descriptors.py

# https://dev.to/dawranliou/writing-descriptors-in-python-36
# https://developer.ibm.com/tutorials/os-pythondescriptors/

# A descriptor is an object attribute with “binding behavior”,
# one whose attribute access has been overridden by methods in the descriptor protocol.
# Those methods are __get__(), __set__(), and __delete__().
# If any of those methods are defined for an object, it is said to be a descriptor.

# Why descriptors
class Order:
    def __init__(self, name, price, quantity):
        self.name = name
        self.price = price
        self.quantity = quantity

    def total(self):
        return self.price * self.quantity


apple_order = Order('apple', 1, 10)
print(apple_order.total())

# Bug
apple_order.quantity = -10
print(apple_order.total())


# Instead of using getter and setter methods and break the APIs,
# use property to enforce quantity be positive.
class Order:
    def __init__(self, name, price, quantity):
        self._name = name
        self.price = price  # price attribute cannot be negative neither.
        self._quantity = quantity

    @property
    def quantity(self):
        return self._quantity

    @quantity.setter
    def quantity(self, value):
        if value < 0:
            raise ValueError('Cannot be negative.')
        self._quantity = value

    def total(self):
        return self.price * self.quantity


apple_order = Order('apple', 1, 10)
try:
    apple_order.quantity = -10
except ValueError:
    print("Raised exeception")
print(apple_order.total())


# 'price' attribute cannot be negative neither. It might be attempting to just create another property for price,
# but remember the DRY principle. Also, in our example, there might be more attributes need to be added into this class in the future.


# Use descriptors, How to write descriptors?

# Define the NonNegative class and implement the descriptor protocols.
class NonNegative:
    def __init__(self, name):
        self.name = name

    def __get__(self, instance, owner):
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        if value < 0:
            raise ValueError('Cannot be negative.')
        instance.__dict__[self.name] = value


# The main thing to remember is that descriptors are linked to classes and not to instances.
class Order:
    price = NonNegative('price')  # Similar to SQLAlchemy
    quantity = NonNegative('quantity')

    def __init__(self, name, price, quantity):
        self._name = name
        self.price = price
        self.quantity = quantity

    def total(self):
        return self.price * self.quantity


# when we go to set price/quantity, Python notices that it is a descriptor.
# Python knows that price/quantity is a descriptor because we defined it as such when we created it as a class attribute.
# So when we go to set price/quantity, we actually call our descriptor’s __set__ method which
# passes in the instance and the value that we are trying to set.

apple_order = Order('apple', 1, 10)
apple_order.total()
try:
    apple_order.price = -10
except ValueError:
    print("Raised exeception")

try:
    apple_order.quantity = -10
except ValueError:
    print("Raised exeception")


# In Python 3.6
# object.__set_name__(self, owner, name)
# Called at the time the owning class owner is created. The descriptor has been assigned to name.

class NonNegative:
    def __get__(self, instance, owner):  # Instance of the Order class, owner is the Order class
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        print("In set ",instance,value)
        if value < 0:
            raise ValueError('Cannot be negative.')
        instance.__dict__[self.name] = value

    # Called automatically with the name of the attribute, on the LHS. [price,quantity]
    def __set_name__(self, owner, name):
        print("__set_name__=", name)
        self.name = name


class Order:
    price = NonNegative()
    quantity = NonNegative()

    def __init__(self, name, price, quantity):
        self._name = name
        self.price = price
        self.quantity = quantity

    def total(self):
        return self.price * self.quantity

    # def __setattr__(self, key, value):
    #     print("In setter",key,value)
    #     self.__dict__[key] = value


apple_order = Order('apple', 1, 10)
apple_order.total()
apple_order.price = 5
try:
    apple_order.price = -10
except ValueError:
    print("Raised exeception")

try:
    apple_order.quantity = -10
except ValueError:
    print("Raised exeception")

print("\n\n********Order.__dict__********")
print(Order.__dict__)
print("\n\n********apple_order.__dict__********")
print(apple_order.__dict__)