6  Functions

6.1 Define and call a function

def calculate_percentage(part, total):
    """Return part as a percentage of total."""
    if total == 0:
        return None
    return part / total * 100


result = calculate_percentage(25, 80)
print(result)

31.25

6.2 Parameters with default values

def create_label(name, prefix="dataset"):
    return f"{prefix}: {name}"


print(create_label("population"))
print(create_label("roads", prefix="layer"))

dataset: population
layer: roads

6.3 Type hints

def normalize_name(name: str) -> str:
    """Return a simple lowercase identifier."""
    return name.strip().lower().replace(" ", "_")


print(normalize_name(" Total Population "))

total_population

6.4 Things to remember

• A function should have one clear responsibility.
• return sends a value back to the caller.
• A docstring explains the function’s purpose.
• Type hints improve clarity but do not enforce types automatically.