Keyword-Only Arguments in Python

Keyword-only arguments are function parameters that must be passed using their keyword names. They cannot be passed as positional arguments.

Syntax

Use the * symbol in the function definition to indicate that all parameters after it are keyword-only:

python

def function_name(param1, param2, *, keyword_only1, keyword_only2):
    # function body

Simple Examples

Example 1: Basic Keyword-Only Arguments

python

def create_profile(name, age, *, email, country):
    print(f"Name: {name}, Age: {age}")
    print(f"Email: {email}, Country: {country}")
    print("-" * 30)

# Valid call (must use keywords for email and country):
create_profile("Alice", 25, email="alice@email.com", country="USA")

# Invalid calls:
# create_profile("Bob", 30, "bob@email.com", "UK")  # ERROR: must use keywords
# create_profile("Charlie", 35, "charlie@email.com")  # ERROR

Example 2: With Default Values

python

def order_product(product, quantity, *, discount_code=None, gift_wrap=False):
    print(f"Order: {quantity} x {product}")
    if discount_code:
        print(f"Discount: {discount_code}")
    if gift_wrap:
        print("Gift wrap: Yes")
    print("-" * 25)

# Valid calls:
order_product("Book", 2, discount_code="SAVE10")
order_product("Pen", 5, gift_wrap=True)
order_product("Notebook", 1, discount_code="WELCOME", gift_wrap=True)
order_product("Pencil", 10)  # Uses default values for keyword-only args

Example 3: Only Keyword-Only Arguments

python

def send_message(*, to, subject, body, cc=None):
    print(f"To: {to}")
    print(f"Subject: {subject}")
    print(f"Body: {body}")
    if cc:
        print(f"CC: {cc}")
    print("Message sent!")
    print("-" * 20)

# Must use keywords for all parameters:
send_message(to="friend@email.com", subject="Hello", body="How are you?")
send_message(to="boss@work.com", subject="Report", body="Attached report", cc="team@work.com")

Practical Examples

Example 4: User Registration

python

def register_user(username, password, *, email, phone=None, newsletter=False):
    print(f"Username: {username}")
    print(f"Password: {'*' * len(password)}")
    print(f"Email: {email}")
    if phone:
        print(f"Phone: {phone}")
    print(f"Newsletter: {'Yes' if newsletter else 'No'}")
    print("Registration complete!")
    print("-" * 35)

register_user("john_doe", "secret123", email="john@email.com")
register_user("sarah", "mypass", email="sarah@mail.com", phone="123-4567", newsletter=True)

Example 5: File Operations

python

def open_file(filename, *, mode="read", encoding="utf-8", create_if_missing=False):
    print(f"Opening: {filename}")
    print(f"Mode: {mode}")
    print(f"Encoding: {encoding}")
    print(f"Create if missing: {create_if_missing}")
    print("File ready!")
    print("-" * 20)

open_file("data.txt", mode="write", encoding="ascii")
open_file("notes.txt", create_if_missing=True)
open_file("config.ini", mode="append", encoding="utf-8")

Example 6: Math Calculations

python

def calculate(operation, *, precision=2, show_steps=False):
    if operation == "add":
        result = 5 + 3
    elif operation == "multiply":
        result = 4 * 6
    
    final_result = round(result, precision)
    print(f"Result: {final_result}")
    
    if show_steps:
        print(f"Calculation steps shown for {operation}")

calculate("add", precision=3, show_steps=True)
calculate("multiply", show_steps=False)

Using * Alone for Keyword-Only

Example 7: Empty * for All Keyword-Only

python

def configure_settings(*, theme="light", font_size=12, language="en"):
    print(f"Theme: {theme}")
    print(f"Font size: {font_size}")
    print(f"Language: {language}")
    print("Settings configured!")
    print("-" * 20)

# All arguments must be keyword-only:
configure_settings(theme="dark", font_size=14)
configure_settings(language="es", theme="light")

Mixed with Positional Arguments

Example 8: Positional + Keyword-Only

python

def create_post(title, content, *, author, tags=None, published=True):
    print(f"Title: {title}")
    print(f"Content: {content[:30]}...")  # Show first 30 characters
    print(f"Author: {author}")
    if tags:
        print(f"Tags: {tags}")
    print(f"Published: {published}")
    print("-" * 25)

create_post("Python Tips", "Learn Python programming with these tips...", 
            author="John", tags=["python", "coding"])
create_post("Hello World", "My first blog post...", 
            author="Sarah", published=False)

Key Benefits

1. Clarity: Makes function calls more readable
2. Flexibility: Parameters can be specified in any order
3. Safety: Prevents accidental misuse of parameters
4. Maintainability: Easier to add new parameters later

When to Use Keyword-Only Arguments

• When you have many optional parameters
• When parameter names make the code more readable
• When you want to enforce explicit naming for certain parameters
• For configuration settings or options

Remember These Rules

1. Keyword-only arguments come after *
2. They must be passed using their keyword names
3. They can have default values
4. They make function calls more explicit and readable

Keyword-only arguments help create cleaner, more maintainable code!

Positional-Only & Keyword-Only Mixed Arguments

You can combine positional-only, regular, and keyword-only arguments in a single function using both / and * symbols.

Syntax

python

def function_name(pos_only1, pos_only2, /, regular, *, keyword_only1, keyword_only2):
    # function body

Simple Examples

Example 1: Basic Mixed Arguments

python

def process_data(name, age, /, country, *, email, phone):
    print(f"Name: {name}, Age: {age}")
    print(f"Country: {country}")
    print(f"Email: {email}, Phone: {phone}")
    print("-" * 30)

# Valid call:
process_data("Alice", 25, "USA", email="alice@email.com", phone="123-4567")

# Invalid calls:
# process_data("Alice", 25, "USA", "alice@email.com", "123-4567")  # ERROR: keyword-only args must use keywords
# process_data(name="Bob", age=30, country="UK", email="bob@email.com", phone="987-6543")  # ERROR: name/age are positional-only

Example 2: Math Function with Mixed Arguments

python

def calculate(x, y, /, operation="add", *, precision=2):
    if operation == "add":
        result = x + y
    elif operation == "multiply":
        result = x * y
    else:
        result = x - y
    
    print(f"Result: {round(result, precision)}")

calculate(5, 3)                              # Output: Result: 8
calculate(5, 3, "multiply")                  # Output: Result: 15
calculate(10, 4, operation="subtract")       # Output: Result: 6
calculate(7, 3, precision=4)                 # Output: Result: 10.0
calculate(2.5, 1.3, "multiply", precision=3) # Output: Result: 3.25

Example 3: User Registration

python

def register_user(username, password, /, country="Unknown", *, email, newsletter=False):
    print(f"Username: {username}")
    print(f"Password: {'*' * len(password)}")
    print(f"Country: {country}")
    print(f"Email: {email}")
    print(f"Newsletter: {newsletter}")
    print("Registration successful!")
    print("-" * 40)

register_user("john_doe", "secret123", "USA", email="john@email.com")
register_user("sarah", "mypass", email="sarah@mail.com", newsletter=True)
register_user("mike", "pass123", country="Canada", email="mike@test.com")

Example 4: File Operations

python

def handle_file(filename, /, mode="read", *, encoding="utf-8", buffer_size=1024):
    print(f"File: {filename}")
    print(f"Mode: {mode}")
    print(f"Encoding: {encoding}")
    print(f"Buffer Size: {buffer_size} bytes")
    print("File operation ready!")
    print("-" * 25)

handle_file("data.txt")
handle_file("image.jpg", "write", encoding="binary", buffer_size=2048)
handle_file("document.pdf", mode="append", buffer_size=512)

Visual Breakdown

python

def example(a, b, /, c, d, *, e, f):
    # a, b → Positional-only (must come first, no keywords)
    # c, d → Regular (can be positional or keyword)
    # e, f → Keyword-only (must use keywords)
    pass

More Practical Examples

Example 5: Shopping Cart

python

def add_to_cart(product_id, quantity, /, *, discount_code=None, gift_wrap=False):
    print(f"Adding product #{product_id}, Quantity: {quantity}")
    if discount_code:
        print(f"Discount applied: {discount_code}")
    if gift_wrap:
        print("Gift wrapping selected")
    print("Item added to cart!")
    print("-" * 30)

add_to_cart(101, 2)
add_to_cart(205, 1, discount_code="SUMMER20")
add_to_cart(150, 3, gift_wrap=True)
add_to_cart(300, 5, discount_code="WELCOME10", gift_wrap=True)

Example 6: Drawing Shapes

python

def draw_shape(x, y, /, shape="circle", *, color="black", filled=True, size=1):
    print(f"Drawing {shape} at position ({x}, {y})")
    print(f"Color: {color}, Filled: {filled}, Size: {size}")
    print("Shape drawn!")
    print("-" * 25)

draw_shape(10, 20)
draw_shape(50, 30, "square", color="blue")
draw_shape(100, 40, "triangle", filled=False, size=2)
draw_shape(75, 60, shape="rectangle", color="red", size=1.5)

Key Rules to Remember

1. Positional-only arguments come before /
2. Regular arguments come between / and *
3. Keyword-only arguments come after *
4. Order must be: Positional-only → Regular → Keyword-only
5. Default values can be used with any type of argument

Benefits of Mixed Arguments

• Flexibility: Different calling styles for different parameters
• Clarity: Makes important parameters stand out
• Safety: Prevents misuse of certain parameters
• Maintainability: Easier to extend functions without breaking existing code

This mixed approach gives you maximum control over how your functions are called!