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+')
  data_file.write(contract_data)
  data_file.close()

def testCreateGapLease(self):
  contract_writer.CreateGapLease()
  self.assertEqual(ReadFileContents('/leases/gap'),
         contract_data)