python list的::访问和易出错的[::-1]

list[param1:param2:param3]，遇到param为负数时容易理解错误。

这个结果等价于下面这个函数。

>>> def listColonColon(list, param1, param2, param3=1):
...     begin = param1 if param1 is not None else 0 if param3 > 0 else len(list) - 1
...     end = param2 if param2 is not None else len(list) if param3 > 0 else -1
...     result = []
...     if param3 > 0:
...         while begin < end:
...             result.append(list[begin])
...             begin = begin + param3
...     else:
...        while begin > end:
...            result.append(list[begin])
...            begin = begin + param3       
...     return result

 
 
 

>>> s='abcde'
>>> print(s[::-1])
edcba
>>> listColonColon(s, None, None, -1)
['e', 'd', 'c', 'b', 'a']
>>> print(s[1::-1])
ba
>>> listColonColon(s, 1, None, -1)
['b', 'a']
>>> print(s[::-1][-2:])
ba
>>> listColonColon(s, None, None, -1)[-2:]
['b', 'a']
>>> print(s[0:2][::-1])
ba
>>> listColonColon(s[0:2], None, None, -1)
['b', 'a']
>>> print(s[::1])
abcde
>>> listColonColon(s, None, None, 1)
['a', 'b', 'c', 'd', 'e']
>>> print(s[::2])
ace
>>> listColonColon(s, None, None, 2)
['a', 'c', 'e']
>>> print(s[:3:-1])
e
>>> listColonColon(s, None, 3, -1)
['e']
>>> print(s[1:4:-1])  #这个结果为空

>>> listColonColon(s, 1, 4, -1)
[]
>>> print(s[4:1:-1])
edc
>>> listColonColon(s, 4, 1, -1)
['e', 'd', 'c']
>>> print(s[::-2])
eca
>>> listColonColon(s, None, None, -2)
['e', 'c', 'a']