map(lambda x: x, ['a', 'b', 'c'], [1, 2]) This should be map(lambda x, y: (x, y), ['a', 'b', 'c'], [1, 2]) If you want the same behavior as passing None

map(lambda x: x, ['a', 'b', 'c'], [1, 2]) This should be map(lambda x, y: (x, y), ['a', 'b', 'c'], [1, 2]) If you want the same behavior as passing None as the first argument.