Advanced OOP — Series 2

Preview — 3 of 10 questions

What is the output of the following code?

javascript

from abc import ABC, abstractmethod

class Flyable(ABC):
    @classmethod
    def __subclasshook__(cls, subclass):
        return hasattr(subclass, 'fly') and callable(subclass.fly)

class Bird:
    def fly(self):
        return "flap flap"

class Rock:
    pass

print(issubclass(Bird, Flyable))
print(issubclass(Rock, Flyable))
print(isinstance(Bird(), Flyable))

ATrue, False, True

BTrue, True, True — __subclasshook__ makes every class a virtual subclass of Flyable automatically.

CFalse, False, True

DTrue, False, False

What is the output of the following code?

javascript

class LoggedAttr:
    def __init__(self):
        self.value = None
    def __get__(self, instance, owner):
        return self.value
    def __set__(self, instance, value):
        self.value = value
    def __delete__(self, instance):
        print("deleting attribute")
        self.value = None

class Widget:
    color = LoggedAttr()

w = Widget()
w.color = "red"
print(w.color)
del w.color
print(w.color)

Ared, then del w.color raises AttributeError: __delete__

Bred, then AttributeError: attribute 'color' cannot be deleted (no deleting attribute message)

Cred, deleting attribute, None

Dred, deleting attribute, red (the value is unaffected by deletion)

What is the output of the following code?

javascript

class Proxy:
    def __init__(self):
        self.real = 42
    def __getattr__(self, name):
        return f"fallback for {name}"

p = Proxy()
print(p.real)
print(p.missing)

Afallback for real, fallback for missing

B42, AttributeError: 'Proxy' object has no attribute 'missing'

C42, None

D42, fallback for missing

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