1.名稱修改機(jī)制

大概是會(huì)對形如 __parm 的成員修改為 _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.

試驗(yàn)結(jié)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

而直接調(diào)用 c.__func() 會(huì)報(bào)錯(cuò)，表明這個(gè)屬性并不存在，因?yàn)橐呀?jīng)被改寫成了 _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'

總結(jié)

以上是生活随笔為你收集整理的Python学习小记(4)---class的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網(wǎng)站內(nèi)容還不錯(cuò)，歡迎將生活随笔推薦給好友。