Python Function Default Parameters Explained with Examples

Learn how to define Python functions with default parameter values and call them with fewer arguments than declared.

Easy Python 3.9+ Aug 9, 2026 Functions & basics 14 views 0 copies

functions default-parameters args basics

Python code

14 lines

Python 3.9+

def greet(name, greeting="Hello", punctuation="!"):
    return f"{greeting}, {name}{punctuation}"

def calculate_area(length, width=1, unit="sq units"):
    area = length * width
    return f"Area: {area} {unit}"

if __name__ == "__main__":
    print(greet("Alice"))
    print(greet("Bob", "Hi"))
    print(greet("Charlie", "Hey", "..."))
    print(calculate_area(5))
    print(calculate_area(5, 3))
    print(calculate_area(4, 2, "cm²"))

Output

stdout

Hello, Alice!
Hi, Bob!
Hey, Charlie...
Area: 5 sq units
Area: 15 sq units
Area: 8 cm²

How it works

Default parameters let you define sensible fallback values so callers can omit those arguments. In greet("Alice"), the defaults greeting="Hello" and punctuation="!" are used automatically. When you pass only some arguments, Python fills the remaining ones from left to right using defaults. The calculate_area function shows defaults can also be non-string values like numbers. This pattern reduces repetitive code and makes functions more flexible to call.

Common mistakes

Putting default parameters before non-default ones (SyntaxError)
Using mutable defaults like `def f(x=[])` which share state across calls
Forgetting that defaults are evaluated once at definition time, not each call

Variations

Call functions using keyword arguments: `greet(name="Alice", greeting="Hello")`
Use `None` as default and assign inside the function for mutable defaults

Real-world use cases

Logging functions where log level defaults to INFO but can be overridden per call.
API client methods with timeout or retry defaults that callers can customize.
Configuration parsers with optional flags like pretty-print defaulting to False.

Python Function Default Parameters Explained with Examples

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

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