# Lesson 04-07: Testing Functions¶

Learning Target: I can write thorough tests for functions.

## Why Test Functions?¶

Functions allow you to write your program in parts. For example, if I were to write a simple quiz program, I might have functions that handle the following, one per function:

• Display the question to the user
• Get the user to type in their answer
• Check if their answer is correct
• Calculate their final score

The benefit of writing them in functions is that it allows you to test each part individually.

## Creating Tests¶

Let’s say that we have a program that requires us to check if a number is a natural number (a natural number is a positive whole number - 1,2,3,...), and we were to call it is_natural():

def is_natural(n):
if n > 0 and int(n) == n:
return True
else:
return False


We can test this function by feeding it a bunch of numbers and testing it against our results:

• is_natural(5) should return True
• is_natural(3.6) should return False
• is_natural(0) should return False
• is_natural(1) should return True
• is_natural(0.999) should return False
• is_natural(-4) should return False
• is_natural(-2.6) should return False
• is_natural(9999999) should return True

When creating tests, you generally want to test for every possible case - notice how I chose a positive integer, a negative integer, a positive decimal, a negative decimal, 0, 1, a number between 0 and 1, and a very large number.

So if we run these:

We can see that not all our numbers match! That’s a problem - examine the code and see what’s wrong with it.

Remember what it means to be a natural number!

Which line in the function definition of is_natural contains the error?

The line is on line 2 - I put or when it really should be and. So now let’s fix that:

Now we see that all our tests pass! Score! Now we can use this function safely, knowing that it works for all the examples we need it to handle!

To recap - the big points:
• Functions are good because it allows you to break your code down into parts.
• Testing functions is good because it’s eaiser to debug a small part than an entire program.
• Testing functions is also good because after it passes all tests, you know it works.
• When coming up with tests, make sure you try all the possibilities that you need it to handle.

## Checks For Understanding¶

### Q#1¶

The given function checks to see if a number is between 10 and 20 inclusive. Write at least three test cases along with their predictions, and run your code to make sure that your tests are accurate (or that the function itself works). If the function is broken, fix the function so it works as expected.

### Q#2¶

The given function takes in a string as an argument and prints out the length of the string times 2. Write at least three test cases along with their predictions, and run your code to make sure that your tests are accurate (or that the function itself works). If the function is broken, fix the function so it works as expected.

### Q#3¶

Write a function called repeat5 that that takes in a string as input, and repeats the first two letters of the string three times (example: repeat5('hello') will return 'hehehe'. Write at least three test cases along with their predictions, and run your code to make sure that your tests are accurate (or that the function itself works).

Next Section - Lesson 04-08: Functions within Functions