Python基础学习-列表的常用方法

列表方法

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Python 3.5.2 (default, Sep 14 2016, 11:27:58) 
[GCC 6.2.1 20160901 (Red Hat 6.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> L=['good','boy','!']
>>> L
['good', 'boy', '!']
>>> L.
L.__add__(           L.__format__(        L.__init__(          L.__reduce__(        L.__str__(           L.insert(
L.__class__(         L.__ge__(            L.__iter__(          L.__reduce_ex__(     L.__subclasshook__(  L.pop(
L.__contains__(      L.__getattribute__(  L.__le__(            L.__repr__(          L.append(            L.remove(
L.__delattr__(       L.__getitem__(       L.__len__(           L.__reversed__(      L.clear(             L.reverse(
L.__delitem__(       L.__gt__(            L.__lt__(            L.__rmul__(          L.copy(              L.sort(
L.__dir__(           L.__hash__           L.__mul__(           L.__setattr__(       L.count(             
L.__doc__            L.__iadd__(          L.__ne__(            L.__setitem__(       L.extend(            
L.__eq__(            L.__imul__(          L.__new__(           L.__sizeof__(        L.index(             

1、append

 　　append方法用于在列表末尾追加新的对象：

>>> L=['index0','index1','index2']
>>> L
['index0', 'index1', 'index2']
>>> L.append('index3')
>>> L
['index0', 'index1', 'index2', 'index3']
>>> L.append([1,2,3])
>>> L
['index0', 'index1', 'index2', 'index3', [1, 2, 3]]
>>> L.append(('Hello','World','!'))
>>> L
['index0', 'index1', 'index2', 'index3', [1, 2, 3], ('Hello', 'World', '!')]
>>> L.append('index5')
>>> L
['index0', 'index1', 'index2', 'index3', [1, 2, 3], ('Hello', 'World', '!'), 'index5']
>>> len(L)
7
>>> L.append()
Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    L.append()
TypeError: append() takes exactly one argument (0 given)
>>> L.append(None)
>>> L
['index0', 'index1', 'index2', 'index3', [1, 2, 3], ('Hello', 'World', '!'), 'index5', None]
>>> L.append([])
>>> L
['index0', 'index1', 'index2', 'index3', [1, 2, 3], ('Hello', 'World', '!'), 'index5', None, []]
>>> id(L)
19215544
>>> L.append(1231)
>>> L
['index0', 'index1', 'index2', 'index3', [1, 2, 3], ('Hello', 'World', '!'), 'index5', None, [], 1231]
>>> id(L)
19215544
>>> 

2、count

　　count方法统计某个元素在列表中出现的次数：

>>> help(L.count)
Help on built-in function count:

count(...)
    L.count(value) -> integer -- return number of occurrences of value

>>> L=['to','be','or','not','to','be']
>>> L
['to', 'be', 'or', 'not', 'to', 'be']
>>> L.count('to')
2
>>> x=[[1,2],1,1,[1,2,1,[1,2]]]
>>> x.count(1)
2>>> x.count([1,2])
1
>>> 

3、extend　　

　　官方说明：

>>> help(L.extend)
Help on built-in function extend:

extend(...)
    L.extend(iterable) -> None -- extend list by appending elements from the iterable

 　　extend方法可以在列表的末尾一次性追加别一个序列中的多个值；换句话说，可以用新列表扩展原有的列表：

>>> a=[1,2,3,4]
>>> id(a)
20761256
>>> b=[5,6,7,8,9]
>>> id(b)
20760536
>>> c=a.extend(b)
>>> c
>>> print(c)
None
>>> a.extend(b)
>>> a
[1, 2, 3, 4, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9]
>>> b
[5, 6, 7, 8, 9]

>>> id(a)
20761256
>>> b.extend(a)
>>> b
[5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9]
>>> 
>>> a
[1, 2, 3, 4, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9]
>>> b
[5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9]
>>> a.extend(b)
>>> a
[1, 2, 3, 4, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 5, 6, 7, 8, 9]

 4、index

　　index方法用于从列表中找出某个值第一个匹配项的索引位置：

 　　举例：

>>> s='He who commences many things finishes but a few'
>>> list1=s.split()
>>> list1
['He', 'who', 'commences', 'many', 'things', 'finishes', 'but', 'a', 'few']
>>> list1.index('who')
1
>>> list1.index('Few')
Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    list1.index('Few')
ValueError: 'Few' is not in list
>>> list1.index('who',2)
Traceback (most recent call last):
  File "<pyshell#17>", line 1, in <module>
    list1.index('who',2)
ValueError: 'who' is not in list
>>> list1.index('but',2)
6
>>> list1.index('things',4,5)
4
>>> list1.index('things',5,len(list1))
Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    list1.index('things',5,len(list1))
ValueError: 'things' is not in list
>>> list1.index('things',3,len(list1))
4
>>> list1.index('a',-2,-1)
7
>>> len(list1)
9
>>> 

5、insert

　　insert方法用于将对象插入到列表中；

>>> list1
['He', 'commences', 'many', 'things', 'finishes', 'but', 'a', 'few']
>>> list1.insert(1,'who')
>>> list1
['He', 'who', 'commences', 'many', 'things', 'finishes', 'but', 'a', 'few']
>>> list1.insert(0,'who')
>>> list1
['who', 'He', 'who', 'commences', 'many', 'things', 'finishes', 'but', 'a', 'few']
>>> 

6、pop

　　pop方法会移除列表中的一个元素（默认是最后一个），并且返回该元素的值:

>>> x=[1,2,3,4]
>>> x.pop()
4
>>> x
[1, 2, 3]
>>> x.pop(0)
1
>>> x
[2, 3]
>>> x.pop(1)
3
>>> x
[2]
>>> help(x.pop)
Help on built-in function pop:

pop(...)
    L.pop([index]) -> item -- remove and return item at index (default last).
    Raises IndexError if list is empty or index is out of range.

>>> 

注意： pop方法是唯一一个既能修改列表又返回元素（除了None）的列表方法。

7、remove

　　remove方法用于移除列表中某个值的第一个匹配项：

>>> x
['to', 'be', 'or', 'not', 'to', 'be']
>>> x.remove('be')
>>> x
['to', 'or', 'not', 'to', 'be']
>>> print(x.remove('bee'))
Traceback (most recent call last):
  File "<pyshell#65>", line 1, in <module>
    print(x.remove('bee'))
ValueError: list.remove(x): x not in list
>>> print(x.remove('to'))
None
>>> x
['or', 'not', 'to', 'be']
>>> x.insert(0,'to')
>>> x
['to', 'or', 'not', 'to', 'be']
>>> x.remove()
Traceback (most recent call last):
  File "<pyshell#70>", line 1, in <module>
    x.remove()
TypeError: remove() takes exactly one argument (0 given)
>>> 

8、reverse

　　reverse方法将列表中的元素反向存放；

>>> help(list.reverse)
Help on method_descriptor:

reverse(...)
    L.reverse() -- reverse *IN PLACE*

>>> x=[1,2,3,4]
>>> x
[1, 2, 3, 4]
>>> x.reverse()
>>> x
[4, 3, 2, 1]
>>> x.reverse()
>>> x
[1, 2, 3, 4]
>>> 

 9、sort

sort方法用于在原位置对列表进行排序，在‘原位置排序’意味着改变原来的列表，从而让其中的元素能按一定的顺序排列，而不是简单地返回一个已经排序

 的列表的副本；

>>> x=[2,55,123,89,-2,23]
>>> x.sort()
>>> x
[-2, 2, 23, 55, 89, 123]
>>> x.sort()
>>> x
[-2, 2, 23, 55, 89, 123]

 sort方法修改原列表并返回了空值（None)：

示例：

>>> help(list.sort)
Help on method_descriptor:

sort(...)
    L.sort(key=None, reverse=False) -> None -- stable sort *IN PLACE*

>>> x=[23,44,1,5,-100]
>>> y=x.sort()  #千万不要这样做！
>>> x
[-100, 1, 5, 23, 44]
>>> y
>>> print(y)
None
>>> 

那么当用户需要一个排好序的列表副本，同时又保留原来列表不变的时候，问题就出现了，为了实现这个功能的正解方法是，首先把x的副本赋值给y,然后对y进行排序，

>>> x=[23,12,9,4,1231]
>>> x
[23, 12, 9, 4, 1231]
>>> y=x[:]
>>> y
[23, 12, 9, 4, 1231]
>>> id(x)
20712184
>>> id(y)
20711584
>>> z=x
>>> id(z)
20712184
>>> y.sort()
>>> x
[23, 12, 9, 4, 1231]
>>> y
[4, 9, 12, 23, 1231]
>>> 

　　再次调用x[:]得到的是包含了x所有元素的分片，这是一种很有效率的复制整个列表的方法

假如只是简单把x赋值给y是没用的，因为这样做让x和y都指向同一个列表了。