Python学习小记(4)---class

　1.名称修改机制

大概是会对形如 __parm 的成员修改为 _classname__spam

9.6. Private Variables

“Private” instance variables that cannot be accessed except from inside an object don’t exist in Python. However, there is a convention that is followed by most Python code: a name prefixed with an underscore (e.g. _spam) should be treated as a non-public part of the API (whether it is a function, a method or a data member). It should be considered an implementation detail and subject to change without notice.

Since there is a valid use-case for class-private members (namely to avoid name clashes of names with names defined by subclasses), there is limited support for such a mechanism, called name mangling. Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.

Name mangling is helpful for letting subclasses override methods without breaking intraclass method calls. For example:

class Mapping:
    def __init__(self, iterable):
        self.items_list = []
        self.__update(iterable)

    def update(self, iterable):
        for item in iterable:
            self.items_list.append(item)

    __update = update   # private copy of original update() method

class MappingSubclass(Mapping):

    def update(self, keys, values):
        # provides new signature for update()
        # but does not break __init__()
        for item in zip(keys, values):
            self.items_list.append(item)

The above example would work even if MappingSubclass were to introduce a __update identifier since it is replaced with _Mapping__update in the Mapping class and _MappingSubclass__update in the MappingSubclass class respectively.

Note that the mangling rules are designed mostly to avoid accidents; it still is possible to access or modify a variable that is considered private. This can even be useful in special circumstances, such as in the debugger.

Notice that code passed to exec() or eval() does not consider the classname of the invoking class to be the current class; this is similar to the effect of the global statement, the effect of which is likewise restricted to code that is byte-compiled together. The same restriction applies to getattr(), setattr() and delattr(), as well as when referencing __dict__ directly.

试验结果如下

class Mapping:
    def func(self):
        print('function_in_Mapping')
    __func = func
class MappingSubclass(Mapping):
    def func(self):
        print('function_in_MappingSubclass')
    __func = func

c = Mapping()
c.func()
e = MappingSubclass()
e.func()
e._Mapping__func()
e._MappingSubclass__func()

E:\Coding\Python>python class_test.py
function_in_Mapping
function_in_MappingSubclass
function_in_Mapping
function_in_MappingSubclass

而直接调用 c.__func() 会报错，表明这个属性并不存在，因为已经被改写成了 _Mapping__func 或 _MappingSubclass__func

class Mapping:
    def func(self):
        print('function_in_Mapping')
    __func = func
class MappingSubclass(Mapping):
    def func(self):
        print('function_in_MappingSubclass')
    __func = func
c = Mapping()
c.func()
e = MappingSubclass()
e.func()
e._Mapping__func()
e._MappingSubclass__func()

c.__func()
e.__func()

E:\Coding\Python>python class_test.py
function_in_Mapping
function_in_MappingSubclass
function_in_Mapping
function_in_MappingSubclass
Traceback (most recent call last):
  File "class_test.py", line 16, in <module>
    c.__func()
AttributeError: 'Mapping' object has no attribute '__func'