Python descriptors | Rustam Zokirov

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What are the descriptors?

Descriptors are Python objects that implement a method of the descriptor protocol, which gives you the ability to create objects that have special behavior when they’re accessed as attributes of other objects.

Examples:

We can use @property decorator, used to make pre-build validation before saving.
staticmethod, classmethod are also descriptors
functions also use get, set, delete dunder methods. And get returns the bound version of the method

Another example:

# descriptors.py
class Verbose_attribute():
    def __get__(self, obj, type=None) -> object:
        print("accessing the attribute to get the value")
        return 42
    def __set__(self, obj, value) -> None:
        print("accessing the attribute to set the value")
        raise AttributeError("Cannot change the value")

class Foo():
    attribute1 = Verbose_attribute() # <- this is how descriptors should be created

my_foo_object = Foo()
x = my_foo_object.attribute1
print(x)

# lazy_properties.py
import time

class LazyProperty:
    def __init__(self, function):
        self.function = function
        self.name = function.__name__

    def __get__(self, obj, type=None) -> object:
        obj.__dict__[self.name] = self.function(obj)
        return obj.__dict__[self.name]

class DeepThought:
    @LazyProperty
    def meaning_of_life(self):
        time.sleep(3)
        return 42

my_deep_thought_instance = DeepThought()
print(my_deep_thought_instance.meaning_of_life)
print(my_deep_thought_instance.meaning_of_life)
print(my_deep_thought_instance.meaning_of_life)

# properties2.py Usefull for following DRY design principle
class EvenNumber:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, obj, type=None) -> object:
        return obj.__dict__.get(self.name) or 0

    def __set__(self, obj, value) -> None:
        obj.__dict__[self.name] = (value if value % 2 == 0 else 0)

class Values:
    value1 = EvenNumber()
    value2 = EvenNumber()
    value3 = EvenNumber()
    value4 = EvenNumber()
    value5 = EvenNumber()

my_values = Values()
my_values.value1 = 1
my_values.value2 = 4
print(my_values.value1)
print(my_values.value2)