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Subprograms, parameters, return values and scope

A subprogram is a named, reusable section of code. A procedure performs an action; a function returns a value. Parameters receive values supplied by arguments in a call. Splitting a problem into subprograms supports decomposition and separation of concerns: each part has one clear responsibility.

1. Model procedure and function roles

def show_label(name):
    print("Name: {}".format(name))

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

show_label("Mina")
area = rectangle_area(4, 3)
print(area)

show_label performs display as a procedure. rectangle_area supplies a result that the caller stores. name, width and height are parameters; the values in the calls are arguments.

Fill in the blanks3 marks

A value written in a call is an gap 1. The name that receives it in the definition is a gap 2. A function sends a gap 3 back to its caller.

argument
global
parameter
return value

2. Predict returned values

Read this code without running it.

def adjusted(score, bonus):
    result = score + bonus
    return result

first = adjusted(8, 2)
second = adjusted(first, 3)
print(second)

Multiple choice1 mark

What exact value is printed?

Written answer2 marks

Explain the flow of arguments, parameters and return values through both calls.

Use the values 8, 2, 10 and 3 in order.

Students type their answer here.

3. Keep scope controlled

A local variable is available only inside the subprogram where it is created. A global variable is declared in the main program and is available more widely. Local names reduce unintended interference and make a subprogram easier to test. Pearson expects global variables to be minimised. The Python global keyword is not part of the Programming Language Subset; pass required values as parameters and return results instead.

Multiple choice1 mark

Why is a local variable often preferable for an intermediate calculation?

AIt can never hold a number.
BIt limits the name to the subprogram and reduces unintended changes elsewhere.
CIt automatically prints the result.
DIt is available before the function is called.

Written answer2 marks

Explain why a program should minimise global variables.

Give a mechanism and consequence, not only 'they are bad'.

Students type their answer here.

4. Build a reusable function

Define delivery_cost(distance, rate) to return distance * rate. Call it with 12 and 1.5, store the returned value in cost, and print cost. Keep the intermediate result local to the function; do not use the global keyword.

Coding task5 marks

# Define delivery_cost(distance, rate), call it and print the returned cost.

5. Separate concerns

A well-decomposed solution might use one function to calculate a value and a separate procedure to display it. This makes each part independently testable. A subprogram should not unexpectedly read or change unrelated global state when its required data can be supplied explicitly.

Written answer3 marks

A program validates an age, calculates a ticket price and displays a receipt. Propose three subprogram responsibilities and explain one benefit of the separation.

Give one clear responsibility per subprogram.

Students type their answer here.

Multiple choice1 mark

Which design has the clearest separation of concerns?

AOne long block that inputs, calculates and prints repeatedly.
BA calculation function that returns a price and a separate procedure that formats the receipt.
CA function that changes many unrelated globals.
DCopying the same calculation into every branch.

Route forward

You can distinguish procedures, functions, parameters, arguments, return values, local and global scope. Next you will use Pearson's permitted maths, time and formatting methods inside these structures.