Subprograms: functions, procedures, parameters and arguments

Procedures and functions (spec 7.1.4)

Both are defined with def. A function returns a value with return; a procedure carries out a task (often printing) and returns nothing (Python gives it the value None).

def show_banner(title):
    print("*" * len(title))
    print(title)

def area(width, height):
    return width * height

show_banner("Report")
a = area(3, 4)
print(a + area(2, 5))

Output:

******
Report
22

Parameters and arguments

A parameter is the name in the def line (width, height). An argument is the value passed in the call (3, 4). Arguments are matched to parameters by position, or by name: area(height=4, width=3). A parameter can have a default value: def greet(name, greeting="Hello").

return ends the function

As soon as return runs, the function stops and sends the value back. A function can return several values as a tuple.

def min_max(values):
    low = values[0]
    high = values[0]
    for v in values:
        if v < low:
            low = v
        if v > high:
            high = v
    return low, high

smallest, largest = min_max([7, 2, 9, 4])
print(smallest, largest)

Output:

2 9

Local variables

Parameters and variables assigned inside a subprogram are local: they exist only while it runs and cannot be seen outside. Pass data in through parameters and get results out with return, rather than using global variables. This is covered further in the program design lesson.

Worked example

Write a function is_leap(year) that returns True for leap years: divisible by 4, except century years, which must be divisible by 400. Test it on 2024, 1900 and 2000.

1. def is_leap(year):
2. return (year % 4 == 0 and year % 100 != 0) or year % 400 == 0
3. Results: True False True

Answer: True, False, True

Check yourself

1. What is displayed when this code runs? ```python def triple(n): return n * 3 print(triple(4) + triple(2)) ```
   Show answer

   18. triple(4) = 12 and triple(2) = 6, so 18.

2. What is displayed when this code runs? ```python def check(x): if x > 10: return "big" return "small" print("never") print(check(15), check(3)) ```
   Show answer

   big small. return ends the function, so the print inside is never reached: big small.

3. What is displayed when this code runs? (Output lines are separated by / in the options.) ```python def greet(name): print("Hi", name) result = greet("Sam") print(result) ```
   • Hi Sam / Hi Sam
   • Hi Sam / None
   • Hi Sam
   • Sam / None
   Show answer

   Hi Sam / None. greet is a procedure: it prints 'Hi Sam' and returns nothing, so result is None: Hi Sam, then None.

4. What is displayed when this code runs? ```python def power(base, exp=2): return base ** exp print(power(5), power(2, 5)) ```
   Show answer

   25 32. power(5) uses the default exp = 2, giving 25; power(2, 5) gives 32: 25 32.

5. What is displayed when this code runs? ```python def change(n): n = n + 10 return n x = 5 y = change(x) print(x, y) ```
   Show answer

   5 15. n is a local copy of the value 5; changing it does not change x. The function returns 15: 5 15.

6. In def total(price, qty): and the call total(4.5, 3), what are 4.5 and 3?
   • Parameters
   • Local constants
   • Arguments
   • Return values
   Show answer

   Arguments. Values passed in a call are arguments; price and qty are the parameters.

7. This function should return the average of a list. Which line completes it? ```python def average(values): total = 0 for v in values: total = total + v __________ print(average([4, 6, 11])) ```
   • `return total / len(values)`
   • `print(total / len(values))`
   • `return total / len(v)`
   • `return total // len(values)`
   Show answer

   `return total / len(values)`. The function must return the total divided by the number of values: 21 / 3 = 7.0.

8. This code should display 16 but it does not. Which line number contains the error? ``` 1 def rectangle_perimeter(length, width): 2 return 2 * (length + length) 3 4 print(rectangle_perimeter(5, 3)) ```
   Show answer

   2. Line 2 uses length twice and ignores the width parameter. The fix is return 2 * (length + width).

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (XCP01/YCP01), first teaching 2026
• Pearson IAL Computer Science Unit 2 Programming Language Subset (PLS), Python 3
• Pearson IAL Computer Science IAS sample assessment materials (Unit 2 paper and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.