Attribute Descriptors

Describing Attribute Descriptors

• Sometimes, attributes are wrapped with an extra layer of logic that interact with:

  • Get operations
  • Set operations
  • Delete operations
• This kind of wrapping is accomplished by creating a descriptor object

• An object is considered a descriptor if any of the following are defined:

  • __get__()
  • __set__()
  • __delete__()
• For example, properties implement the descriptor protocol

Describing Special Methods of a Descriptor

• Our three special methods of a descriptor are meant to interact with the default implementation of the following:

  • __getattribute__()
  • __setattr__()
  • __delattr__()

• Specifically, there are two types of descriptors:

  • Data Descriptor: Has a __set__ and/or __delete__
  • Non-Data Descriptor: Neither __set__ nor __delete__
• Classmethods, staticmethods, and functions are examples of non-data descriptors
• Properties are an example of a data descriptor
• In general, we should try to use a simple class attribute
• If necessary, then we could use the property decorator
• The following is an example of a descriptor:

>>> class Descriptor(object):
...     def __init__(self, name):
...         self.name = name
...     def __get__(self, obj, objtype):
...         return '{}for{}'.format(self.name, self.name)
...     def __set__(self, obj, name):
...         if isinstance(name, str):
...             self.name = name
...         else:
...             raise TypeError('Name should be string')

>>> class GFG(object):
...     name = Descriptor() 
    
>>> g = GFG()
>>> g.name = 'Todd'
>>> print(g.name)
'ToddforTodd'

Creating a Descriptor using Properties

>>> class Alphabet:
...     def __init__(self, value):
...         self._value = value
...
... # getting the values
... @property
... def value(self):
...     print('Getting value')
...     return self._value
... # setting the values
... @value.setter
... def value(self, value):
...     print('Setting value to ' + value)
...     self._value = value
... # deleting the values
... @value.deleter
... def value(self):
...     print('Deleting value')
...     del self._value

>>> x = Alphabet('Peter')
>>> print(x.value)
>>> x.value = 'Diesel'
>>> del x.value

Summarizing Special Methods

References

• Python Descriptors
• Python Essential Reference
• Understanding get and set Special Methods
• Details about Descriptors in Python