Python基本切片
示例
对于任何可迭代的对象（例如字符串，列表等），Python允许您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片的最后一个索引之后的一个。默认为len(iterable)
step是步长（下面的示例可以更好地说明）
例子：
a = "abcdef"
a # "abcdef"
#与a[：]或a[::]相同，因为它对所有三个索引使用默认值
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，上述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)
索引可以为负，在这种情况下，它们是从序列末尾开始计算的
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())
步长也可以为负，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项很有用
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值为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)
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