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  1. def find_descriptor1(instance, attrname):
  2.     '''Find the descriptor handling a given attribute, if any.
  3.  
  4.     If the attribute named attrname of the given instance is handled by a
  5.     descriptor, this will return the descriptor object handling the attribute.
  6.     Otherwise, it will return None.
  7.     '''
  8.     def hasspecialmethod(obj, name):
  9.         return any(name in klass.__dict__ for klass in type(obj).__mro__)
  10.     for klass in type(instance).__mro__:
  11.         if attrname in klass.__dict__:
  12.             descriptor = klass.__dict__[attrname]
  13.             if not (hasspecialmethod(descriptor, '__get__') or
  14.                     hasspecialmethod(descriptor, '__set__') or
  15.                     hasspecialmethod(descriptor, '__delete__')):
  16.                 # Attribute isn't a descriptor
  17.                 return None
  18.             if (attrname in instance.__dict__ and
  19.                 not hasspecialmethod(descriptor, '__set__') and
  20.                 not hasspecialmethod(descriptor, '__delete__')):
  21.                 # Would be handled by the descriptor, but the descriptor isn't
  22.                 # a data descriptor and the object has a dict entry overriding
  23.                 # it.
  24.                 return None
  25.             return descriptor
  26.     return None
  27.  
  28. def find_descriptor2(obj, name:str):
  29.     """ Find the descriptor, if any, an attribute retrieval on an instance would go through
  30.  
  31.         :param obj: instance object
  32.         :param name: attribute which would be retrieved
  33.         :returns: unbound descriptor ``__get__``, or ``None`` if no descriptor would be used
  34.     """
  35.     # lookup attribute in class hierarchy
  36.     for base in type(obj).__mro__:
  37.         base_dict = vars(base) 
  38.         if name in base_dict:
  39.             # we look for descriptor interface on value's class, not value itself
  40.             value_clazz = type(base_dict[name])
  41.             descr = getattr(value_clazz, '__get__', None)
  42.             if descr:
  43.                 # data descriptor?
  44.                 if (hasattr(value_clazz, '__set__') or hasattr(value_clazz, "__delete__")):
  45.                     return descr
  46.                 # instance variable can be used instead?
  47.                 if hasattr(obj, '__dict__') and name in vars(obj):
  48.                     return
  49.             # non-data descriptor?
  50.             # if None, it indicates a class variable
  51.             return descr
  52.  
  53. class WeirdDescriptor:
  54.     def __set_name__(self, owner, name):
  55.         self.name = name
  56.     def __set__(self, instance, value):
  57.         print('setter invoked!')
  58.         instance.__dict__[self.name] = value
  59.  
  60. class Foo:
  61.     x = WeirdDescriptor()
  62.  
  63. x = Foo()
  64. x.x = 2
  65. print(x.x)
  66. print(find_descriptor1(x, 'x'))
  67. print(find_descriptor2(x, 'x'))
Success #stdin #stdout 0.03s 9620KB
comments (?)
stdin
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Standard input is empty
stdout
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setter invoked!
2
<__main__.WeirdDescriptor object at 0x15284288bd00>
None
https://ideone.com/FlXM8f
language:
Python 3 (python  3.12)
created:
1 year ago
visibility:
public

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