def _transpose(group):
    """
    Given a list of 3-tuples from _grouper, return 3 lists.
    Also filter out possible None values from _grouper
    """
    a, b, c = [], [], []
    for g in group:
        # g can be None
        if g is not None:

            x, y, z = g
            #if x is not None and y is not None and z is not None:
            a.append(x)
            b.append(y)
            c.append(z)
    return a, b, c