3)
**kwargs	Multiple keyword arguments	dict	func(name="Alice", age=25)	{'name': 'Alice', 'age': 25}

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🚀 Best Practices

✔ Use *args when you don’t know how many positional arguments will be passed.

✔ Use **kwargs when you don’t know how many keyword arguments will be passed.

✔ Use both when creating flexible Django views or utility functions.

🔹 Understanding *args and `kwargs` in Flask & FastAPI**

Both Flask and FastAPI allow you to use *args and **kwargs to handle dynamic arguments in request handling.

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🔹 Example in Flask

Flask allows dynamic routes, and *args and **kwargs can help handle unknown parameters.

✅ Example: Flask Route with *args and `kwargs`**

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/user/<int:user_id>', methods=['GET'])
def get_user(user_id, *args, **kwargs):
    extra_params = request.args  # Get query parameters
    return jsonify({
        "user_id": user_id,
        "args": args,
        "kwargs": kwargs,
        "query_params": extra_params.to_dict()  # Convert to dictionary
    })

if __name__ == '__main__':
    app.run(debug=True)

📌 How It Works?

• The URL pattern /user/<int:user_id> captures user_id as a positional argument.
• *args and **kwargs capture additional data (if provided).
• request.args retrieves query parameters.

✅ Testing in Postman

Request:

GET http://127.0.0.1:5000/user/123?name=Alice&age=30

Response:

{
    "user_id": 123,
    "args": [],
    "kwargs": {},
    "query_params": {
        "name": "Alice",
        "age": "30"
    }
}

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🔹 Example in FastAPI

FastAPI uses *args and **kwargs in path operations and dependency injection.

✅ Example: FastAPI with *args and `kwargs`**

from fastapi import FastAPI, Query
from typing import Dict

app = FastAPI()

@app.get("/items/{item_id}")
async def get_item(item_id: int, *args, **kwargs):
    return {
        "item_id": item_id,
        "args": args,
        "kwargs": kwargs
    }

@app.get("/query/")
async def query_example(name: str = Query(None), age: int = Query(None), **kwargs: Dict):
    return {
        "name": name,
        "age": age,
        "kwargs": kwargs
    }

📌 How It Works?

1. *args and **kwargs capture additional parameters that may be sent in the request.
2. Query(None) allows extracting query parameters dynamically.
3. `kwargs: Dict`** allows capturing additional keyword parameters.

✅ Testing in Postman

Request:

GET http://127.0.0.1:8000/query/?name=Alice&age=30&city=NewYork

Response:

{
    "name": "Alice",
    "age": 30,
    "kwargs": {
        "city": "NewYork"
    }
}

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🔹 Summary of *args and `kwargs` in Flask & FastAPI**

Framework	Use Case	Example Call	Result
Flask	Capture extra route parameters and query params	/user/123?name=Alice&age=30	Returns user_id, args, kwargs, and query_params
FastAPI	Handle query parameters dynamically	/query/?name=Alice&age=30&city=NewYork	Captures name, age, and additional kwargs

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🚀 Best Practices

✔ Use *args for positional arguments (though rarely needed in APIs).

✔ Use `kwargs to capture additional query params dynamically**.
✔ **In Flask, use request.args.to_dict()** to handle query parameters.
✔ **In FastAPI, use Query() and kwargs: Dict` for flexibility.