Testing on the Toilet: Tests Too DRY? Make Them DAMP!

This article was adapted from a Google Testing on the Toilet (TotT) episode. You can download a printer-friendly version of this TotT episode and post it in your office.

By Derek Snyder and Erik Kuefler

The test below follows the DRY principle (“Don’t Repeat Yourself”), a best practice that encourages code reuse rather than duplication, e.g., by extracting helper methods or by using loops. But is it a well-written test?
def setUp(self):
self.users = [User('alice'), User('bob')] # This field can be reused across tests.
self.forum = Forum()

def testCanRegisterMultipleUsers(self):
for user in self.users: # Use a for-loop to verify that all users are registered.
def _RegisterAllUsers(self): # This method can be reused across tests.
for user in self.users:

While the test body above is concise, the reader needs to do some mental computation to understand it, e.g., by following the flow of self.users from setUp() through _RegisterAllUsers(). Since tests don't have tests, it should be easy for humans to manually inspect them for correctness, even at the expense of greater code duplication. This means that the DRY principle often isn’t a good fit for unit tests, even though it is a best practice for production code.

In tests we can use the DAMP principle (“Descriptive and Meaningful Phrases”), which emphasizes readability over uniqueness. Applying this principle can introduce code redundancy (e.g., by repeating similar code), but it makes tests more obviously correct. Let’s add some DAMP-ness to the above test:

def setUp(self):
self.forum = Forum()

def testCanRegisterMultipleUsers(self):
# Create the users in the test instead of relying on users created in setUp.
user1 = User('alice')
user2 = User('bob')

# Register the users in the test instead of in a helper method, and don't use a for-loop.
# Assert each user individually instead of using a for-loop.

Note that the DRY principle is still relevant in tests; for example, using a helper function for creating value objects can increase clarity by removing redundant details from the test body. Ideally, test code should be both readable and unique, but sometimes there’s a trade-off. When writing unit tests and faced with a choice between the DRY and DAMP principles, lean more heavily toward DAMP.