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How to write a function

Updated September 2026 · Beginner-friendly guide

Define and call a function

def creates a function. Its indented body runs only when the function is called.

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

message = greet("Ada")
print(message)

Use parameters and defaults

Parameters receive inputs. A default value makes an argument optional; required parameters must come first.

def calculate_total(price, quantity=1, tax_rate=0.0):
    subtotal = price * quantity
    return subtotal * (1 + tax_rate)

print(calculate_total(10, quantity=2, tax_rate=0.1))

Return useful values

return ends the call and sends a value back. Without an explicit return, a function returns None.

def divide_with_remainder(number, divisor):
    return number // divisor, number % divisor

quotient, remainder = divide_with_remainder(17, 5)
print(quotient, remainder)

Document and scope functions

Local variables exist inside their function. Prefer returning results to modifying global state, and add a concise docstring to reusable functions.

def celsius_to_fahrenheit(celsius):
    """Convert a Celsius temperature to Fahrenheit."""
    fahrenheit = celsius * 9 / 5 + 32
    return fahrenheit

print(celsius_to_fahrenheit(20))

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