9.3.2 类对象 Class Objects

Class objects support two kinds of operations: attribute references and instantiation.

Attribute references use the standard syntax used for all attribute references in Python: obj.name. Valid attribute names are all the names that were in the class's namespace when the class object was created. So, if the class definition looked like this:

属性引用使用和Python中所有的属性引用一样的标准语法： obj.name。类对象创建后，类命名空间中所有的命名都是有效属性名。所以如果类定义是这样：

class MyClass:
    "A simple example class"
    i = 12345
    def f(self):
        return 'hello world'

then MyClass.i and MyClass.f are valid attribute references, returning an integer and a method object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class:

"A
simple example class"

那么 MyClass.i 和 MyClass.f 是有效的属性引用，分别返回一个整数和一个方法对象。也可以对类属性赋值，你可以通过给 MyClass.i 赋值来修改它。 __doc__ 也是一个有效的属性，返回类的文档字符串：

"A simple example
class"

。

Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example (assuming the above class):

类的实例化使用函数符号。只要将类对象看作是一个返回新的类实例的无参数函数即可。例如（假设沿用前面的类）：

x = MyClass()

creates a new instance of the class and assigns this object to the local variable x.

The instantiation operation (``calling'' a class object) creates an empty object. Many classes like to create objects in a known initial state. Therefore a class may define a special method named __init__(), like this:

这个实例化操作（“调用”一个类对象）来创建一个空的对象。很多类都倾向于将对象创建为有初始状态的。因此类可能会定义一个名为 __init__() 的特殊方法，像下面这样：

    def __init__(self):
        self.data = []

When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:

类定义了 __init__() 方法的话，类的实例化操作会自动为新创建的类实例调用 __init__() 方法。所以在下例中，可以这样创建一个新的实例：

x = MyClass()

Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,

当然，出于弹性的需要， __init__() 方法可以有参数。事实上，参数通过 __init__() 传递到类的实例化操作上。例如：

>>> class Complex:
...     def __init__(self, realpart, imagpart):
...         self.r = realpart
...         self.i = imagpart
...
>>> x = Complex(3.0, -4.5)
>>> x.r, x.i
(3.0, -4.5)