Choosing Test Cases

Choosing tests for `count_lowercase_vowels`

def count_lowercase_vowels(s):
    """ (str) -> int
	
    Return the number of vowels (a, e, i, o, and u) in s.
	
    >>> count_lowercase_vowels('Happy Anniversary!')
    4
    >>> count_lowercase_vowels('xyz')
    0
    """
	
    num_vowels = 0
	
    for char in s:
        if char in 'aeiou':		
            num_vowels = num_vowels + 1
	return num_vowels

To test count_lowercase_vowels, we need to:

pick values for the string argument, and
call the function to ensure it returns what we expect for each case.

It is not realistic to test using every single possible string argument. Instead, we create relevant categories, and choose one representative string argument from each category. To choose the string argument, consider:

the length of the string, and
the characters that make up the string.

There are many possible for string lengths. For this example, we'll consider strings that have these lengths:

0 (empty)
1 (single character)
6 (longer)

Which characters should we use? For this example, we'll choose characters based on whether they are vowels or non-vowels. The actual character doesn't matter.
If we want a non-vowel, we could use 'b', 'n', '?', or any other character that is not a vowel.

We will make a table of the test cases with the following 3 columns:

value of the argument to the function
expected return value of the function
a description of the test case


					
						
							s
							Expected Value
							Description
						
						
							''
							0
							empty string
						
						
							'a'
							1
							single char, vowel
						
						
							'b'
							0
							single char, non-vowel
						
						
							'pfffft'
							0
							several chars, no vowels
						
						
							'bandit'
							2
							several chars, some vowels
						
						
							'aeioua'
							6
							several chars, all vowels*

* we included all 5 vowels to ensure each one is properly counted.

Choosing tests for is_palindrome

def is_palindrome(s):
    """ (str) -> bool

    Return True if and only if s is a palindrome.

    >>> is_palindrome('noon')
    True
    >>> is_palindrome('racecar')
    True
    >>> is_palindrome('dented')
    False
    """

Because the function returns a Boolean value, we need at least 2 test cases: one that returns True and one that returns False. In this case, we actually need quite a few more than 2 test cases.

As for the previous example, we need to choose different values for the string argument that represent different categories of strings.

Dichotomy

When we developed the code for is_palindrome(), we found that whether a string was even or odd affected the code. Therefore, the tests should consider strings that have even and odd lengths. For this example, we'll consider strings that have these lengths:

0 and 1 (which are both considered palindromes)
2 (smallest possible non-empty even length palindrome, smallest possible non-palindrome)
3 (smallest possible multiple-character odd length palindrome)
6 (longer, even length string)
7 (longer, odd length string)

The test cases are summarized in this table:


						
							
								s
								Expected Value
								Description
							
							
								''
								True
								empty string
							
							
								'a'
								True
								single character
							
							
								'aa'
								True
								2 chars, palindrome
							
							
								'ab'
								False
								2 chars, not palindrome
							
							
								'aba'
								True
								3 chars, palindrome
							
							
								'abc'
								False
								3 chars, not palindrome
							
							
							
								'redder'
								True
								longer, even, palindrome
							
							
								'renter'
								False
								longer, even, not palindrome
							
							
								'racecar'
								True
								longer, odd, palindrome
							
							
								'banana'
								False
								longer, odd, not palindrome

General Tips

There is often overlap between the categories, so one test case may fall into more than 1 category.

Choosing Test Cases

Choosing tests for count_lowercase_vowels

Choosing tests for is_palindrome

Dichotomy

General Tips

Choosing tests for `count_lowercase_vowels`

Choosing tests for `is_palindrome`