def binary_str(num):
    """ Return a binary string representation from the posive interger 'num'
    :type num: int
    :return:

    Examples:
    >>> binary_str(2)
    '10'
    >>> binary_str(5)
    '101'
    """

    # Store mod 2 operations results as '0' and '1'
    bnum = ''
    while num > 0:
        bnum = str(num & 0x1) + bnum
        num = num >> 1

    return bnum