Project: a toolbox of functions

What you will be able to do

After this lesson you can

• plan a set of small functions that each do one job,
• write each one with parameters, return and a docstring,
• test each one alone, and then use them together, and
• extend the toolbox with your own tools.

The goal

You use the same little calculations again and again: the area of a room, the area of a circle, a temperature in degrees Celsius changed into Fahrenheit, whether a year is a leap year, a letter for a mark. Instead of writing them again in each program, you put them in functions, in a toolbox that you can use in every program.

Step 1: plan the tools

Write each tool as a sentence: its name, what comes in, what comes out.

Also write one expected answer for each, by hand, from what you know: a 6 by 4 room has an area of 24. A circle of radius 3 has an area of about 28.27. 37 degrees Celsius is 98.6 Fahrenheit. 2024 is a leap year, and 1900 is not. A mark of 82 is a B.

Step 2: the toolbox

Step 3: test each tool

Use the idea of the last lesson. For each tool, write a table of cases, and run a loop. For area_rectangle: (6, 4) gives 24, and also a zero side and a decimal side. For is_leap_year: 1900 (no), 2000 (yes), 2023 (no), 2024 (yes), 2100 (no). For celsius_to_fahrenheit: 0 gives 32, 100 gives 212, and -40 gives -40. That last one is a famous case: at -40, both scales show the same number.

Each tool does one job and returns its answer. The program combines them, like the buttons of a calculator.

Step 4: extend it

Pick one or two, and do them without looking at a solution. Write the expected answers first.

1. fahrenheit_to_celsius. The reverse tool. A good test: converting there and back gives the start value.
2. area_triangle. Half of base times height.
3. bmi. A weight in kg and a height in metres give a body mass index.
4. is_prime. A number is prime when only 1 and itself divide it. Use a loop inside the function.
5. average. A list of numbers in, the average out. What should it do for an empty list?

Common mistakes

• A tool that prints instead of returning. You cannot use its answer.
• A tool that reads an outside variable. It only works in one program.
• A tool that does two jobs. Split it.
• No test for the edges (the zero, the negative, the empty).
• Forgetting to run the tests again after a change.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab A toolbox of functions, if you have not done it, and the lab A temperature table.
2. Build. Open the lab Project: your toolbox. The checker calls your functions with many values.
3. Stretch. Add is_prime to your toolbox, test it with the numbers 1 to 20, and write down which numbers are prime.

Remember these

• Plan each tool: its name, what comes in, what goes out. Work out one expected answer by hand.
• Each function does one job, uses parameters in and return out, and has a docstring.
• The main program is a few short lines that use the tools.
• Test each tool alone, with the edges.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does a tool return its answer and not print it?

2. What is celsius_to_fahrenheit(100)?

3. What is special about -40?