I came up with a similar solution; using True/None/False instead of 1/0/-1 allows for more self-documenting code

    def monotonics(seq):
        run = []
        last_v = None
        increasing = None
        for v in seq:
            if last_v is None:
                pass
            elif increasing is None:
                if v != last_v:
                    increasing = v > last_v
            elif increasing:
                if v < last_v:
                    increasing = None
                    yield run
                    run = []
            else:
                if v > last_v:
                    increasing = None
                    yield run[::-1]
                    run = []
            last_v = v
            run.append(v)

        if run:
            yield run

and, I'd add a two more test cases: [], and [1, 2, 3, 1, 2, 3]