Return a new return_value for each call when mocking tempfile in python

Here is a python snippet that mocks out tempfile.NamedTemporaryFile() to use a StringIO and add it to an array of file handles.

The somewhat unintuitive bit here is that by passing side_effect a function mock will use the result of the function as the return_value for the mocked tempfile.NamedTemporaryFile(). Or as the docs say: "The function is called with the same arguments as the mock, and unless it returns DEFAULT, the return value of this function is used as the return value."

def _GetTempFile(self, *args, **kwargs):
  tempfile = StringIO.StringIO() 
  tempfile.flush = mock.MagicMock()
  self.output_streams.append(tempfile)
  return tempfile

with mock.patch.object(tempfile, "NamedTemporaryFile", side_effect=self._GetTempFile):
  # mocked code...