Python3笔记：列表方法详解

摘要：如果我想删除第个元素，但是第个元素并不存在，而系统帮你删除了列表的最后一个元素，我觉得这也是无法容忍的。所以在使用这些方法时，务必确认该偏移量的元素是否存，否则可能会报错。

user = []
user = list()

# 将字符串转成列表
>>> list("abcde")
["a", "b", "c", "d", "e"]

# 将元祖转成列表
>>> list(("a","b","c"))
["a", "b", "c"]

>>> users = ["a","b","c","d","e"]
# 可以使用整数来获取某个元素
>>> users[0]
"a"
# 可以使用负整数来表示从尾部获取某个元素
>>> users[-1]
"e"

# 数组越界会报错
>>> users[100]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range
>>> users[-100]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range

# 修改某个元素
>>> users[0] = "wdd"
>>> users
["wdd", "b", "c", "d", "e"]
>>> 

列表的切片或者提取之后仍然是一个列表
形式如：list[start:end:step]

>>> users
["wdd", "b", "c", "d", "e"]
# 正常截取 注意这里并不会截取到users[2]
>>> users[0:2]
["wdd", "b"]
# 也可从尾部截取
>>> users[0:-2]
["wdd", "b", "c"]
# 这样可以获取所有的元素
>>> users[:]
["wdd", "b", "c", "d", "e"]
# 也可以加上步长参数
>>> users[0:4:2]
["wdd", "c"]
# 也可以通过这种方式去将列表取反
>>> users[::-1]
["e", "d", "c", "b", "wdd"]

# 注意切片时，偏移量可以越界，越界之后不会报错，仍然按照界限来处理 例如开始偏移量如果小于0，那么仍然会按照0去计算。
>>> users
["wdd", "b", "c", "d", "e"]
>>> users[-100:3]
["wdd", "b", "c"]
>>> users[-100:100]
["wdd", "b", "c", "d", "e"]
>>> 

形式如：list.append(item)

>>> users
["wdd", "b", "c", "d", "e"]
>>> users.append("ddw")
>>> users
["wdd", "b", "c", "d", "e", "ddw"]

形式如：list1.extend(list2)
这两个方法都会直接修改原列表

>>> users
["wdd", "b", "c", "d", "e", "ddw"]
>>> names
["heihei", "haha"]
>>> users.extend(names)
>>> users
["wdd", "b", "c", "d", "e", "ddw", "heihei", "haha"]
>>> users += names
>>> users
["wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "haha"]

形式如：list.insert(offset, item)
insert也不存在越界的问题，偏移量正负都行，越界之后会自动伸缩到界限之内，并不会报错

>>> users
["wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "haha"]
>>> users.insert(0,"xiaoxiao")
>>> users
["xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "haha"]
>>> users.insert(-1,"-xiaoxiao")
>>> users
["xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha"]
# 下面-100肯定越界了
>>> users.insert(-100,"-xiaoxiao")
>>> users
["-xiaoxiao", "xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha"]
# 下面100也是越界了
>>> users.insert(100,"-xiaoxiao")
>>> users
["-xiaoxiao", "xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]

形式如：del list[offset]
del是python的语句，而不是列表的方法，del删除不存在的元素时，也会提示越界

>>> users
["-xiaoxiao", "xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]
>>> del users[0]
>>> users
["xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]
>>> del users[100]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list assignment index out of range
>>> del users[-100]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list assignment index out of range

形式如：list.remove(value)

>>> users
["xiaoxiao", "wdd", "b", "c", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]
# 删除指定值"c"
>>> users.remove("c")
>>> users
["xiaoxiao", "wdd", "b", "d", "e", "ddw", "heihei", "haha", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]
# 删除不存在的值会报错
>>> users.remove("alsdkfjalsdf")
Traceback (most recent call last):
  File "", line 1, in 
ValueError: list.remove(x): x not in list
# 如果该值存在多个，那么只能删除到第一个
>>> users.remove("haha")
>>> users
["xiaoxiao", "wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]

形式如：list.pop(offset=-1) 偏移量默认等于-1，也就是删除最后的元素

>>> users
["xiaoxiao", "wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha", "-xiaoxiao"]
# 删除最后的元素
>>> users.pop()
"-xiaoxiao"
>>> users
["xiaoxiao", "wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
# 如果列表本身就是空的，那么pop时会报错
>>> user.pop(0)
Traceback (most recent call last):
  File "", line 1, in 
IndexError: pop from empty list
>>> users.pop(0)
"xiaoxiao"
>>> users
["wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
# 越界时也会报错
>>> users.pop(100)
Traceback (most recent call last):
  File "", line 1, in 
IndexError: pop index out of range

形式如：list.index(value)

# index只会返回第一遇到该值得位置
>>> users
["wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
>>> users.index("heihei")
5

# 如果该值不存在，也会报错
>>> users.index("laksdf")
Traceback (most recent call last):
  File "", line 1, in 
ValueError: "laksdf" is not in list

形式如：value in list

>>> users
["wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
>>> "wdd" in users
True

形式如：list.count(value)

>>> users
["wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
>>> users.count("heihei")
2
>>> users.count("h")
0

形式如：string.join(list)

>>> users
["wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
>>> ",".join(users)
"wdd,b,d,e,ddw,heihei,heihei,-xiaoxiao,haha"
>>> user
[]
>>> ",".join(user)
""

形式如：list.sort()

>>> users
["wdd", "b", "d", "e", "ddw", "heihei", "heihei", "-xiaoxiao", "haha"]
# 默认是升序排序
>>> users.sort()
>>> users
["-xiaoxiao", "b", "d", "ddw", "e", "haha", "heihei", "heihei", "wdd"]
# 加入reverse=True, 可以降序排序
>>> users.sort(reverse=True)
>>> users
["wdd", "heihei", "heihei", "haha", "e", "ddw", "d", "b", "-xiaoxiao"]

# 通过匿名函数，传入函数进行自定义排序
>>> students
[{"name": "wdd", "age": 343}, {"name": "ddw", "age": 43}, {"name": "jik", "age": 90}]
>>> students.sort(key=lambda item: item["age"])
>>> students
[{"name": "ddw", "age": 43}, {"name": "jik", "age": 90}, {"name": "wdd", "age": 343}]
>>> students.sort(key=lambda item: item["age"], reverse=True)
>>> students
[{"name": "wdd", "age": 343}, {"name": "jik", "age": 90}, {"name": "ddw", "age": 43}]
>>> 

形式如：list.reverse()

>>> users
["wdd", "heihei", "heihei", "haha", "e", "ddw", "d", "b", "-xiaoxiao"]
>>> users.reverse()
>>> users
["-xiaoxiao", "b", "d", "ddw", "e", "haha", "heihei", "heihei", "wdd"]

形式如：list2 = list1.copy()
list2 = list1 这种并不是列表的复制，只是给列表起了别名。实际上还是指向同一个值。

>>> users
["-xiaoxiao", "b", "d", "ddw", "e", "haha", "heihei", "heihei", "wdd"]
>>> users2 = users.copy()
>>> users2
["-xiaoxiao", "b", "d", "ddw", "e", "haha", "heihei", "heihei", "wdd"]
>>> 

形式如： list.clear()

>>> users2
["-xiaoxiao", "b", "d", "ddw", "e", "haha", "heihei", "heihei", "wdd"]
>>> users2.clear()
>>> users2
[]

形式如：len(list)

>>> users
["-xiaoxiao", "b", "d", "ddw", "e", "haha", "heihei", "heihei", "wdd"]
>>> len(users)
9

写了这些方法后，我有一些疑问，为什么有些操作会提示越界，有些则不会呢？

会提示偏移量越界的操作有

list[offset] 读取或者修改某个元素

del list[offset] 删除指定位置的元素

list.remove(value) 删除指定值的元素

list.pop(offset) 删除指定位置的元素

如果偏移量越界，这些方法会报错的。我的个人理解是:
假如我想读取偏移量为10的元素，但是该元素并不存在，如果系统自动给你读取了列表的最后一个元素，而且不报错，这是无法容忍的bug。 如果我想删除第10个元素，但是第10个元素并不存在，而系统帮你删除了列表的最后一个元素，我觉得这也是无法容忍的。

所以在使用这些方法时，务必确认该偏移量的元素是否存，否则可能会报错。