9. 类 Classes

Python's class mechanism adds classes to the language with a minimum of new syntax and semantics. It is a mixture of the class mechanisms found in C++ and Modula-3. As is true for modules, classes in Python do not put an absolute barrier between definition and user, but rather rely on the politeness of the user not to ``break into the definition.'' The most important features of classes are retained with full power, however: the class inheritance mechanism allows multiple base classes, a derived class can override any methods of its base class or classes, a method can call the method of a base class with the same name. Objects can contain an arbitrary amount of private data.

Python在尽可能不增加新的语法和语义的情况下加入了类机制。这种机制是C++和Python's Modula-3的混合。Python中的类没有在用户和定义之间建立一个绝对的屏障，而是依赖于用户自觉的不去“破坏定义”。然而，类机制最重要的功能都完整的保留下来。类继承机制允许多继承，派生类可以覆盖（override）基类中的任何方法，方法中可以调用基类中的同名方法。对象可以包含任意数量的私有成员。

In C++ terminology, all class members (including the data members) are public, and all member functions are virtual. There are no special constructors or destructors. As in Modula-3, there are no shorthands for referencing the object's members from its methods: the method function is declared with an explicit first argument representing the object, which is provided implicitly by the call. As in Smalltalk, classes themselves are objects, albeit in the wider sense of the word: in Python, all data types are objects. This provides semantics for importing and renaming. Unlike C++ and Modula-3, built-in types can be used as base classes for extension by the user. Also, like in C++ but unlike in Modula-3, most built-in operators with special syntax (arithmetic operators, subscripting etc.) can be redefined for class instances.

用 C++ 术语来讲，所有的类成员（包括数据成员）都是公有（ public ）的，所有的成员函数都是虚拟（ virtual ）的。用Modula-3的术语来讲，在成员方法中没有什么简便的方式（ shorthands ）可以引用对象的成员：方法函数在定义时需要以引用的对象做为第一个参数，调用时则会隐式引用对象。这样就形成了语义上的引入和重命名。（ This provides semantics for importing and renaming. ）但是，像 C++ 而非 Modula-3 中那样，大多数带有特殊语法的内置操作符（算法运算符、下标等）都可以针对类的需要重新定义。