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