Theory: Assertions

Each check we have written for the capitalize() function is commonly referred to in testing as a statement. We'll learn more about this concept in this lesson.

What is a statement

Assertions are an integral part of tests. They are the ones that check the functionality of your code:

from capitalize import capitalize

if capitalize('hello') != 'Hello':
    raise Exception('The function is not working correctly!')

if capitalize('') != '':
    raise Exception('The function is not working correctly!')

print('All tests are passed!')

You may notice that all the checks follow the same structure — they have two components:

• Condition
• Exception if the condition isn't true

The function suggests calling itself here, but you don't have to. Python already includes a dedicated `assert' construct that does the same thing.

The assert construct looks like a function, but it isn't. It is a specific language statement that takes as input an expression whose value must be True, or else an exception occurs. Therefore, all the checks in the tests are inverted:

from capitalize import capitalize

assert capitalize('') == ''
assert capitalize('hello') == 'Hello'

In such tests, assert true means everything is fine, and assert false indicates an error. The latter is that if the capitalize() function doesn't work correctly, it throws an exception with the following message:

Traceback (most recent call last):
      File "main.py", line 11, in <module>
        assert capitalize('hello') == 'Hello'
    AssertionError

Here's the actual statement that threw the exception. Besides the message, there is a backtrace that you can use to find the statement.