Python Descriptor Classes: Hook into Attribute Access

Python allows us to execute code when an attribute of our class gets accessed, set, or removed through Descriptor Classes. A descriptor class is just a simple class that defines either __get__ , __set__ or __delete__ methods. For example:

class MyDescriptor(object):
    def __init__(self, *args, **kwargs):
        self.value = 1
  
    def __get__(self, obj, owner=None):
        print('get is called')
        try:
            return self.value
        except AttributeError:
            # do something
            raise AttributeError
   
    def __set__(self, obj, value):
        print('set is called')
        self.value = value
  
    def __delete__(self, obj):
        print('delete is called')
        del self.value

To use the descriptor class, you need to make an instance of it an attribute of your class like:

class Foo(object):
    bar = MyDescriptor()

Now every time the bar attribute of the instances of your class Foo is accessed, set, or deleted the corresponding function you defined in MyDescriptor is executed.

>>> a = Foo()
>>> print(a.bar)
get is called
1
>> a.bar = 2
set is called
>> print(a.bar)
get is called
2
>> del a.bar
delete is called

Descriptor classes are simple yet very powerful. Use it.

Thanks for reading.