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Quote: Characterization tests

Apr 18, 2016 - 2 minute read - Comments - quotewewlcmichael-featherslegacy-codecharacterization-testtestlegacy

In Working effectively with legacy code, Michael Feathers talks a type of tests to define or characterize the current behavior of the legacy system:

In nearly every legacy system, what the system does is more important than what it is supposed to do.

A characterization test is a test that characterizes the actual behavior of a piece of code. […] The test document the actual current behavior of the system. Here is a little algorithm for writing algorithm for writing characterization tests:

  • Use a piece of code in a test harness
  • Write an assertion that you know will fail
  • Let the failure tell you what the behavior is.
  • Change the test so that it expects the behavior that the code produces
  • Repeat

M Feathers, in the chapter 131, section “Characterization tests” of Working effectively with legacy code. Actual page, 186.

Later on, he goes to describe them in a bigger environment:

Characterization tests record the actual behavior of a piece of code. If we find something unexpected when we write them, it pays to get some clarification. It could be a bu. That doesn’t mean that we don’t include the test in our test suite; instead, we should mark it as suspicious an find out what th effect would be of fixing it.

M Feathers, in the chapter 132, section “Characterization tests” of Working effectively with legacy code. Actual page, 188.


  1. Chapter 13, “I need to make a change, but I don’t know what tests to write” [return]
  2. Chapter 13, “I need to make a change, but I don’t know what tests to write” [return]

Brownish Greenfield Gilded Rose Kata Self-Study in May 2016

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