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21.标准Type的分类
作者将标准Type分成三个Model，以搞清相互之间的关系，以及如何动作。
①Storage Model：
| Storage Model Category | Python Types That Fit Category |
| Scalar/atom | Numbers (all numeric types), strings (all are literals) |
| Container | Lists, tuples, dictionaries |
String不被看成Container是因为没有Character这个基本Type。
②Update Model：
| Update Model Category | Python Types That Fit Category |
| Mutable | Lists, dictionaries |
| Immutable | Numbers, strings, tuples |
Number和String也是Immutable，尽管如下语句：
x = 'Python numbers and strings'
x = 'are immutable?!? What gives?'
i = 0
i = i + 1
但其实x、i并非被更改了值，而是系统生成了新的对象，然后将x、i指向这个新的对象，原来的对象将被垃圾回收。通过查看id(x)可以看出其前后的差别。
③Access Model：
| Access Model Category | Python Types That Fit Category |
| Direct | Numbers |
| Sequence | Strings, lists, tuples |
| Mapping | Dictionaries |
综合：
| Data-Type | Storage-Model | Update-Model | Access-Model |
| Numbers | Scalar | Immutable | Direct |
| Strings | Scalar | Immutable | Sequence |
| Lists | Container | Mutable | Sequence |
| Tuples | Container | Immutable | Sequence |
| Dictionaries | Container | Mutable | Mapping |
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