python - Unit-testing with dependencies between tests

Question

How do you do unit testing when you have

• some general unit tests
• more sophisticated tests checking edge cases, depending on the general ones

To give an example, imagine testing a CSV-reader (I just made up a notation for demonstration),

def test_readCsv(): ...

@dependsOn(test_readCsv)
def test_readCsv_duplicateColumnName(): ...

@dependsOn(test_readCsv)
def test_readCsv_unicodeColumnName(): ...

I expect sub-tests to be run only if their parent test succeeds. The reason behind this is that running these tests takes time. Many failure reports that go back to a single reason wouldn't be informative, either. Of course, I could shoehorn all edge-cases into the main test, but I wonder if there is a more structured way to do this.

I've found these related but different questions,

• How to structure unit tests that have dependencies?
• Unit Testing - Is it bad form to have unit test calling other unit tests

UPDATE:

I've found TestNG which has great built-in support for test dependencies. You can write tests like this,

@Test{dependsOnMethods = ("test_readCsv"))
public void test_readCsv_duplicateColumnName() {
   ...
}

Answer 1

Personally, I wouldn't worry about creating dependencies between unit tests. This sounds like a bit of a code smell to me. A few points:

• If a test fails, let the others fail to and get a good idea of the scale of the problem that the adverse code change made.
• Test failures should be the exception rather than the norm, so why waste effort and create dependencies when the vast majority of the time (hopefully!) no benefit is derived? If failures happen often, your problem is not with unit test dependencies but with frequent test failures.
• Unit tests should run really fast. If they are running slow, then focus your efforts on increasing the speed of these tests rather than preventing subsequent failures. Do this by decoupling your code more and using dependency injection or mocking.