Writing Functions

Ms. Roy’s practice check-in came back for a second round. She wanted the week’s total, the longest single session, and the number of days off — and she wanted the totals in hours and minutes, not decimals, because “3.1 hours” means nothing when you are planning a Tuesday. The first draft had the hours-and-minutes arithmetic written out three times. This one has it written once.

The program

# Practice-week summary for Ms. Roy, who plans next week from last
# week's minutes. Three small functions, each with one job.
 
def total_minutes(sessions):
    total = 0
    for minutes in sessions:
        total = total + minutes
    return total
 
 
def longest_session(sessions):
    longest = sessions[0]
    for minutes in sessions:
        if minutes > longest:
            longest = minutes
    return longest
 
 
def as_hours_and_minutes(minutes):
    hours = minutes // 60
    rest = minutes % 60
    return f"{hours} h {rest} min"
 
 
week = [45, 0, 60, 30, 0, 90, 25]
 
print("Last week's practice")
print(f"Total:    {as_hours_and_minutes(total_minutes(week))}")
print(f"Longest:  {as_hours_and_minutes(longest_session(week))}")
print(f"Days off: {week.count(0)}")

Last week's practice
Total:    4 h 10 min
Longest:  1 h 30 min
Days off: 2

How it works

Nothing runs when Python reads a def — it only remembers the definition. The program really begins at week = [...], and the three print lines are the whole of it. That is the shape to aim for: a main part short enough to read aloud, standing on functions with honest names.

Each function takes what it needs as a parameter and hands back an answer with return. None of them prints anything, which is what lets them be combined: as_hours_and_minutes(total_minutes(week)) works because the inner call is finished and replaced by 250 before the outer call begins.

as_hours_and_minutes is the one that earns its keep. // gives whole hours, % gives the leftover minutes, and this awkward little pair of lines now exists in exactly one place. When Ms. Roy asks for 4h10 instead, you edit one function and every line of output changes.

week.count(0) is a reminder that you are not obliged to write everything yourself — count is a method that lists already have, like len, max, and abs. Reaching for the tool that exists is a skill, not a shortcut.¹

Change it

1. One line. Change as_hours_and_minutes to return f"{hours}h{rest:02d}", and the total prints as 4h10. The :02d pads the minutes to two digits, so five past four is 4h05 rather than 4h5.
2. A few lines. Write a fourth function, days_practised(sessions), that counts the sessions above zero and returns the count — 5 for this week. Then use it instead of week.count(0) and say “Days practised” in the output, which is the sentence a coach would rather read.
3. A real change. Add last_week = [30, 30, 60, 0, 45, 60, 0] and print a summary for both weeks by calling the same three functions again with the other list. Nothing inside any function changes — that is what “reusable” means, and it is the reason Decomposition and Design treats functions as the unit you design in.

The ideas behind this page are in Functions and Parameters, Returns, and Scope; the reps are in Functions Practice.

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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Footnotes

1. Every one of these functions is a subprogram in the curriculum’s vocabulary, whether you wrote it or Python did. The only difference is who maintains it. ↩