def my_bin(num):
    la = []
    if num < 0:
        return '-' + my_bin(abs(num))
    while True:
        num, remainder = divmod(num, 2)
        la.append(str(remainder))
        if num == 0:
            return ''.join(la[::-1])
print my_bin(5)
print my_bin(0)
print my_bin(-5)

输出结果: