What is a Unit Test

• Allows automated testing of independent code units
• No replacement for other test categories
• Write them as early as possible - if possible before the code it tests
• Tests can serve as useful requirements specification
• Create a test for each bug you encounter (refacturing!)

Which problems does unit testing solve?

Code without tests (aka. legacy code) is extremely difficult to work with

Which problems does unit testing cause?

Essentially none, but writing unit tests ...

• seems to cost time
• demands discipline
• is far from trivial
• does not substitute other kinds of testing

What is a Test(case)

• Answers a single question about the code it is testing
• Run completely by itself (automation)
• Results must not require human interpretation
• Should be independent of other tests and their order of execution
• Can serve as documentation of how (not) to use your code

Flaky Tests

• Have different outcomes without changes to the code
• Erode confidence in testing processes
• Fix them (mute / skip them until they are fixed) or delete them

User's view (visible to the user)

• Functionality
• Reliability
• Usability
• Performance/ efficiency

Producer's view

• Portability
• Maintainability
• Testability (vs. legacy code)
• Transparency (is the code readable/ understandable?)

def add_to(n):
    """
    Return the sum of all values from 1 to n.
    """
    sum = 0
    for i in range(n+1):
        sum += i
    return sum

# after refacturing
def add_to(n):
    """
    Return the sum of all values from 1 to n.
    """
    return (n / 2.0) * (n + 1)

How can we apply the change without risking to destroy our application?

1. Define good test cases
2. Implement test cases using your project's unit testing library
3. Run the tests before the change to ensure they pass
4. Apply the change, then run the tests again

1. Define Good Test Cases

Using pencil and paper / brainstorming: define relevant test cases

• All code paths should be used
• Add one or more common cases
• Include relevant edge cases

25 is a possible common case. Add 26 as an even input. The edge cases should include 0 and 4294967295.
Expected results for these are 325, 351, 0 and 9223372034707292160.

Let's also add 500000000 (expected result 125000000250000000) as 4294967295 does not terminate with the current implementation.

2. Implement defined test cases

from before_refactoring import add_to
# from after_refactoring import add_to

def test_add_to():
    assert add_to(0) == 0
    assert add_to(25) == 325
    assert add_to(26) == 351
    assert add_to(500000000) == 125000000250000000

def test_add_to_slow():
    assert add_to(4294967295) == 9223372034707292160

3. Run the Unittests Before the Change

# takes rather long; can be interrupted via [Ctrl]+[C]
# tests are discovered automatically
pytest  # or `python -m pytest`

What to do now?

pytest --help
pytest --collect-only  # list all available tests
pytest -k "not slow"  # ignore tests with `slow` in their name

4. Apply the Change, then Run the Tests Again

For the sake of the presentation, instead of applying the change, we use another source file.

Adjust the test to import from the new implementation and run the tests.

pytest

Installation

pip install --user --upgrade pip
pip install --user --upgrade pytest

sudo apt install python3-pytest

Documentation

Comprehensive documentation can be found on pytest.org.