Positional-Only Arguments in Python

Positional-only arguments are function parameters that must be passed by position (order) and cannot be passed by keyword name.

Syntax

Use the / symbol in the function definition to indicate that all parameters before it are positional-only:

python

def function_name(param1, param2, /, param3, param4):
    # function body

Simple Examples

Example 1: Basic Positional-Only Arguments

python

def calculate_area(length, width, /):
    area = length * width
    print(f"Area: {area} square units")

# These work (positional arguments):
calculate_area(5, 3)    # Output: Area: 15 square units
calculate_area(10, 2)   # Output: Area: 20 square units

# This will cause an ERROR (keyword arguments not allowed):
# calculate_area(length=5, width=3)

Example 2: Mixing Positional-Only and Regular Arguments

python

def create_person(name, age, /, country):
    print(f"Name: {name}, Age: {age}, Country: {country}")

# Valid calls:
create_person("Alice", 25, "USA")          # Positional for all
create_person("Bob", 30, country="Canada") # Positional for first two, keyword for last

# Invalid calls:
# create_person(name="Charlie", age=35, country="UK")  # ERROR: name and age are positional-only

Example 3: Practical Example – Math Operations

python

def power(base, exponent, /):
    result = base ** exponent
    print(f"{base} to the power of {exponent} = {result}")

power(2, 3)     # Output: 2 to the power of 3 = 8
power(5, 2)     # Output: 5 to the power of 2 = 25

# This would cause an error:
# power(base=2, exponent=3)

Example 4: With Default Arguments

python

def greet(name, /, message="Hello"):
    print(f"{message}, {name}!")

# Valid calls:
greet("John")                   # Output: Hello, John!
greet("Sarah", "Hi")            # Output: Hi, Sarah!
greet("Mike", message="Hey")    # Output: Hey, Mike!

# Invalid call:
# greet(name="Emma")  # ERROR: name is positional-only

Real-World Use Cases

Example 5: File Processing

python

def process_file(file_path, /, mode="read"):
    print(f"Processing {file_path} in {mode} mode")

process_file("data.txt")                # Output: Processing data.txt in read mode
process_file("image.jpg", "write")      # Output: Processing image.jpg in write mode

# This would be invalid:
# process_file(file_path="document.pdf")

Example 6: Coordinate System

python

def plot_point(x, y, /, color="black", size=1):
    print(f"Point at ({x}, {y}) with {color} color and size {size}")

plot_point(10, 20)                      # Output: Point at (10, 20) with black color and size 1
plot_point(5, 15, "red", 2)             # Output: Point at (5, 15) with red color and size 2
plot_point(3, 7, color="blue", size=3)  # Output: Point at (3, 7) with blue color and size 3

# Invalid:
# plot_point(x=1, y=2)

Key Points to Remember

1. Positional-only arguments must come before the /
2. They cannot be passed using keyword syntax
3. They make function APIs more strict and predictable
4. Useful when parameter names might change in the future
5. Helps prevent users from relying on specific parameter names

When to Use Positional-Only Arguments

• When parameter names are not descriptive or might change
• For mathematical functions where order matters (like coordinates)
• When you want to enforce a specific calling convention
• For internal functions where you want to prevent keyword usage

Positional-only arguments give you more control over how your functions are called!