Shared base fixture

Q: How do you know you can trust your test set-up?
A: You reuse a well-known test set up in every test.

Example code

def test_simple():
  req = set_up_working_request()
  resp = invoke(req)
  assert resp.status == OK

def test_invalid_page_size():
  req = set_up_working_request()
  req.page_size = -5
  resp = invoke(req)
  assert resp.status == INVALID_ARGUMENT

In the test,

• We call set_up_working_request() and we know this results in “working” behavior. Then we purposefully inject a defective negative page size into the working request.
• We exercise the system under test in the same way.
• We expect a different result.

Because the only difference in set up between test_simple and test_invalid_page_size was in the differing page size, we can easily reason that the different response status is resulting from just that.

Related techniques

This works well together with these techniques:

• Structure your test methods with separate set up, exercise and verify phases.¹
• Use explicit helper functions for set up and possibly name them according to expected behavior such as “working” in the example above.
• Keep the shared set up code in test methods as short as possible, so that it’s easy to spot the equal parts.
• 1:N: Use one base scenario (e.g. “working”) and use its set up as a base for multiple derived scenarios.

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1. This is called the “Four-Phase test” in the xUnit test pattern book. (Meszaros07, p358) ↩︎

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