def find_period(l):
    """ Finds the period of list of numbers.
    Parameters
    ----------
    l: integer[]
        The sequence of numbers.
    Return
    ------
    steps: integer
        The period.
    Returns None, if no period is found.
    """
    steps = 1
    for i in range(1, len(l)):
        if l[i] == l[0]:
            if l[:i] == l[i:i+i]:
                return steps
        steps += 1
    return None