Python Language Tutorial => Simple descriptor

Example

There are two different types of descriptors. Data descriptors are defined as objects that define both a __get__() and a __set__() method, whereas non-data descriptors only define a __get__() method. This distinction is important when considering overrides and the namespace of an instance's dictionary. If a data descriptor and an entry in an instance's dictionary share the same name, the data descriptor will take precedence. However, if instead a non-data descriptor and an entry in an instance's dictionary share the same name, the instance dictionary's entry will take precedence.

To make a read-only data descriptor, define both get() and set() with the set() raising an AttributeError when called. Defining the set() method with an exception raising placeholder is enough to make it a data descriptor.

descr.__get__(self, obj, type=None) --> value
descr.__set__(self, obj, value) --> None
descr.__delete__(self, obj) --> None

An implemented example:

class DescPrinter(object):
    """A data descriptor that logs activity."""
    _val = 7
    
    def __get__(self, obj, objtype=None):
        print('Getting ...')
        return self._val

    def __set__(self, obj, val):
        print('Setting', val)
        self._val = val
    
    def __delete__(self, obj):
        print('Deleting ...')
        del self._val


class Foo():
    x = DescPrinter()       

i = Foo()
i.x
# Getting ...
# 7

i.x = 100
# Setting 100
i.x
# Getting ...
# 100

del i.x
# Deleting ...
i.x
# Getting ...
# 7

PDF - Download Python Language for free

Previous Next

Python Language

Fastest Entity Framework Extensions

Example

Got any Python Language Question?

Python Language

Python Language Descriptor Simple descriptor

Fastest Entity Framework Extensions

Example

Got any Python Language Question?