old_contents: import itertools
  import traceback
  
  
  class MockDateTime(object):
      def __init__(self, ret):
          try:
              self.ret = iter(ret)
          except TypeError:
              self.ret = iter(itertools.repeat(ret))
  
      def utcnow(self):
          return next(self.ret)
  
new_contents: class MockDateTime(object):
      def __init__(self, ret):
          try:
              self.ret = iter(ret)
          except TypeError:
              self.ret = iter((ret,))
  
      def utcnow(self):
          try:
              return next(self.ret)
          except StopIteration:
              raise ValueError('No more mock values!')
  
subject: Raise error in mock if there are not enough mock datetime values