By the end of this lesson
Write a function with a clear input and output contract.
Before you start
- Python 3.12 or a compatible newer Python 3 interpreter; no external packages.
- Complete lesson 10 in this route, or be comfortable with its examples.
Three words to know
- Parameter
- A name in a function definition.
- Argument
- A value supplied in a function call.
- Return value
- The result sent back to the caller.
1. Separate calculation from display
A function begins with def. Its body runs when called, not merely when defined. return sends a value to the caller; print writes text to output. Returning data allows the same rule to be tested or displayed in different ways.
def count_open(statuses):
return sum(1 for status in statuses if status == "open")
result = count_open(["open", "closed", "open"])
print(result)
What you should see · example output
2
2. Local names and default values
A name assigned inside a function is normally local to that call. Use explicit parameters instead of changing unrelated global values. Defaults such as a string or integer are straightforward; avoid a mutable list default because calls would share the same list.
def greeting(name="learner"):
message = f"Hello, {name}!"
return message
print(greeting())
print(greeting("Sam"))
What you should see · example output
Hello, learner! Hello, Sam!
3. State the contract
count_open accepts an iterable of statuses and counts exact lowercase matches. It does not validate that every value is a supported status. Decide whether a function validates input or assumes a caller already did. A short docstring can state that decision, expected results and possible errors.
YOUR TURN
A small check before you continue
Write double(number) that returns twice the argument. How can you verify it without printing inside the function?
Show a suggested answer
Define def double(number): return number * 2. Compare double(3) with 6 in a test. Keep display in the caller so the function remains reusable.
Sources and scope
Documentation and examples reviewed 2026-10-18. Provider screens, package versions and output wording can differ. Examples show expected outcomes; they are not a claim that OpenViST connected to your server or verified your setup.