Thursday, April 17, 2008

TotT: Avoiding Flakey Tests

Flaky tests make your life more difficult. You get failure notifications that aren't helpful. You might become numb to failures and miss an actual failure condition. Your changes might get unfairly blamed for causing a flaky test to fail.

Unfortunately, a myriad of factors can make a test flaky. Today, we tackle a simple example: file access from a unit test. Take this function and its test:

def CreateGapLease(self):
  data_file = open('/leases/gap', 'w+')

def testCreateGapLease(self):

What if /leases/gap already exists and contains some data? testCreateGapLease will fail. This is a general problem where preconditions are assumed to be correct. This could just as easily happen by assuming a database contains the proper information (or no information). What if another test that uses that file was running concurrently?

If you really want to test your code using live resources, always check your assumptions. In this case, clearing the file at the start of the test can reduce its brittleness:

def testCreateGapLease(self):
  if os.path.exists(lease_file):

Unfortunately, this doesn't completely eliminate the flakiness of our test. If /leases/gap is an NFS path or can be written to by a different test, our test can still fail unexpectedly. It's better for the test to use a unique resource. This can be accomplished with a small refactoring of CreateGapLease:

def CreateGapLease(self, lease_path=None):
  if lease_path is None:
    lease_path = '/leases/gap'

The callers of CreateGapLease can continue invoking it as usual, but the unit test can pass in a different path:

def testCreateGapLease(self):
  lease_file = os.path.join(FLAGS.test_tmpdir, 'gap')

Of course, to make your test as fast as possible, it would be better to forgo disk access altogether by using a mock file system.

Remember to download this episode of Testing on the Toilet and post it in your office.

Due to illness availability of the PDF will be slightly delayed

The PDF is now available at the above link


  1. What was wrong with "def CreateGapLease(self, lease_path='/leases/gap'):"? Immutable types like strings aren't a risk in default args. My version probably runs slightly faster and will produce much better automatically-extracted docs.

    The related trap, which you might have been trying to avoid, looks like this:

    def func(x=[]):
    . x.append(5)
    . return x

    (Except without the silly dots to workaround blogger's formatting)

    That program only has one list in it which will keep growing every time you call func(). That's where the "x=None .. if x is None: x = []" idiom is important.

  2. Nice post.
    Do yuo have some example of good file system mocking? :)

  3. I assume FLAGS.test_tmpdir is a randomly-named temporary directory (e.g. created with tempfile.mkdtemp), otherwise you still might have problems if you run the test suite concurrently and both instances try to write to the same file at the same time.


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