贝斯狸的 Python 之旅 -- 深入切片操作及原理

s = input()
if s == s[::-1]:
    print("True")
else:
    print("False")

>>> abcba
True

li=["A","B","C","D"]
# 格式：li[start : end : step]    
# start 是切片起点索引，end 是切片终点索引，但切片结果不包括终点索引的值。step 是步长默认是 1。
# [start : end : step) 左开右闭
t=li[0:3]        ["A","B","C"]       #起点的0索引可以省略，t=li[:3]
t=li[2: ]        ["C","D"]           #省略end，则切到末尾
t=li[1:3]        ["B","C"]
t=li[0:4:2]      ["A","C"]            #从li[0]到li[3],设定步长为2。          

x=[1,2,3]
y=x[:]
x[0]=-1
print(y) #输出[1,2,3]

#case 1
>>> x=[1,2,3,4,5]
>>> x[2:0]=100  #在2后面插入若干个元素，应该用列表
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: can only assign an iterable
>>> x[2:0]=[100]
>>> x
[1, 2, 100, 3, 4, 5]

#case 2
>>> x[2:4]=None
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: can only assign an iterable
>>> x[2:4]=[None]
>>> x
[1, 2, None, 5]

>>> del x[2:3]  #删除切片
>>> x
[1, 2, 3, 4, 5]

>>> x=[1,2,3,4,5]
>>> x[2:1]=[100] #对于切片x[from:to]，会进行预处理to=max(from+1,to)
>>> x
[1, 2, 100, 3, 4, 5]

>>> del x[2:0] 
>>> x
[1, 2, 100, 3, 4, 5]
>>> del x[2:1]
>>> x
[1, 2, 100, 3, 4, 5]
>>> del x[2:3]
>>> x
[1, 2, 3, 4, 5]

a = [1,2,3,4,5,6]
x = a [ 1 : 5 ]                # x = a.__getitem__(slice( 1, 5, None))
a [ 1 : 3 ] = [10, 11, 12 ]    # a.__setitem__(slice(1, 3, None), [ 10, 11, 12 ])
del a [ 1 : 4 ]                # a.__delitem__(slice(1, 4, None))

def between(beg, end, mid):
    # 判断mid是否位于begin和end之间
    return end > mid >= beg or end < mid <= beg

def get_slice(a, beg, end, delta=1):
     # 数组切片get方式
     if delta == 0: 
        raise ValueError("slice step cannot be 0")
     # 将负数下标转化一下
     if beg < 0: 
        beg += len(a)
     if end < 0: 
        end += len(a)
     # 如果转化完成之后依然不在合法范围内，则返回空列表
     if beg < 0 and end < 0 or beg >= len(a) and end >= len(a): 
        return []
     # 如果方向不同，则返回空列表
     if (end - beg) * delta <= 0: 
        return []
     # 将越界的部分进行裁剪
     beg = max(0, min(beg, len(a) - 1))
     end = max(-1, min(end, len(a)))
     ans = []
     i = beg
     while between(beg, end, i):
          ans.append(a[i])
          i += delta
     return ans

def set_slice(a, li, beg, end, delta=1):
     if delta == 0: 
        raise ValueError("slice step cannot be 0")
     if delta == 1:
      # 如果delta==1，那么li的长度可以随意
      if beg < 0: 
        beg += len(a)
      if end < 0: 
        end += len(a)
      beg = max(0, min(beg, len(a) - 1))
      end = max(-1, min(end, len(a)))
      for i in range(beg, end):
           del a[beg]
      for i in reversed(li):
           a.insert(beg, i)
    else:
      # delta!=1,相当于替换
      if len(get_slice(a, beg, end, delta)) != len(li): 
        raise ValueError("array don't match")
      if len(li) == 0: 
        return
      if beg < 0: 
        beg += len(a)
      if end < 0: 
        end += len(a)
      beg = max(0, min(beg, len(a) - 1))
      # 用li中的全部元素逐一替换
      for ind, value in enumerate(li):
           a[ind * delta + beg] = value