Unit Testing – Testing Assembler Code on Different Architectures

We have all unit tests (for a module) in one executable. The tests are put into groups. I can execute a single test (or some tests) or a group of tests by specifying a (test/group) name on the command line of the test runner. The build system can run group "Build", the test department can run "All". The developer can put some tests into a group like "BUG1234" with 1234 being the issue tracker number of the case he's working on.

I'm going to use Python-like pseudocode based around your test cases to (hopefully) help explain this answer a little, rather than just expositing. But in short: This answer is basically just an example of small, single-function-level dependency injection / dependency inversion, instead of applying the principle to an entire class/object.

Right now, it sounds like your tests are structured as such:

def test_3_minute_confirm():
    req = make_request()
    sleep(3 * 60)  # Baaaad
    assertTrue(confirm_request(req))

With implementations something along the lines of:

def make_request():
    return { 'request_time': datetime.now(), }

def confirm_request(req):
    return req['request_time'] >= (datetime.now() - timedelta(minutes=5))

Instead, try forgetting about "now" and tell both functions what time to use. If it's most of the time going to be "now", you can do one of two main things to keep the calling code simple:

• Make time an optional argument that defaults to "now". In Python (and PHP, now that I reread the question) this is simple - default "time" to None (null in PHP) and set it to "now" if no value was given.
• In other languages without default arguments (or in PHP if it becomes unwieldy), it might be easier to pull the guts of the functions out into a helper that does get tested.

For example, the two functions above rewritten in the second version might look like this:

def make_request():
    return make_request_at(datetime.now())

def make_request_at(when):
    return { 'request_time': when, }

def confirm_request(req):
    return confirm_request_at(req, datetime.now())

def confirm_request_at(req, check_time):
    return req['request_time'] >= (check_time - timedelta(minutes=5))

This way, existing parts of the code don't need to be modified to pass in "now", while the parts you actually want to test can be tested much more easily:

def test_3_minute_confirm():
    current_time = datetime.now()
    later_time = current_time + timedelta(minutes=3)

    req = make_request_at(current_time)
    assertTrue(confirm_request_at(req, later_time))

It also creates additional flexibility in the future, so less needs to change if the important parts need to be reused. For two examples that pop into mind: A queue where requests might need to be created moments too late but still use the correct time, or confirmations have to happen to past requests as part of data sanity checks.

Technically, thinking ahead like this might violate YAGNI, but we're not actually building for those theoretical cases - this change would just be for easier testing, that just happens to support those theoretical cases.

Is there any reason you would recommend one of these solutions over another?

Personally, I try to avoid any sort of mocking as much as possible, and prefer pure functions as above. This way, the code being tested is identical to the code being run outside of tests, instead of having important parts replaced with mocks.

We've had one or two regressions that no one noticed for a good month because that particular part of the code was not often tested in development (we weren't actively working on it), was released slightly broken (so there were no bug reports), and the mocks used were hiding a bug in the interaction between helper functions. A few slight modifications so mocks weren't necessary and a whole lot more could be easily tested, including fixing the tests around that regression.

Change the system date and time for the test, using exec(). This can work out if you are not afraid to mess up with other things on the server.

This is the one that should be avoided at all costs for unit testing. There's really no way of knowing what kinds of inconsistencies or race conditions might be introduced to cause intermittent failures (for unrelated applications, or co-workers on the same development box), and a failed/errored test might not set the clock back correctly.