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如何用好Python的切片操作?

Python 切片 slice

如何用好Python的切片操作?


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2020-01-12

共2个答案

python大咖 python大咖

简单用法:

a[start:stop]  # items start through stop-1
a[start:]      # items start through the rest of the array
a[:stop]       # items from the beginning through stop-1
a[:]           # a copy of the whole array

使用步长

a[start:stop:step] # start through not past stop, by step

要记住的关键点是该:stop值表示不在所选切片中的第一个值。所以,之间的差stop和start是选择的元素的数量(如果step是1,默认值)。

另一个功能是start或stop可能是负数,这意味着它从数组的末尾而不是开头开始计数。所以:

a[-1]    # last item in the array
a[-2:]   # last two items in the array
a[:-2]   # everything except the last two items

step可能为负数:

a[::-1]    # all items in the array, reversed
a[1::-1]   # the first two items, reversed
a[:-3:-1]  # the last two items, reversed
a[-3::-1]  # everything except the last two items, reversed

如果项目数量少于您的要求,Python对程序员很友好。例如,如果您要求a[:-2]并且a仅包含一个元素,则会得到一个空列表,而不是一个错误。有时您会更喜欢该错误,因此您必须意识到这种情况可能会发生。

与slice()对象的关系

[]上面的代码中实际上将切片运算符与slice()使用:符号的对象一起使用(仅在中有效[]),即:

a[start:stop:step]

等效于:

a[slice(start, stop, step)]

切片对象也表现略有不同,这取决于参数的个数,同样range(),即两个slice(stop)slice(start, stop[, step])支持。要跳过指定给定参数的操作,可以使用None,例如a[start:]等于a[slice(start, None)]a[::-1]等于a[slice(None, None, -1)]

尽管: 基于的符号对于简单切片非常有帮助,但是slice()对象的显式使用简化了切片的程序生成。

2020-01-12
python大咖 python大咖

列举语法允许的可能性:

>>> seq[:]                # [seq[0],   seq[1],          ..., seq[-1]    ]
>>> seq[low:]             # [seq[low], seq[low+1],      ..., seq[-1]    ]
>>> seq[:high]            # [seq[0],   seq[1],          ..., seq[high-1]]
>>> seq[low:high]         # [seq[low], seq[low+1],      ..., seq[high-1]]
>>> seq[::stride]         # [seq[0],   seq[stride],     ..., seq[-1]    ]
>>> seq[low::stride]      # [seq[low], seq[low+stride], ..., seq[-1]    ]
>>> seq[:high:stride]     # [seq[0],   seq[stride],     ..., seq[high-1]]
>>> seq[low:high:stride]  # [seq[low], seq[low+stride], ..., seq[high-1]]

当然,如果(high-low)%stride != 0,则终点将比稍低high-1

如果stride为负,则由于我们递减计数,因此顺序有所更改:

>>> seq[::-stride]        # [seq[-1],   seq[-1-stride],   ..., seq[0]    ]
>>> seq[high::-stride]    # [seq[high], seq[high-stride], ..., seq[0]    ]
>>> seq[:low:-stride]     # [seq[-1],   seq[-1-stride],   ..., seq[low+1]]
>>> seq[high:low:-stride] # [seq[high], seq[high-stride], ..., seq[low+1]]

扩展切片(带有逗号和省略号)通常仅由特殊的数据结构(如NumPy)使用;基本序列不支持它们。

>>> class slicee:
...     def __getitem__(self, item):
...         return repr(item)
...
>>> slicee()[0, 1:2, ::5, ...]
'(0, slice(1, 2, None), slice(None, None, 5), Ellipsis)'
2020-01-12

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