Two Roles: Writer and Consumer of Functions

When working with functions, there are two distinct perspectives to keep in mind: the writer of the function and the consumer (or caller) of the function.

The Writer

The writer is responsible for:

• Choosing a clear, descriptive name.

• Deciding what parameters the function needs.

• Writing the body so it correctly produces the intended result.

• Documenting what the function does (its contract) in a docstring.

A docstring is a string literal placed immediately after the def line. Python tools can display it as help text:

def weekly_wages(hours: float, rate: float) -> float:
    """Return total weekly wages including 1.5x overtime above 40 hours."""
    if hours <= 40:
        return hours * rate
    return 40 * rate + (hours - 40) * rate * 1.5

>>> help(weekly_wages)
Help on function weekly_wages:

weekly_wages(hours, rate)
    Return total weekly wages including 1.5x overtime above 40 hours.
    ...

The Consumer

The consumer does not need to know how the function is implemented — only what it does (its contract):

• What are the parameters and their expected types?

• What does it return?

• Are there any preconditions (e.g., hours must be non-negative)?

A well-written function with a clear docstring can be used correctly without reading the body. This is the power of abstraction — you hide the details so the consumer can focus on their own problem.

The Contract

The docstring defines the contract between writer and consumer:

• The consumer promises to call the function with valid arguments.

• The writer promises to return the correct result for valid arguments.

If the consumer violates the preconditions (e.g., passes a negative number of hours), the function is not obligated to produce a sensible result.

Separation of Concerns

Good programs separate the computation (what is calculated) from the presentation (how results are shown). Functions help with this:

# start: weekly_wages
def weekly_wages(hours: float, rate: float) -> float:
    """Return total weekly wages including 1.5x overtime above 40 hours."""
    if hours <= 40:
        return hours * rate
    return 40 * rate + (hours - 40) * rate * 1.5
# end: weekly_wages


def main() -> None:
    hours = float(input("Hours worked: "))
    rate = float(input("Hourly rate: $"))
    pay = weekly_wages(hours, rate)
    print(f"Weekly wages: ${pay:.2f}")


if __name__ == '__main__':
    main()

The weekly_wages function only computes; main handles input and output.