This is the third in a short series of articles about Test Driven Development.

This article was written with and is also published by Matthew Atkinson.

Having done a few practice attempts at TDDAIYMI, it’s clear that the hardest part is writing the first test. Unlike a more traditional approach to TDD, you can’t leverage the ability to assume the existence of classes and methods to write the test; and then implement them to make it pass. This can lead to some rather contrived test failures.

The Starting Commits

To illustrate the challenges of starting with TDDAIYMI, we are going to go through the first few of our RED-GREEN-REFACTOR cycles step-by-step and explain in detail our thought process at each stage. We’ve included timestamps for the commits to make it easy to see how long each step took. These commits form part of the flamboyantly-named attempt_006_tddaiymi_2 branch on GitHub.

Each commit is presented, followed by the commentary on it.

RED Test that a legal move is legal - 2015-02-19 13:25

def test_legal_move_is_legal():
    move = 0
    assert move > 0

As mentioned last time, we are going to be tackling the problem of noughts and crosses (tic-tac-toe). We decided, somewhat arbitrarily, to start with a method to check the legality of a move. It seems more natural to approach TDDAIYMI in a bottom-up, as opposed to a top-down, manner.

We have decided to use a numeric position for the move. What that exactly means will be determined by the tests we write (the idea being that there are nine squares, which we will identify as 0 to 8 – interestingly, and as was discussed in Keith Braithwaite’s workshop, this means there is no notion of ‘a board’ specifically).

Our first test is somewhat contrived due to the restrictions of the process. One could argue that the inclusion of the move variable is taking things a step too far and that the initial test should have been assert 0 > 0 followed by using the Extract Variable refactoring to get to the current state, after having made the test pass.

GREEN - 2015-03-02 13:09

def test_legal_move_is_legal():
    move = 0
    assert move == 0  # changed

Making the test pass is trivial; as, of course, it should be. We change the contrived ‘less than’ test to use an equals comparison. As we only have the one test, there is no duplication to refactor, so we move straight onto the next test.

RED Test that an illegal move is not legal - 2015-03-02 13:20

def test_legal_move_is_legal():
    move = 0
    assert move == 0

# added function
def test_illegal_move_is_not_legal():
    move = -1
    assert move == 0

We are essentially starting from scratch again with test number 2. That said, we want to try and keep going in the same direction to triangulate some behaviour to extract in the form of duplicate code.

We have settled on move == 0 being the standard test for legality, so we now want to use the same test, when presented with an invalid move. This will take us a step closer to being able to remove duplication via Extract Method in future.

GREEN - 2015-03-02 13:25

def test_legal_move_is_legal():
    move = 0
    assert move == 0


def test_illegal_move_is_not_legal():
    move = -1
    assert (move == 0) is False  # changed

Note that the GREEN step is supposed to take the least-complex means to make the tests pass – it is the tests, not their solutions, that are intended to direct the overall design of the code. This is why, in this case, the GREEN step does seem rather simplistic. Arguably, we should have changed the == to != to make the test pass. By making the test pass in this way, we are introducing some duplication that needs addressing and moves us nearer to our goal.

REFACTOR Extract is_legal function - 2015-03-02 13:28

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move)  # changed


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False  # changed


# added function
def is_legal(move):
    return move == 0

This REFACTOR step may seem somewhat ‘aggressive’, in the sense that it lengthens the code, but it’s also vital (for any form of TDD) to refactor as early as possible, in order to keep the code – and any emerging avenues for good design – as ‘clean’ as possible. In this case, we had a DRY violation, which needed to be fixed; introducing is_legal() solves that problem, and starts giving some structure to the code. Using the tests to come at problems from both sides, in a sort of pincer strategy, can enable us to tease out structure such as this, and is a technique we used often.

RED Test a different legal move - 2015-03-03 12:40

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move)


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


# added function
def test_different_legal_move_is_legal():
    move = 8
    assert is_legal(move)


def is_legal(move):
    return move == 0

We need to expand the range of legal moves. We start by testing the highest legal value. Of course, there are many possible moves in the game and, whilst we’d not want to test all of them separately, we do need to add at least one more in order to pin down the range of valid moves – think of it along the lines of needing at least three points in order to be reasonably confident you can draw a meaningful line through them.

GREEN - 2015-03-03 12:48

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move)


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) or move > 0  # changed


def is_legal(move):
    return move == 0

We make this test pass, in the test method, and by using the simplest possible means which, again, may seem somewhat insufficient at first glance. We can use refactoring to improve the design. For some reason we changed our test case to ‘1’ instead of ‘8’ at this point; which was rather naughty.

REFACTOR Improve design by moving code - 2015-03-03 12:50

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move)


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move)  # changed


def is_legal(move):
    return move == 0 or move > 0  # changed

Next, for the first time, we have a few consecutive refactorings, and thus make two separate REFACTOR commits. In the first, we move the extra legality check to the is_legal() function, which is allowed per the rules, as it ‘improves the design of the code’ and the rules allow us to move code from a test function to an existing function. This is not strictly a refactoring, as it changes the behaviour of the code (whereas refactoring should preserve behaviour). However, the tests all still pass and this has given us a way to improve the design.

REFACTOR Simplify expression - 2015-03-03 12:52

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move)


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move)


def is_legal(move):
    return move >= 0  # changed

Now we are able to simplify the legality check, because all of the code is together…

REFACTOR Explicitly state expected result of tests - 2015-03-03 12:53

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move) is True  # changed


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) is True  # changed


def is_legal(move):
    return move >= 0

At this point, we realised that, as per The Zen of Python and in the interests of making it easy to read and understand the tests, ‘explicit is better than implicit’ – we had not been clearly stating what we expected the results of the tests to be, so we add the cases where we want a True result.

RED Test the first illegal move on upper boundary - 2015-03-03 12:55

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move) is True


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) is True


# added function
def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move) is False


def is_legal(move):
    return move >= 0

The next logical failing test we can write is to use an illegal move at the other end of the range. When we are no longer able to write a failing test, we know we are done. (Note that if we wrote a test for checking a move of ‘8’ was legal, it would pass, so it doesn’t need to be written.)

GREEN - 2015-03-03 12:58

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move) is True


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) is True


def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move) or move < 9 is False  # changed


def is_legal(move):
    return move >= 0

Again, making the test pass is done in the test method itself, using the simplest code.

REFACTOR Improve design by moving code - 2015-03-03 13:05

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move) is True


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) is True


def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move) is False  # changed


def is_legal(move):
    return move >= 0 and move < 9  # changed

We refactor to improve the design by moving the extra code to the is_legal() method. The tests are run automatically after each change we make, so we know if we have broken anything. Once again, this is a pseudo-refactoring as, whilst it doesn’t change the stated (tested) behaviour of the overall program, it does change the behaviour of the function.

RED Test previously-played move is illegal - 2015-03-04 12:39

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move) is True


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) is True


def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move) is False


# added function
def test_move_is_illegal_if_already_played():
    moves_played = [0]
    move = 0
    assert is_legal(move) is False


def is_legal(move):
    return move >= 0 and move < 9

We have now run out of the basic failing tests that we can think of for the is_legal() method. We move on to expand its behaviour by considering moves that have been played previously to be illegal. Again, this is somewhat contrived as we are inventing some state that we aren’t initially using in the test.

This direction of exploration seems like the most natural extension of the existing test sequence. We are not attempting to force any sort of design; we are just writing tests and will see what ‘design’ emerges.

GREEN - 2015-03-04 12:44

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move) is True


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move) is True


def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move) is False


def test_move_is_illegal_if_already_played():
    moves_played = [0]
    move = 0
    assert (is_legal(move) and move not in moves_played) is False  # changed


def is_legal(move):
    return move >= 0 and move < 9

We now use the state information to make the test pass in the test method.

REFACTOR Introduce Parameter - 2015-03-04 12:56

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move, []) is True  # changed


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move, []) is False  # changed


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move, []) is True  # changed


def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move, []) is False  # changed


def test_move_is_illegal_if_already_played():
    moves_played = [0]
    move = 0
    assert (is_legal(move, moves_played) and move not in moves_played)
is False

def is_legal(move, moves_played):  # changed
    return move >= 0 and move < 9

We want to improve the design of the code by moving the new code that made the test pass to the is_legal() method but to do this we need to pass the state in too. We proceed cautiously and do this refactoring in a couple of steps. First we add the extra parameter to the is_legal() method (which requires us to update all the other tests that use is_legal()).

REFACTOR Move history-checking code to is_legal() - 2015-03-04 12:59

def test_legal_move_is_legal():
    move = 0
    assert is_legal(move, []) is True


def test_illegal_move_is_not_legal():
    move = -1
    assert is_legal(move, []) is False


def test_different_legal_move_is_legal():
    move = 1
    assert is_legal(move, []) is True


def test_high_illegal_move_is_not_legal():
    move = 9
    assert is_legal(move, []) is False


def test_move_is_illegal_if_already_played():
    moves_played = [0]
    move = 0
    assert is_legal(move, moves_played) is False  # changed


def is_legal(move, moves_played):
    return move >= 0 and move < 9 and move not in moves_played  # changed

Now that we have passed in the parameter, we can move the extra check into is_legal() and make sure that all of the tests still pass, which they do.

Next Time

We could continue on but this gives sufficient flavour of the thought process as we worked our way through the exercise. The rest of the development continues in the same manner; looping through the RED-GREEN-REFACTOR cycle. In the next article in the series we shall pick out some interesting moments from the rest of the development and discuss them.