彻底理解Python list切片原理

print str[0:3] #截取第一位到第三位的字符
print str[:] #截取字符串的全部字符
print str[6:] #截取第七个字符到结尾
print str[:-3] #截取从头开始到倒数第三个字符之前
print str[2] #截取第三个字符
print str[-1] #截取倒数第一个字符
print str[::-1] #创造一个与原字符串顺序相反的字符串
print str[-3:-1] #截取倒数第三位与倒数第一位之前的字符
print str[-3:] #截取倒数第三位到结尾
print str[:-5:-3] #逆序截取

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

>>> 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]
>>> del x[2:3]  #删除切片
>>> 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]  #对于切片del操作，如果from>to,不执行任何操作
>>> 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]
>>> 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]

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


def test_getSlice():
 a = list(range(10))
 import random
 for i in range(10):
  beg = random.randint(-15, 15)
  end = random.randint(-15, 15)
  delta = 0
  while delta == 0: delta = random.randint(-15, 15)
  print(len(get_slice(a, beg, end, delta)) == len(a[beg:end:delta]), beg, end, delta)


def test_setSlice():
 import random
 for i in range(10):
  a = list(range(10))
  beg = random.randint(-15, 15)
  end = random.randint(-15, 15)
  delta = 0
  while delta == 0: delta = random.randint(-5, 5)
  sz = len(a[beg:end:delta])
  if delta == 1: sz = random.randint(0, 4)
  li = [random.randint(0, 100) for i in range(sz)]
  set_slice(a, li, beg, end, delta)
  mine = a
  a = list(range(10))
  a[beg:end:delta] = li
  print(a == mine)


test_setSlice()