170 lines
5.1 KiB
Python
170 lines
5.1 KiB
Python
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"""
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Overriding descriptor (a.k.a. data descriptor or enforced descriptor):
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>>> obj = Model()
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>>> obj.over # doctest: +ELLIPSIS
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Overriding.__get__() invoked with args:
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self = <descriptorkinds.Overriding object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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owner = <class 'descriptorkinds.Model'>
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>>> Model.over # doctest: +ELLIPSIS
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Overriding.__get__() invoked with args:
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self = <descriptorkinds.Overriding object at 0x...>
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instance = None
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owner = <class 'descriptorkinds.Model'>
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An overriding descriptor cannot be shadowed by assigning to an instance:
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>>> obj = Model()
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>>> obj.over = 7 # doctest: +ELLIPSIS
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Overriding.__set__() invoked with args:
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self = <descriptorkinds.Overriding object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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value = 7
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>>> obj.over # doctest: +ELLIPSIS
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Overriding.__get__() invoked with args:
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self = <descriptorkinds.Overriding object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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owner = <class 'descriptorkinds.Model'>
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Not even by poking the attribute into the instance ``__dict__``:
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>>> obj.__dict__['over'] = 8
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>>> obj.over # doctest: +ELLIPSIS
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Overriding.__get__() invoked with args:
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self = <descriptorkinds.Overriding object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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owner = <class 'descriptorkinds.Model'>
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>>> vars(obj)
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{'over': 8}
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Overriding descriptor without ``__get__``:
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>>> obj.over_no_get # doctest: +ELLIPSIS
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<descriptorkinds.OverridingNoGet object at 0x...>
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>>> Model.over_no_get # doctest: +ELLIPSIS
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<descriptorkinds.OverridingNoGet object at 0x...>
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>>> obj.over_no_get = 7 # doctest: +ELLIPSIS
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OverridingNoGet.__set__() invoked with args:
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self = <descriptorkinds.OverridingNoGet object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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value = 7
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>>> obj.over_no_get # doctest: +ELLIPSIS
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<descriptorkinds.OverridingNoGet object at 0x...>
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Poking the attribute into the instance ``__dict__`` means you can read the new
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value for the attribute, but setting it still triggers ``__set__``:
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>>> obj.__dict__['over_no_get'] = 9
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>>> obj.over_no_get
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9
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>>> obj.over_no_get = 7 # doctest: +ELLIPSIS
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OverridingNoGet.__set__() invoked with args:
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self = <descriptorkinds.OverridingNoGet object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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value = 7
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>>> obj.over_no_get
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9
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Non-overriding descriptor (a.k.a. non-data descriptor or shadowable descriptor):
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>>> obj = Model()
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>>> obj.non_over # doctest: +ELLIPSIS
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NonOverriding.__get__() invoked with args:
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self = <descriptorkinds.NonOverriding object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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owner = <class 'descriptorkinds.Model'>
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>>> Model.non_over # doctest: +ELLIPSIS
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NonOverriding.__get__() invoked with args:
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self = <descriptorkinds.NonOverriding object at 0x...>
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instance = None
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owner = <class 'descriptorkinds.Model'>
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A non-overriding descriptor can be shadowed by assigning to an instance:
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>>> obj.non_over = 7
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>>> obj.non_over
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7
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Methods are non-over descriptors:
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>>> obj.spam # doctest: +ELLIPSIS
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<bound method Model.spam of <descriptorkinds.Model object at 0x...>>
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>>> Model.spam # doctest: +ELLIPSIS
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<function Model.spam at 0x...>
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>>> obj.spam() # doctest: +ELLIPSIS
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Model.spam() invoked with arg:
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self = <descriptorkinds.Model object at 0x...>
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>>> obj.spam = 7
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>>> obj.spam
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7
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No descriptor type survives being overwritten on the class itself:
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>>> Model.over = 1
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>>> obj.over
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1
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>>> Model.over_no_get = 2
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>>> obj.over_no_get
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2
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>>> Model.non_over = 3
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>>> obj.non_over
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7
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"""
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# BEGIN DESCRIPTORKINDS
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def print_args(name, *args): # <1>
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cls_name = args[0].__class__.__name__
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arg_names = ['self', 'instance', 'owner']
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if name == 'set':
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arg_names[-1] = 'value'
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print('{}.__{}__() invoked with args:'.format(cls_name, name))
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for arg_name, value in zip(arg_names, args):
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print(' {:8} = {}'.format(arg_name, value))
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class Overriding: # <2>
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"""a.k.a. data descriptor or enforced descriptor"""
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def __get__(self, instance, owner):
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print_args('get', self, instance, owner) # <3>
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def __set__(self, instance, value):
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print_args('set', self, instance, value)
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class OverridingNoGet: # <4>
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"""an overriding descriptor without ``__get__``"""
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def __set__(self, instance, value):
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print_args('set', self, instance, value)
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class NonOverriding: # <5>
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"""a.k.a. non-data or shadowable descriptor"""
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def __get__(self, instance, owner):
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print_args('get', self, instance, owner)
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class Model: # <6>
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over = Overriding()
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over_no_get = OverridingNoGet()
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non_over = NonOverriding()
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def spam(self): # <7>
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print('Model.spam() invoked with arg:')
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print(' self =', self)
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#END DESCRIPTORKINDS
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