Descriptors

descriptors.py

# Descriptor protocol
# Special objects with that show magic functionality when they are the type of 
# other's class attribute.

## Methods

def __get__(self, obj, type = None) -> object:
    """
        self: descriptor itself
        obj: object the descriptor is attached (as attribute) to
        type: type of the obj
    """
    pass

def __set__(self, obj, value) -> None:
    pass

def __delete__(self, obj) -> None:
    pass

def __set_name__(self, owner, name):
    """
        This is for python > 3.6
        name: is the name assigned as attribute
    """
    pass

## Type of Descriptors

# If only __get__ -> Non-data descriptor
# If __get__ + __set__ / __delete__ -> Data descriptor

# ⛓️ The lookup chain

# When we do OBJECT.attribute/method:

# 1st) Data descriptor (__get__ + __set__ / __delete__): attribute.__get__()
# 2nd) OBJECT.__dict__['attribute/method']
# 3rd) Non-Data descriptor (only __get__): attribute.__get__()
# 4th) type(OBJECT).__dict__['attribute/method'] # Search in the Class attributes
# 5th) type(OBJECT).__base__.__dict__['attribute/method'] # Search in the Base class attributes

## Example of use: Lazy attributes.
# Loads the attribute when it is first accessed

# lazy_properties.py
import time

class LazyProperty:
    """ Non data descriptor for a lazy property
    """
    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) # this is like call meaning_of_life(self), where self == obj
        return obj.__dict__[self.name]

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

my_deep_thought_instance = DeepThought() # At this point, my_deep_thought_instance.meaning_of_life is a method
print(my_deep_thought_instance.meaning_of_life) # Step 1 (miss), Step 2 (miss), Step 3 (found! Non data descriptor)
print(my_deep_thought_instance.meaning_of_life) # Step 1 (miss), Step 2 (found!, already set) -> no execution of function, just attribute
print(my_deep_thought_instance.meaning_of_life) # At this point my_deep_thought_instance.meaning_of_life is an integer (42)