简单总结Python中序列与字典的相同和不同之处
共同点:
1.它们都是python的核心类型，是python语言自身的一部分
核心类型与非核心类型
多数核心类型可通过特定语法来生成其对象，比如"dave"就是创建字符串类型的对象的表达式；
非核心类型需要内置函数来创建，比如文件类型需要调用内置函数open()来创建。
类也可以理解成自定义的非核心类型。
2.边界检查都不允许超越索引边界
>>> a = 'dave' >>> a[3] 'e' >>> a[4]
Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: string index out of range
3.索引方法都使用[ ]
>>> a = 'dave'
>>> print a[0]
d
>>> a = ['d','a','v','e']
>>> print a[0]
d
>>> a = ('d','a','v','e')
>>> print a[0]
d
4.大部分都支持迭代协议
不同点:
1.有序性
字符串,列表,元组是序列,元组可理解成不可变的列表
字典是python中唯一的映射类型
序列有有序性,字典没有顺序
2.可变性
列表,字典具有可变性.
字符串,元组,数字具有不可变性,即复制后对应存储空间的内容不可变,除非这个对象被销毁.
例如:
>>> s = 'dave' >>> id(s)
140105068006304
>>> s = 'python' + s[1:] >>> s
'pythonave'
>>> id(s)
140105067997536
第一行的s指向内存空间140105068006304,存储了字符串'dave'
第二行的s变成了'pythonave',但它只是重新指向了一个存储着'pythonave'字符串的内存地址.原来140105068006304的内存空间可能还是'dave'，也可能被系统清理销毁.
3.序列操作
序列(字符串,列表,元组)支持,映射(字典)不支持
注意:序列操作生成的类型不变
>>> seq = 'dave' >>> print seq[1:]
ave
>>> seq = ['d','a','v','e'] >>> print seq[1:]
['a', 'v', 'e']
>>> seq = ('d','a','v','e')
>>> print seq[1:]
('a', 'v', 'e')
4.列表解析表达式
表达式中支持序列(字符串,列表,元组),也支持映射(字典)
和序列操作不同的是,列表解析表达式生成的只能是一个列表.
>>> str = [ a * 2 for a in 'dave' ] >>> print str
['dd', 'aa', 'vv', 'ee']
>>> L1 = [1,2,3] >>> L2 = [4,5,6] >>> L3 = [(a+b) for (a,b) in zip(L1,L2)] >>> print L3
[5, 7, 9]
>>> tup = (1,2,3) >>> tups = [ a * 2 for a in tup] >>> print tups
[2, 4, 6]
>>> dic = {'a':'dave','b':'emily'}
>>> dics = [ i *2 for i in dic.items()]
>>> print dics
[('a', 'dave', 'a', 'dave'), ('b', 'emily', 'b', 'emily')]
字典的items(),keys(),values()方法都是返回的列表,这里的for i in a写for i ina.keys(),效果等同。
>>> dic = {'a':'dave','b':'emily'}
>>> dicts = [ i * 2 for i in dic]
>>> print dicts
['aa', 'bb']
5.嵌套
除了字符串外,列表,元组,字典可以相互多层嵌套
嵌套与可变性是不冲突的。
tup = (4, 5, a)中所记忆,录入的b是个变量指针,或者叫内存地址,是个数字。
>>> a = [1,2,3] >>> tup = (4,5,a) >>> print tup[2]
[1, 2, 3]
>>> a[0] = 'x' >>> print tup[2]
['x', 2, 3]
a指向一个列表,列表具有可变性,所以b[0]='x'的操作后,对元组的索引可以看到变化
>>> a = '123' >>> tup = (4,5,a) >>> print tup[2]
123
>>> a = 'xxx' >>> print tup[2]
123
a指向一个字符串，字符串具有不可变性,所以a = 'xxx'的操作后,a这个指针本身就改变了,重新指向另一段内存空间,而tup元组中记忆的内存地址所指空间并没有变,还是字符串'123'
上面2段,元组tup始终保持不可变性,要变化也是其元素作为指针所指向内存的变化.
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