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]=-1print(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]=NoneTraceback (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 <= begdef 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 ansdef 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] = valuedef 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()