The Python Tutorial之Class(二)

###1. Class Objects

When a class definition is entered, a new namespace is created, and used as the local scope — thus, all assignments to local variables go into this new namespace. In particular, function definitions bind the name of the new function here.

特别回顾上一篇的命名空间，class的定义开辟新的命名空间，该空间为local scope，但是从module的角度来看，class的name属于该module的global namespaces。该结论并没有出处，是笔者自己的理解。

>>> 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)

官网给出的例子，然后配合下面的说明理解class的定义：

• Class objects support two kinds of operations: attribute references and instantiation.属性引用或者实例化
• 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.有效的属性名字必须属于class对象的命名空间

• Class attributes can also be assigned to, _doc_ is also a valid attribute, returning the docstring belonging to the class.class属性可以被赋值
• Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class.
• a class may define a special method named _init_(), class instantiation automatically invokes _init_() for the newly-created class instance.实例化的时候自动调用_init_()
• 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_().实例化时传递的参数最终给_init_

2. Instance Objects

• Data attributes need not be declared; like local variables, they spring into existence when they are first assigned to.class的数据属性并不需要在class的定义中声明，如下小实验验证：

>>> class change:
...     def sum(self):
...         z=1
...     i=1
...
>>> x=change()
>>> x.j=2
>>> x.j
2
>>>

• The other kind of instance attribute reference is a method.

• Valid method names of an instance object depend on its class. By definition, all attributes of a class that are function objects define corresponding methods of its instances.实例化之后，原来的函数对象变成该实例的method对象，两者是有区别的

3. Method Objects

• In general, calling a method with a list of n arguments is equivalent to calling the corresponding function with an argument list that is created by inserting the method’s instance object before the first argument.x.sum()和change.sum(x)效果一样，理解这个需要理解深刻理解一切东西都是对象的含义
• If you still don’t understand how methods work, a look at the implementation can perhaps clarify matters. When an instance attribute is referenced that isn’t a data attribute, its class is searched. If the name denotes a valid class attribute that is a function object, a method object is created by packing (pointers to) the instance object and the function object just found together in an abstract object: this is the method object.method object生成的过程

• When the method object is called with an argument list, a new argument list is constructed from the instance object and the argument list, and the function object is called with this new argument list.比较复杂的陈述了这个method调用的过程，该部分进一步解释上面x.sum()=change.sum(x)

4. Class and Instance Variables

class Dog:

    kind = 'canine'         # class variable shared by all instances

    def __init__(self, name):
        self.name = name    # instance variable unique to each instance

官网给的例子上的注释很清楚的解释了什么是Class and Instance Variables,然后又给出下面的例子：

class Dog:

    tricks = []             # mistaken use of a class variable

    def __init__(self, name):
        self.name = name

    def add_trick(self, trick):
        self.tricks.append(trick)

>>> d = Dog('Fido')
>>> e = Dog('Buddy')
>>> d.add_trick('roll over')
>>> e.add_trick('play dead')
>>> d.tricks                # unexpectedly shared by all dogs
['roll over', 'play dead']

如果从namespace思考的话，所有实例共享class Dog的命名空间，其trick被所有实例共享，而name只有实例自己可以访问，如果改动代码如下：

>>> class Dog:

    name = 'a' 

    def __init__(self, name):
        self.name = name

    def add_trick(self, trick):
        self.tricks.append(trick)


>>> x=Dog('b')
>>> x.name
'b'
>>>

如果应用namespaces的规则，那么name首先在local scope查找，所以x.name=b而不是a。

5. 总结

从内容编排来看介绍了Class Objects, Instance Objects, Method Objects，特别强调了object，最后又给出了Class and Instance Variables，该特性在之后的private详细讨论。总之理解上一篇的命名空间是基础，之后才能更深的理解其他概念，目前笔者感觉用The Python Tutorial入门是错误的决定。