Lesson 04-07: Testing 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:

def is_natural(n):

    if n > 0 or int(n) == n:

        return True

    else:

        return False

​

#tests

print(is_natural(5))       #True

print(is_natural(3.6))     #False

print(is_natural(0))       #False

print(is_natural(1))       #True

print(is_natural(0.999))   #False

print(is_natural(-4))      #False

print(is_natural(-2.6))    #False

print(is_natural(9999999)) #True

​

True
True
True
True
True
True
False
True

(ex_0407_1)

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

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

Spoiler alert! Don’t read below until you’ve finished!

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

def is_natural(n):

 if n > 0 and int(n) == n:

     return True

 else:

     return False

​

#tests

print(is_natural(5))       #True

print(is_natural(3.6))     #False

print(is_natural(0))       #False

print(is_natural(1))       #True

print(is_natural(0.999))   #False

print(is_natural(-4))      #False

print(is_natural(-2.6))    #False

print(is_natural(9999999)) #True

​

True
False
False
True
False
False
False
True

(ex_0407_2)

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¶

def mystery(n):

    if n >= 10 or n <= 20:

        return True

    else:

        return False

​

#write tests below

​

def mystery(s):

    return len(s) * 2 + 1

​

#write tests below

​

#write function below

​

​

#write tests below

​