Descriptors

Describing Descriptors in Python

• Properties provide access to attributes using a series of user-defined get, set, and delete methods
• This form of attribute control can be further generalized through the use of a descriptor object
• A descriptor is an object representing the value of an attribute

• A descriptor implements the following special methods:

  • __get__()
  • __set__()
  • __delete__{}
• By doing this, it can customize the operations of attributes

Illustrating a Descriptor Object

• The following is an example of a descriptor:

>>> class TypedProperty(object):  # descriptor
...     def __init__(self, name, type, default=None)
...         self.name = '_' + name
...         self.type = type
...         self.default = default if default else type()
...     def __get__(self, inst, cls):
...         return getattr(inst, self.name, self.default)
...     def __set__(self, instance, value):
...         if not isinstance(value, self.type):
...             raise TypeError('wrong type')
...         setattr(instance, self.name, value)
...     def __delete__(self, instance):
...         raise AttributeError('cannot del attribute')

>>> class Foo(object):
...     name = TypedProperty('name', str)
...     name = TypedPropert('num', int, 42)

• In this example, TypedProperty defines a descriptor
• It performs type cheecking when the attribute is assigned
• Then, an error is produced if an attempt is made to delete the attribute

References

• Python Essential Reference