def create_mock_Popen(x_success='', x_error=''):
"""Return mock subprocess.Popen class.
stdout and stderr will be returned by communicate() of its returned
instance.
"""
mock_Popen_instance = Mock()
mock_Popen_instance.communicate = Mock(return_value=(x_success, x_error))
mock_Popen = Mock(return_value=mock_Popen_instance)
return mock_Popen
