Python Keyword-Only Arguments (Complete Guide for Beginners)

In Python, keyword-only arguments are parameters that must be passed using their name (keyword), not by position. This means you cannot pass values normally like 10, 20; you must use name=value.

Keyword-only arguments help make your functions clear, safe, and less confusing.

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🔹 What are Keyword-Only Arguments?

Keyword-only arguments are function parameters that:

Must be passed using key=value format

They are defined after a special symbol * in a function.

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🔹 Syntax of Keyword-Only Arguments

def function_name(*, param1, param2):
    pass

🔍 Important:

• Everything after * becomes keyword-only
• You must use param=value when calling

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🔹 Simple Example of Keyword-Only Arguments

def greet(*, name, message):
    print(message, name)

greet(name="Alex", message="Hello")

🔸 Output:

Hello Alex

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🔹 ❌ Wrong Usage (Positional Not Allowed)

def greet(*, name, message):
    print(name, message)

greet("Alex", "Hello")

❌ This gives an error

🔍 Why?

• After *, arguments cannot be passed positionally

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🔹 Correct Usage (Keyword Required)

def user_info(*, username, age):
    print("Username:", username)
    print("Age:", age)

user_info(username="John", age=25)

🔸 Output:

Username: John
Age: 25

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🔹 Why Use Keyword-Only Arguments?

They are useful when:

• You want clear function calls
• You want to avoid argument confusion
• You want to force users to be explicit

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🔹 Example: Payment System

def payment(*, amount, currency):
    print("Amount:", amount)
    print("Currency:", currency)

payment(amount=100, currency="USD")

🔸 Output:

Amount: 100
Currency: USD

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🔹 Mixing Positional and Keyword-Only Arguments

You can combine normal and keyword-only arguments:

def order(product, *, price, quantity):
    print(product)
    print(price)
    print(quantity)

order("Laptop", price=1200, quantity=2)

🔸 Output:

Laptop
1200
2

🔍 Explanation:

• "Laptop" → positional argument
• price, quantity → keyword-only

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🔹 Invalid Example

def order(product, *, price, quantity):
    print(product, price, quantity)

order("Laptop", 1200, 2)

❌ Error occurs

🔍 Reason:

• price and quantity must be written as keywords

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🔹 Real-Life Example: User Registration

def register(username, *, email, country):
    print(username)
    print(email)
    print(country)

register("alex", email="alex@gmail.com", country="Cambodia")

🔸 Output:

alex
alex@gmail.com
Cambodia

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🔹 Real-Life Example: Hotel Booking System

def book_room(room_type, *, nights, price_per_night):
    total = nights * price_per_night
    print("Room:", room_type)
    print("Total:", total)

book_room("Deluxe", nights=3, price_per_night=50)

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🔹 Keyword-Only with Default Values

def profile(*, name="Guest", age=0):
    print(name, age)

profile(name="John", age=30)
profile()

🔸 Output:

John 30
Guest 0

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🔹 Difference Between Keyword Arguments and Keyword-Only Arguments

Feature	Keyword Arguments	Keyword-Only Arguments
Order matters	No	No
Positional allowed	Yes	No
Must use name=value	Optional	Required
Defined using *	No	Yes

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🔹 Why Keyword-Only Arguments Are Important

✔ Prevents mistakes in argument order
✔ Makes code more readable
✔ Forces clarity in function calls
✔ Useful in APIs and large applications
✔ Improves code safety and structure

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🔹 Key Rule to Remember

Everything after * in function definition becomes keyword-only.

def example(a, b, *, c, d):
    pass

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🚀 Conclusion

Python keyword-only arguments are a powerful feature that improves code clarity and safety. They ensure that important parameters are always passed explicitly using names, reducing confusion and bugs.

They are widely used in:

• APIs
• Frameworks
• Large software systems
• Professional Python development