基本切片

对于任何可迭代的（例如，字符串，列表等），Python 允许你切片并返回其数据的子字符串或子列表。

切片格式：

iterable_name[start:stop:step]

哪里，

• start 是切片的第一个索引。默认为 0（第一个元素的索引）
• stop 超过切片的最后一个索引。默认为 len（可迭代）
• step 是步长（下面的例子更好地解释）

例子：

a = "abcdef"
a            # "abcdef" 
             # Same as a[:] or a[::] since it uses the defaults for all three indices
a[-1]        # "f"
a[:]         # "abcdef" 
a[::]        # "abcdef" 
a[3:]        # "def" (from index 3, to end(defaults to size of iterable)) 
a[:4]        # "abcd" (from beginning(default 0) to position 4 (excluded)) 
a[2:4]       # "cd" (from position 2, to position 4 (excluded)) 

此外，以上任何一种都可以与定义的步长一起使用：

a[::2]       # "ace" (every 2nd element)
a[1:4:2]     # "bd" (from index 1, to index 4 (excluded), every 2nd element)

指数可以是负数，在这种情况下，它们是从序列的末尾计算出来的

a[:-1]     # "abcde" (from index 0 (default), to the second last element (last element - 1))
a[:-2]     # "abcd" (from index 0 (default), to the third last element (last element -2))
a[-1:]     # "f" (from the last element to the end (default len()) 

步长也可以是负数，在这种情况下，切片将以相反的顺序遍历列表：

a[3:1:-1]   # "dc" (from index 2 to None (default), in reverse order)

此构造对于反转可迭代非常有用

a[::-1]     # "fedcba" (from last element (default len()-1), to first, in reverse order(-1))

请注意，对于否定步骤，默认 end_index 是 None（请参阅 http://stackoverflow.com/a/12521981 ）

a[5:None:-1] # "fedcba" (this is equivalent to a[::-1])
a[5:0:-1]    # "fedcb" (from the last element (index 5) to second element (index 1)