Chapter 5 · Part 2 of 3Lists, Functions and Projects

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Functions: Name a Recipe

Name: Functions: Name a Recipe
Author: StemPy Lab

You have used functions made by others: print(), len(), range(). Now you write your own. A function is a group of lines with a name, so that you write them once and use them as often as you like. In this course you define and call a function, give it values to work with, get an answer back with return, set default values, make functions that answer yes or no, and let one function use another. You learn where a variable lives and why a variable made inside a function is not known outside it, and how to test a function with a table of cases. A toolbox of functions is the project at the end. The practice runs your functions on many inputs and tells you which ones fail.

6 lessons, about 2 h 55 min6 simulations14 problems12 practice questionsPython in every lesson

Created by StemPy Lab

Course outline (6 lessons)

Lesson 1about 30 min
Defining and calling a function
A function is a group of lines with a name. The word def starts it, then comes the name, brackets and a colon, and the indented lines are its body. Defining a function only stores it, and calling it with its name and brackets runs the lines. A function can be called as often as you like, so you write the lines once. Functions avoid repeating, make a fix easy to do in one place, and make a program clear to read. A function that is defined but never called does nothing.
1 simulation2 practice questions2 Python programs2 problems1 Python lab
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Lesson 2about 35 min
Parameters and return
Most functions take values in and give an answer back. The names in the def line are parameters, and the values in the call are arguments. return sends a value back to the place that called the function, and ends the function at once. print only shows something on the screen, and the program cannot use it. A function without a return gives back None. A call must give a value for every parameter, or Python shows a TypeError.
1 simulation2 practice questions4 Python programs6 problems2 Python labs1 question set
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Lesson 3about 25 min
Using functions well
A parameter can have a default value, which Python uses when the call gives no argument for it. Arguments can be named, so the order is free and the call reads clearly. A function can return True or False to answer a question, and a name that starts with is_ tells the reader that it does. One function can call another, so small functions that do one job can be put together like building blocks. A function that returns a value should return it in every path.
1 simulation2 practice questions2 Python programs4 problems2 Python labs
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Lesson 4about 30 min
Where a variable lives
A variable made inside a function is local: it lives only while the function runs, and Python does not know its name outside, so using it there gives a NameError. To get a result out, return it. A function can read a variable from outside, but if it assigns to the same name, it makes a new local variable and the outside one stays. Parameters are local variables too. Rely on parameters and return, and not on outside variables, so that a function works alone and is easy to test.
1 simulation2 practice questions3 Python programs1 problem1 Python lab
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Lesson 5about 30 min
Testing your functions
A function is only useful if it is right, so test it. A test is a table of cases: an input, and the answer that you expect from the rules, not from the function. Include the edges, such as 49, 50 and 51 for a grade, the smallest and the biggest allowed input, and a case with an unusual value. A loop over the table prints PASS or FAIL for each case. A short note in triple quotes, the docstring, says what the function does and can be read back with help. When a test fails, follow the function by hand for that input.
1 simulation2 practice questions2 Python programs1 problem1 Python lab
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Lesson 6about 25 min
Project: a toolbox of functions
In this project we build a toolbox of small functions that each do one job: the area of a rectangle and of a circle, a temperature conversion, a leap year test and a letter grade. Each function takes its values as parameters and gives its answer with return, has a docstring, and is tested with a table of cases. The main program is then a few short lines that use the tools. We plan the tools first, build and test them one by one, and extend the toolbox with tools of our own.
1 simulation2 practice questions1 Python program1 Python lab
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Functions: Predict the Output

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Functions: Name a Recipe

Course outline (6 lessons)

Defining and calling a function

Parameters and return

Using functions well

Where a variable lives

Testing your functions

Project: a toolbox of functions

Test Your Learning

Your teacher

Discussion (0)