2-python代码坑点

#切片：
# L = ['aaa', 'bbb', 'ccc', 'ddd']
# print(L[1 : 3])  #取[1, 3):下标
# L = list(range(100))
# print(L[:10])
# print(L[-10:])
#
# print(L[2:10:4])       #从2开始取，步长为4，小于10
# # [2, 6]
# print('abcdefg'[:3])
# # abc
# print('abcdefg'[::2])
# # aceg
 
#取出字符串的首位空格的函数
# def trim(s):
#     if s == '':
#         return ''
#     len1 = len(s)
#     i = 0
#     j = len1 - 1
#     while i < len1  and s[i] == ' ':
#         i += 1
#         print(i)
#     while j > 0 and s[j] == ' ' :
#         j -= 1
#         print(j)
#     if i <= j:
#         print(i, (j))
#         return s[i:j+1]
#     else:
#         return '' 
 
#递归写法：
# def trim(s):
#     if s == '':
#         return s
#     if s[:1] == ' ':
# #     if s[0] == ' ':
#         return trim(s[1:])
#     if s[-1:] == ' ':
# #     if s[-1] == ' ':
#         return trim(s[:-2])
#     else:
#         return s
#递归写法2：
def trim(s):
    if s == '':
        return s
    if s[0] == ' ':
        s = trim(s[1: ])   #如果这里不用返回值的话，必须要在下面判空，否则可能s已经为空了，再去做下一个if判断，就会越界
#         return trim(s[1: ])
    if s == '':
        return s
    if s[-1] == ' ':
        s = trim(s[ :-2])
#         return trim(s[ :-2])
    return s
#
# # 测试:
t = [1, 2, 3, 4, 5]
print(t[:-2])
 
if trim(' ') != '':
    print('ys' + trim('    hello') + 'ys')
    print('no')
#
if trim('hello  ') != 'hello':
    print('a' + trim('hello  ') + 'b')
    print('1测试失败!')
elif trim('  hello') != 'hello':
    print('2测试失败!')
elif trim('  hello  ') != 'hello':
    print('3测试失败!')
elif trim('  hello  world  ') != 'hello  world':
    print('4测试失败!')
elif trim('') != '':
    print('5测试失败!')
elif trim('    ') != '':
    print('6测试失败!')
else:
    print('测试成功!')