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
- Clarity: Makes function calls more readable
- Flexibility: Parameters can be specified in any order
- Safety: Prevents accidental misuse of parameters
- 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
- Keyword-only arguments come after
* - They must be passed using their keyword names
- They can have default values
- 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
- Positional-only arguments come before
/ - Regular arguments come between
/and* - Keyword-only arguments come after
* - Order must be: Positional-only → Regular → Keyword-only
- 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!
