切片

# 利用切片操作，实现一个trim()函数，去除字符串首尾的空格，注意不要调用str的strip()方法

def trim(string):

    a = 0
    b = len(string)
    if isinstance(string, str):    # 检查输入参数是否为字符串

        for i in string:
            if i == ' ':     # ' ' != ''  空格不等于None
                a += 1
            else:
                break

        for j in string[:: -1]:
            if j == ' ':
                b -= 1

            else:
                break

    else:
        raise TypeError("bad operand")

    return string[a:b]


def simple_trim(string):
    if isinstance(string, str):
        while string[0:1] == ' ':
            string = string[1:]

        while string[-1:] == ' ':
            string = string[:-1]

    else:
        raise TypeError('bad operand')

    return string

test_string = '    hello world   '

print(simple_trim(test_string))