9.1 有关术语的话题 A Word About Terminology

Lacking universally accepted terminology to talk about classes, I will make occasional use of Smalltalk and C++ terms. (I would use Modula-3 terms, since its object-oriented semantics are closer to those of Python than C++, but I expect that few readers have heard of it.)

由于没有什么关于类的通用术语，我从 Smalltalk 和 C++ 中借用一些（我更希望用 Modula-3 的，因为它的面向对象机制比 C++ 更接近Python，不过我想没多少读者听说过它）。

I also have to warn you that there's a terminological pitfall for object-oriented readers: the word ``object'' in Python does not necessarily mean a class instance. Like C++ and Modula-3, and unlike Smalltalk, not all types in Python are classes: the basic built-in types like integers and lists are not, and even somewhat more exotic types like files aren't. However, all Python types share a little bit of common semantics that is best described by using the word object.

我要提醒读者，这里有一个面向对象方面的术语陷阱，在 Python 中“对象”这个词不一定指类实例。Python中并非所有的类型都是类：例如整型、链表这些内置数据类型就不是，甚至某些像文件这样的外部类型也不是，这一点类似于 C++ 和Modula-3，而不像Smalltalk。然而，所有的Python类型在语义上都有一点相同之处：描述它们的最贴切词语是“对象”。

Objects have individuality, and multiple names (in multiple scopes) can be bound to the same object. This is known as aliasing in other languages. This is usually not appreciated on a first glance at Python, and can be safely ignored when dealing with immutable basic types (numbers, strings, tuples). However, aliasing has an (intended!) effect on the semantics of Python code involving mutable objects such as lists, dictionaries, and most types representing entities outside the program (files, windows, etc.). This is usually used to the benefit of the program, since aliases behave like pointers in some respects. For example, passing an object is cheap since only a pointer is passed by the implementation; and if a function modifies an object passed as an argument, the caller will see the change -- this eliminates the need for two different argument passing mechanisms as in Pascal.

对象是被特化的，多个名字（在多个作用域中）可以绑定同一个对象。这相当于其它语言中的别名。通常对 Python 的第一印象中会忽略这一点，使用那些不可变的基本类型（数值、字符串、元组）时也可以很放心的忽视它。然而，在 Python 代码调用字典、链表之类可变对象，以及大多数涉及程序外部实体（文件、窗体等等）的类型时，这一语义就会有影响。这通用有助于优化程序，因为别名的行为在某些方面类似于指针。例如，很容易传递一个对象，因为在行为上只是传递了一个指针。如果函数修改了一个通过参数传递的对象，调用者可以接收到变化－－在 Pascal 中这需要两个不同的参数传递机制。