Unit tests for automatically generated code: automatic or manual?

Question

I know similar questions have been asked before but they don't really have the information I'm looking for - I'm not asking about the mechanics of how to generate unit tests, but whether it's a good idea.

I've written a module in Python which contains objects representing physical constants and units of measurement. A lot of the units are formed by adding on prefixes to base units - e.g. from m I get cm, dm, mm, hm, um, nm, pm, etc. And the same for s, g, C, etc. Of course I've written a function to do this since the end result is over 1000 individual units and it would be a major pain to write them all out by hand ;-) It works something like this (not the actual code):

def add_unit(name, value):
    globals()[name] = value
    for pfx, multiplier in prefixes:
        globals()[pfx + name] = multiplier * value

add_unit('m', <definition of a meter>)
add_unit('g', <definition of a gram>)
add_unit('s', <definition of a second>)
# etc.

The problem comes in when I want to write unit tests for these units (no pun intended), to make sure they all have the right values. If I write code that automatically generates a test case for every unit individually, any problems that are in the unit generation function are likely to also show up in the test generation function. But given the alternative (writing out all 1000+ tests by hand), should I just go ahead and write a test generation function anyway, check it really carefully and hope it works properly? Or should I only test, say, one series of units (m, cm, dm, km, nm, um, and all other multiples of the meter), just enough to make sure the unit generation function seems to be working? Or something else?

Answer 1

You're right to identify the weakness of automatically generating test cases. The usefulness of a test comes from taking two different paths (your code, and your own mental reasoning) to come up with what should be the same answer -- if you use the same path both times, nothing is being tested.

In summary: Never write automatically generated tests, unless the algorithm for generating the test results is dramatically simpler than the algorithm that you are testing. (Testing of a sorting algorithm is an example of when automatically generated tests would be a good idea, since it's easy to verify that a list of numbers is in sorted order. Another good example would be a puzzle-solving program as suggested by ChrisW in a comment. In both cases, auto-generation makes sense because it is much easier to verify that a given solution is correct than to generate a correct solution.)

My suggestion for your case: Manually test a small, representative subset of the possibilities.

[Clarification: Certain types of automated tests are appropriate and highly useful, e.g. fuzzing. I mean that that it is unhelpful to auto-generate unit tests for generated code.]

Answer 2

If you auto-generate the tests:

• You might find it faster to then read all the tests (to inspect them for correctness) that it would have been to write them all by hand.

• They might also be more maintainable (easier to edit, if you want to edit them later).