In an ideal world, everyone will know how to write unit tests, understand their value, and want to write them. In an organization transitioning to agile having all three parts in place is not a given. Most people understand why unit tests are useful, in principle. The problem is the execution. In my experience, there are two reasons people who claim that that want to write unit tests give for not writing them:
- They are too hard. The overhead of the test can make the cost of developing a feature excessive.
- They are too easy. Some functionality appears to be trivial, so why would we want to test it.
Both of these reasons can have merit at times. Testing getters and setters, and trivial calls to a system library and other simple coding constructs really don't add value. And writing a complicated test using a hard to use framework to verify something non business critical that could be quickly validated by visual inspection ("make the background color of the home screen orange") may well be not worth the effort. The problem is that most people have bad intuitions about where the lines line until they start practicing the skill of unit testing. Most teams need experience with testing to get a true feel of what is a trivial test, and what is a seemingly trivial test that can unmask a major problem
Rather than frame the testing challenge with the default being the old way of not testing:
Write a test when it makes sense.
Change your perspective to the default being to test:
Write a test unless you can explain why you did not.
The key to this approach is to make encourage people to think through their rationale for not testing. There are a few ways to do this, but one approach is to tie the explanation mechanism into something every developer works with every day: your source code repository. Have the team agree that, in addition to the source files, every commit will have a change to a test or a rationale for why you didn't write a test. For example if I do a commit without a test I could write:
ISSUE-23: Fixed the spelling of the company name. NO TEST: it was a typo.
ISSUE-26: Fixed the rendering mechanism. NO TEST: we don't have a good framework for testing this sort of thing
ISSUE-28: Fixed a serious logic issue. NO TEST: I didn't feel like writing one.
By developing a team agreement to add tests or explain why not, you are starting with a small change of behavior that paves the way for a greater change based on understanding. Even if a message like the last one is acceptable, many will be uncomfortable admitting to laziness, and think harder for a reason. By reviewing the commit messages later on you can get a sense of impediments to testing (technology, organizational, or attitude), and use that data in a retrospective to decide how to improve.
By being creative you can help people on your team understand the value of process changes, and start a conversation about how to evolve practices to suit your team.