python学习-20-descriptor

""" Overriding descriptor (a.k.a. data descriptor or enforced descriptor): # BEGIN DESCR_KINDS_DEMO1 >>> obj = Managed() # <1> >>> obj.over # <2> -> Overriding.__get__(<Overriding object>, <Managed object>, <class Managed>) >>> Managed.over # <3> -> Overriding.__get__(<Overriding object>, None, <class Managed>) >>> obj.over = 7 # <4> -> Overriding.__set__(<Overriding object>, <Managed object>, 7) >>> obj.over # <5> -> Overriding.__get__(<Overriding object>, <Managed object>, <class Managed>) >>> obj.__dict__['over'] = 8 # <6> >>> vars(obj) # <7> {'over': 8} >>> obj.over # <8> -> Overriding.__get__(<Overriding object>, <Managed object>, <class Managed>) # END DESCR_KINDS_DEMO1 Overriding descriptor without ``__get__``: (these tests are reproduced below without +ELLIPSIS directives for inclusion in the book; look for DESCR_KINDS_DEMO2) >>> obj.over_no_get # doctest: +ELLIPSIS <descriptorkinds.OverridingNoGet object at 0x...> >>> Managed.over_no_get # doctest: +ELLIPSIS <descriptorkinds.OverridingNoGet object at 0x...> >>> obj.over_no_get = 7 -> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7) >>> obj.over_no_get # doctest: +ELLIPSIS <descriptorkinds.OverridingNoGet object at 0x...> >>> obj.__dict__['over_no_get'] = 9 >>> obj.over_no_get 9 >>> obj.over_no_get = 7 -> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7) >>> obj.over_no_get 9 Non-overriding descriptor (a.k.a. non-data descriptor or shadowable descriptor): # BEGIN DESCR_KINDS_DEMO3 >>> obj = Managed() >>> obj.non_over # <1> -> NonOverriding.__get__(<NonOverriding object>, <Managed object>, <class Managed>) >>> obj.non_over = 7 # <2> >>> obj.non_over # <3> 7 >>> Managed.non_over # <4> -> NonOverriding.__get__(<NonOverriding object>, None, <class Managed>) >>> del obj.non_over # <5> >>> obj.non_over # <6> -> NonOverriding.__get__(<NonOverriding object>, <Managed object>, <class Managed>) # END DESCR_KINDS_DEMO3 No descriptor type survives being overwritten on the class itself: # BEGIN DESCR_KINDS_DEMO4 >>> obj = Managed() # <1> >>> Managed.over = 1 # <2> >>> Managed.over_no_get = 2 >>> Managed.non_over = 3 >>> obj.over, obj.over_no_get, obj.non_over # <3> (1, 2, 3) # END DESCR_KINDS_DEMO4 Methods are non-overriding descriptors: >>> obj.spam # doctest: +ELLIPSIS <bound method Managed.spam of <descriptorkinds.Managed object at 0x...>> >>> Managed.spam # doctest: +ELLIPSIS <function Managed.spam at 0x...> >>> obj.spam() -> Managed.spam(<Managed object>) >>> Managed.spam() Traceback (most recent call last): ... TypeError: spam() missing 1 required positional argument: 'self' >>> Managed.spam(obj) -> Managed.spam(<Managed object>) >>> Managed.spam.__get__(obj) # doctest: +ELLIPSIS <bound method Managed.spam of <descriptorkinds.Managed object at 0x...>> >>> obj.spam.__func__ is Managed.spam True >>> obj.spam = 7 >>> obj.spam 7 """ """ NOTE: These tests are here because I can't add callouts after +ELLIPSIS directives and if doctest runs them without +ELLIPSIS I get test failures. # BEGIN DESCR_KINDS_DEMO2 >>> obj.over_no_get # <1> <__main__.OverridingNoGet object at 0x665bcc> >>> Managed.over_no_get # <2> <__main__.OverridingNoGet object at 0x665bcc> >>> obj.over_no_get = 7 # <3> -> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7) >>> obj.over_no_get # <4> <__main__.OverridingNoGet object at 0x665bcc> >>> obj.__dict__['over_no_get'] = 9 # <5> >>> obj.over_no_get # <6> 9 >>> obj.over_no_get = 7 # <7> -> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7) >>> obj.over_no_get # <8> 9 # END DESCR_KINDS_DEMO2 Methods are non-overriding descriptors: # BEGIN DESCR_KINDS_DEMO5 >>> obj = Managed() >>> obj.spam # <1> <bound method Managed.spam of <descriptorkinds.Managed object at 0x74c80c>> >>> Managed.spam # <2> <function Managed.spam at 0x734734> >>> obj.spam = 7 # <3> >>> obj.spam 7 # END DESCR_KINDS_DEMO5 """ # BEGIN DESCR_KINDS ### auxiliary functions for display only ### def cls_name(obj_or_cls): cls = type(obj_or_cls) if cls is type: cls = obj_or_cls return cls.__name__.split('.')[-1] def display(obj): cls = type(obj) if cls is type: return '<class {}>'.format(obj.__name__) elif cls in [type(None), int]: return repr(obj) else: return '<{} object>'.format(cls_name(obj)) def print_args(name, *args): pseudo_args = ', '.join(display(x) for x in args) print('-> {}.__{}__({})'.format(cls_name(args[0]), name, pseudo_args)) ### essential classes for this example ### class Overriding: # <1> """a.k.a. data descriptor or enforced descriptor""" def __get__(self, instance, owner): print_args('get', self, instance, owner) # <2> def __set__(self, instance, value): print_args('set', self, instance, value) class OverridingNoGet: # <3> """an overriding descriptor without ``__get__``""" def __set__(self, instance, value): print_args('set', self, instance, value) class NonOverriding: # <4> """a.k.a. non-data or shadowable descriptor""" def __get__(self, instance, owner): print_args('get', self, instance, owner) class Managed: # <5> over = Overriding() over_no_get = OverridingNoGet() non_over = NonOverriding() def spam(self): # <6> print('-> Managed.spam({})'.format(display(self))) # END DESCR_KINDS