You can assign an iterable to side_effect, and the mock will return the next value in the sequence each time it is called:
>>> from unittest.mock import Mock
>>> m = Mock()
>>> m.side_effect = ['foo', 'bar', 'baz']
>>> m()
'foo'
>>> m()
'bar'
>>> m()
'baz'
Quoting the Mock() documentation:
If side_effect is an iterable then each call to the mock will return the next value from the iterable.