dynamic attributes, descriptors and first concurrency examples
This commit is contained in:
Luciano Ramalho
@@ -1,166 +1,199 @@
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"""
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Data descriptor (a.k.a. overriding or enforced descriptor):
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Overriding descriptor (a.k.a. data descriptor or enforced descriptor):
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>>> o = Model()
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>>> o.data # doctest: +ELLIPSIS
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DataDescriptor.__get__() invoked with args:
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self = <descriptorkinds.DataDescriptor 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.data # doctest: +ELLIPSIS
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DataDescriptor.__get__() invoked with args:
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self = <descriptorkinds.DataDescriptor object at 0x...>
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instance = None
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owner = <class 'descriptorkinds.Model'>
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# BEGIN DESCR_KINDS_DEMO1
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>>> obj = Managed() # <1>
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>>> obj.over # <2>
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-> Overriding.__get__(<Overriding object>, <Managed object>, <class Managed>)
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>>> Managed.over # <3>
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-> Overriding.__get__(<Overriding object>, None, <class Managed>)
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>>> obj.over = 7 # <4>
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-> Overriding.__set__(<Overriding object>, <Managed object>, 7)
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>>> obj.over # <5>
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-> Overriding.__get__(<Overriding object>, <Managed object>, <class Managed>)
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>>> obj.__dict__['over'] = 8 # <6>
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>>> vars(obj) # <7>
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{'over': 8}
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>>> obj.over # <8>
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-> Overriding.__get__(<Overriding object>, <Managed object>, <class Managed>)
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A data descriptor cannot be shadowed by assigning to an instance:
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# END DESCR_KINDS_DEMO1
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>>> o.data = 7 # doctest: +ELLIPSIS
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DataDescriptor.__set__() invoked with args:
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self = <descriptorkinds.DataDescriptor object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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value = 7
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>>> o.data # doctest: +ELLIPSIS
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DataDescriptor.__get__() invoked with args:
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self = <descriptorkinds.DataDescriptor 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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Overriding descriptor without ``__get__``:
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(these tests are reproduced below without +ELLIPSIS directives for inclusion in the book;
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look for DESCR_KINDS_DEMO2)
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Not even by poking the attribute into the instance ``__dict__``:
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>>> obj.over_no_get # doctest: +ELLIPSIS
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<descriptorkinds.OverridingNoGet object at 0x...>
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>>> Managed.over_no_get # doctest: +ELLIPSIS
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<descriptorkinds.OverridingNoGet object at 0x...>
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>>> obj.over_no_get = 7
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-> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7)
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>>> obj.over_no_get # doctest: +ELLIPSIS
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<descriptorkinds.OverridingNoGet object at 0x...>
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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
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-> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7)
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>>> obj.over_no_get
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9
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>>> o.__dict__['data'] = 8
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>>> o.data # doctest: +ELLIPSIS
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DataDescriptor.__get__() invoked with args:
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self = <descriptorkinds.DataDescriptor 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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Non-overriding descriptor (a.k.a. non-data descriptor or shadowable descriptor):
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# BEGIN DESCR_KINDS_DEMO3
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Data descriptor without ``__get__``:
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>>> o.data_no_get # doctest: +ELLIPSIS
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<descriptorkinds.DataDescriptorNoGet object at 0x...>
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>>> Model.data_no_get # doctest: +ELLIPSIS
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<descriptorkinds.DataDescriptorNoGet object at 0x...>
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>>> o.data_no_get = 7 # doctest: +ELLIPSIS
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DataDescriptorNoGet.__set__() invoked with args:
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self = <descriptorkinds.DataDescriptorNoGet object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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value = 7
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>>> o.data_no_get # doctest: +ELLIPSIS
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<descriptorkinds.DataDescriptorNoGet 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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>>> o.__dict__['data_no_get'] = 8
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>>> o.data_no_get
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8
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>>> o.data_no_get = 7 # doctest: +ELLIPSIS
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DataDescriptorNoGet.__set__() invoked with args:
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self = <descriptorkinds.DataDescriptorNoGet object at 0x...>
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instance = <descriptorkinds.Model object at 0x...>
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value = 7
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>>> o.data_no_get # doctest: +ELLIPSIS
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8
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Non-data descriptor (a.k.a. non-overriding or shadowable descriptor):
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>>> o = Model()
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>>> o.non_data # doctest: +ELLIPSIS
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NonDataDescriptor.__get__() invoked with args:
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self = <descriptorkinds.NonDataDescriptor 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_data # doctest: +ELLIPSIS
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NonDataDescriptor.__get__() invoked with args:
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self = <descriptorkinds.NonDataDescriptor object at 0x...>
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instance = None
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owner = <class 'descriptorkinds.Model'>
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A non-data descriptor can be shadowed by assigning to an instance:
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>>> o.non_data = 7
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>>> o.non_data
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7
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Methods are non-data descriptors:
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>>> o.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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>>> o.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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>>> o.spam = 7
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>>> o.spam
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>>> obj = Managed()
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>>> obj.non_over # <1>
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-> NonOverriding.__get__(<NonOverriding object>, <Managed object>, <class Managed>)
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>>> obj.non_over = 7 # <2>
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>>> obj.non_over # <3>
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7
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>>> Managed.non_over # <4>
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-> NonOverriding.__get__(<NonOverriding object>, None, <class Managed>)
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>>> del obj.non_over # <5>
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>>> obj.non_over # <6>
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-> NonOverriding.__get__(<NonOverriding object>, <Managed object>, <class Managed>)
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# END DESCR_KINDS_DEMO3
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No descriptor type survives being overwritten on the class itself:
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>>> Model.data = 1
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>>> o.data
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1
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>>> Model.data_no_get = 2
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>>> o.data_no_get
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2
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>>> Model.non_data = 3
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>>> o.non_data
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# BEGIN DESCR_KINDS_DEMO4
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>>> obj = Managed() # <1>
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>>> Managed.over = 1 # <2>
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>>> Managed.over_no_get = 2
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>>> Managed.non_over = 3
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>>> obj.over, obj.over_no_get, obj.non_over # <3>
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(1, 2, 3)
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# END DESCR_KINDS_DEMO4
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Methods are non-overriding descriptors:
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>>> obj.spam # doctest: +ELLIPSIS
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<bound method Managed.spam of <descriptorkinds.Managed object at 0x...>>
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>>> Managed.spam # doctest: +ELLIPSIS
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<function Managed.spam at 0x...>
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>>> obj.spam()
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-> Managed.spam(<Managed object>)
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>>> Managed.spam()
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Traceback (most recent call last):
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...
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TypeError: spam() missing 1 required positional argument: 'self'
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>>> Managed.spam(obj)
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-> Managed.spam(<Managed object>)
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>>> Managed.spam.__get__(obj) # doctest: +ELLIPSIS
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<bound method Managed.spam of <descriptorkinds.Managed object at 0x...>>
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>>> obj.spam.__func__ is Managed.spam
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True
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>>> obj.spam = 7
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>>> obj.spam
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7
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"""
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"""
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NOTE: These tests are here because I can't add callouts after +ELLIPSIS
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directives and if doctest runs them without +ELLIPSIS I get test failures.
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class DataDescriptor:
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"a.k.a. overriding or enforced descriptor"
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# BEGIN DESCR_KINDS_DEMO2
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>>> obj.over_no_get # <1>
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<__main__.OverridingNoGet object at 0x665bcc>
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>>> Managed.over_no_get # <2>
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<__main__.OverridingNoGet object at 0x665bcc>
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>>> obj.over_no_get = 7 # <3>
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-> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7)
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>>> obj.over_no_get # <4>
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<__main__.OverridingNoGet object at 0x665bcc>
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>>> obj.__dict__['over_no_get'] = 9 # <5>
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>>> obj.over_no_get # <6>
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9
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>>> obj.over_no_get = 7 # <7>
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-> OverridingNoGet.__set__(<OverridingNoGet object>, <Managed object>, 7)
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>>> obj.over_no_get # <8>
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9
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# END DESCR_KINDS_DEMO2
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Methods are non-overriding descriptors:
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# BEGIN DESCR_KINDS_DEMO5
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>>> obj = Managed()
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>>> obj.spam # <1>
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<bound method Managed.spam of <descriptorkinds.Managed object at 0x74c80c>>
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>>> Managed.spam # <2>
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<function Managed.spam at 0x734734>
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>>> obj.spam = 7 # <3>
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>>> obj.spam
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7
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# END DESCR_KINDS_DEMO5
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"""
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# BEGIN DESCR_KINDS
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### auxiliary functions for display only ###
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def cls_name(obj_or_cls):
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cls = type(obj_or_cls)
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if cls is type:
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cls = obj_or_cls
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return cls.__name__.split('.')[-1]
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def display(obj):
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cls = type(obj)
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if cls is type:
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return '<class {}>'.format(obj.__name__)
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elif cls in [type(None), int]:
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return repr(obj)
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else:
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return '<{} object>'.format(cls_name(obj))
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def print_args(name, *args):
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pseudo_args = ', '.join(display(x) for x in args)
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print('-> {}.__{}__({})'.format(cls_name(args[0]), name, pseudo_args))
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### essential classes for this example ###
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class Overriding: # <1>
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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('DataDescriptor.__get__() invoked with args:')
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print(' self = ', self)
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print(' instance = ', instance)
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print(' owner = ', owner)
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print_args('get', self, instance, owner) # <2>
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def __set__(self, instance, value):
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print('DataDescriptor.__set__() invoked with args:')
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print(' self = ', self)
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print(' instance = ', instance)
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print(' value = ', value)
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print_args('set', self, instance, value)
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class DataDescriptorNoGet:
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class OverridingNoGet: # <3>
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"""an overriding descriptor without ``__get__``"""
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def __set__(self, instance, value):
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print('DataDescriptorNoGet.__set__() invoked with args:')
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print(' self = ', self)
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print(' instance = ', instance)
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print(' value = ', value)
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print_args('set', self, instance, value)
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class NonDataDescriptor:
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"a.k.a. non-overriding or shadowable descriptor"
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class NonOverriding: # <4>
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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('NonDataDescriptor.__get__() invoked with args:')
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print(' self = ', self)
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print(' instance = ', instance)
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print(' owner = ', owner)
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print_args('get', self, instance, owner)
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class Model:
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data = DataDescriptor()
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data_no_get = DataDescriptorNoGet()
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non_data = NonDataDescriptor()
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class Managed: # <5>
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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):
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print('Model.spam() invoked with arg:')
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print(' self = ', self)
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def spam(self): # <6>
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print('-> Managed.spam({})'.format(display(self)))
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# END DESCR_KINDS
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