Unit testing

Assertions are the basis of modern automated testing. Developers write test code in source files that are separate from the main program code. We have our program code in sample.py and the test code will be in test_sample.py. This is a common naming convention.

In practice, the test code will be kept in a separate directory from the program code.

Testing sample.py

Now, let’s use our assert to test the correctness of the functions in sample.py.

1. Comment out all the code in test_sample.py
2. Add the line import sample. In Python, this makes the content of sample.py accessible to code in test_sample.py.¹
3. Now let’s convert those print(...) statements from sample.py into assert statements in test_sample.py. test_sample.py should now have the following:
   test_sample.py

   
   import sample  # We import the filename without the .py
   
   assert sample.palindrome_check("kayak")  # the function should return True, giving "assert True"
   assert sample.palindrome_check("Kayak")
   assert sample.palindrome_check("moose") is False  # the function should return False, giving "assert False is False", which is True
   
   assert sample.is_prime(1) is False
   assert sample.is_prime(2)
   assert sample.is_prime(8) is False
   
   assert sample.reverse_string("press") == "sserp"  # checking result for equality with expected
   assert sample.reverse_string("alice") == "ecila"
   assert sample.reverse_string("") == ""
   print("All assertions passed!")
   

Point 1: We access the functions in sample.py by calling, e.g., sample.palindrome_check(...). The prefix sample.X tells Python “go into the sample module and call the function named X.” We would get an error if we called only sample.palindrome(...) because Python would be looking in the current running file, which has no such function defined in it.

Point 2: In Python, you should check if a value is True or False using is. The is operator returns a boolean. You could also type x == True or x == False. Either form will work, but is is preferred².

Point 3: Remember that palindrome_check() and is_prime() return True/False themselves. We are simply verifying that they are returning the correct value. reserve_string() returns a string value, so we need to compare using == to an expected value.

Point 4: The program will crash with an AssertionError if any of the assert statements are False. Mess up one of the assertions to verify this.

Exercise

1. Go to sample.py and define a function named power() that takes two parameters, x and y, and returns the computed result of xʸ.
2. Add assert statements to test_sample.py to verify your function behaves correctly.

Unit tests

The file test_sample.py is what software engineers call an automated unit test. Unit tests test individual an individual classes or source files³. Unit tests are usually written by the same developer who wrote the program code.

Our automated unit test now calls functions and use assert statements to verify that they are returning the expected results. If an assertion fails, the test fails.

What does it mean if a test fails? One of two things:

1. Either there is something wrong in the program code. Maybe there is a logic error.
2. The test code itself has a mistake in its logic.

Regardless, if a test fails, you need to figure out why. A good unit test will systematically exercise all the logic of the function or module under test. This can help uncover flaws in the program code. We will discuss strategies to do this in subsequent lessons.

We also need a way to run the test code and accumulate the results in a useful way. We will do this in the next lab.

Knowledge check

• Question: Suppose you wanted to test a function named get_patient_priority(str) in hospital.py. What would you have to do to call the function from your test code?
• Question: The right hand side of an assert statement can be any expression (simple or complex) as long as it evaluates to _____ or _____.
• Question: Who writes unit tests?
• Question: The name for a test that tests an individual module is a ______ test.
• Question: Why do you think we write separate assert statements for each function input, rather than one assert statement that calls the function multiple times with different inputs? That is, why not do assert sample.reverse_string("alice") == "ecila" and sample.reverse_string("") == ""?