Functions

By the end of The Repeated Chunk every group had the same complaint. The program worked, but the eight lines that converted a mark to a letter grade appeared four times, and when the cutoff for a B changed, three of the four copies got updated. The bug was not in any line of code. It was in having four copies at all.

A function is the fix: name a chunk of thinking once, then use the name.

def letter_grade(mark):
    if mark >= 80:
        return "A"
    elif mark >= 70:
        return "B"
    elif mark >= 60:
        return "C"
    elif mark >= 50:
        return "D"
    else:
        return "R"

Now the cutoff lives in exactly one place. Change it there and every part of the program that grades anything changes with it — including the parts you have not written yet.

What the parts are called

Part	In the example	What it does
Definition	def letter_grade(mark):	Names the chunk
Parameter	mark	The information the chunk needs
Body	the indented lines	The thinking itself
Return value	return "A"	The answer it hands back
Call	letter_grade(78)	Asking for that thinking, here, now

A call is not a copy. When Python reaches letter_grade(78) it jumps into the function, runs it with mark set to 78, and comes back holding "B". The function does not print anything and does not know who asked — which is exactly what makes it reusable.

Why “returns” beats “prints”

An early instinct is to make the function print the grade. Resist it. A function that returns can be used in ways you did not anticipate:

grade = letter_grade(78)              # store it
print(f"You earned a {letter_grade(78)}")   # show it
if letter_grade(mark) == "R":         # decide with it
    print("See me — let's make a plan.")

A function that prints can only ever do the one thing. This is the first place in the course where a small design decision quietly decides how useful your code will be to somebody else — which is the whole subject of Who Is This For.

Self-check: why does this function return None?

(click to expand)

def double(number):
    print(number * 2)

It prints, but never returns. result = double(5) displays 10 and then puts None in result, because a function with no return hands back nothing. Swap print for return and the caller decides what to do with the answer.

Where this goes next

Functions are the unit you will design in when the problem gets bigger than one screen — see Decomposition and Design, where a whole program becomes a handful of well-named chunks. Every program you build from here on, including The Community App, is easier to finish and far easier to hand to somebody else when the thinking has names. Practise the mechanics in Functions Practice, then read somebody else’s in Writing Functions.

Curriculum connection

A3.1

demonstrate the ability to use existing subprograms (e.g., random number generator, substring, absolute value) within computer programs;

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A3.2

write subprograms (e.g., functions, procedures) that use parameter passing and appropriate variable scope (e.g., local, global), to perform tasks within programs.

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B2.3

apply the principle of modularity to design reusable code (e.g., subprograms, classes) in computer programs;

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