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Generative Tests

Tuesday, September 16, 2008

Both nose and py.test allow you to write generative tests. These are basically generators that yield the actual test functions and their arguments. The test runners will then test all the yielded functions with their arguments.

However I'm left wondering why this is better then just iterating in the test function and doing many asserts? Let me demonstrate with the py.test example:

def test_generative():
    for x in (42,17,49):
        yield check, x

def check(arg):
    assert arg % 7 == 0   # second generated tests fails!

Why is that code better then:

def test_something():
    for x in (42,17,49):
        assert x % 7 == 0

I know that it could possibly tell you slightly more, but in general I keep testing until all of my tests succeed so I don't really mind if one test includes more then one assert statement. I'll just keep fixing things till the test passes.

What is the motivation for generative tests?

As an aside (and the inspiration for this post thanks to Holger Krekel talking about py.test), PyCon UK was great. I was considering a post titled "Resolver Systems Ltd is Evil since they provided the Sunday morning hangovers, but thought that was a bit too sensationalist.

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