PCPP1 Advanced Object Oriented Programming — Timed 15 Questions

Question 1

In the context of the Descriptor Protocol, what is the primary difference between a data descriptor and a non-data descriptor?

Accepted Answer

Data descriptors define __set__ or __delete__, whereas non-data descriptors do not.. A data descriptor defines both __get__ and __set__ (or __delete__), while a non-data descriptor only defines __get__.

Answer

Non-data descriptors can override instance dictionary lookup.

Answer

Data descriptors only function with class attributes.

Answer

Non-data descriptors are always read-only.

Question 2

How do you correctly call a method from a sibling class in a diamond inheritance structure using super()?

Accepted Answer

Use super() in each class to delegate to the next class in the MRO.. super() follows the Method Resolution Order (MRO), which correctly handles diamond inheritance by delegating calls to the next class in the chain, not just the parent.

Answer

Call the grandparent class directly.

Answer

Use the __bases__ attribute to iterate through parents.

Answer

Explicitly call the sibling class method by name.

Question 3

Which magic method is responsible for providing the informal string representation of an object, typically used for user-facing output?

Accepted Answer

__str__. The __str__ method is used for informal, readable string representations, whereas __repr__ is for formal, unambiguous representations.

Answer

__format__

Answer

__unicode__

Answer

__repr__

Question 4

What is the result of applying the @staticmethod decorator to a method inside a class that also defines a metaclass?

Accepted Answer

The method is correctly bound as a static function regardless of the metaclass.. The @staticmethod decorator creates a static method object which is stored in the class dictionary; it is independent of the metaclass's __init__ logic for instance methods.

Answer

The metaclass must manually convert it back to a static method.

Answer

It raises a TypeError at class definition time.

Answer

The metaclass will treat the method as an instance method.

Question 5

You are implementing a Singleton pattern using the __new__ method. Why is it considered best practice to also define a __call__ method in the metaclass or use a decorator instead of just overriding __new__ in the base class?

Accepted Answer

It prevents the creation of multiple instances during the unpickling process.. Overriding __new__ in a base class can lead to issues with pickling and deserialization, as __new__ is called every time an object is unpickled, potentially creating multiple instances.

Answer

The __new__ method cannot accept variable arguments in Python 3.

Answer

The __new__ method is reserved for static methods only.

Answer

It is required for thread-safe access to the instance variable.

Question 6

You are implementing the Observer pattern. Why should you avoid using a strong reference to the observers in the subject's list?

Accepted Answer

It prevents the observer from being garbage collected when it is no longer needed.. Strong references prevent garbage collection of the observers, leading to memory leaks if observers are meant to be temporary.

Answer

It causes circular dependency issues.

Answer

It makes the notify method thread-unsafe.

Answer

It violates the principle of encapsulation.

Question 7

When using abstract base classes (ABCs) from the abc module, what happens if a class inherits from an ABC but fails to implement one of the abstract methods?

Accepted Answer

The class will be marked as abstract and will raise a TypeError upon instantiation.. Python will raise a TypeError when you attempt to instantiate the class, preventing the creation of an incomplete object.

Answer

The class will be created, but calling the missing method will raise an AttributeError.

Answer

The missing method will default to a 'pass' statement.

Answer

The parent ABC will automatically provide a NotImplementedError.

Question 8

Consider a class that uses a metaclass to automatically register subclasses in a dictionary. If you want to prevent the base class itself from being registered, how should the metaclass be implemented?

Accepted Answer

Check the class name within the metaclass __init__ method before adding it to the registry.. The metaclass __init__ method receives the class object. By checking the class name or using a specific attribute (e.g., _is_base), you can conditionally skip registration.

Answer

Use the __call__ method to raise a TypeError if the base class is instantiated.

Answer

Set the __new__ method of the metaclass to return None if the name is 'Base'.

Answer

Define a decorator on the base class that removes it from the metaclass registry.

Question 9

Which magic method should you implement to make your objects support the 'with' statement context manager?

Accepted Answer

__enter__ and __exit__. The __enter__ and __exit__ methods are required to implement the context manager protocol.

Answer

__context__

Answer

__open__ and __close__

Answer

__init__ and __del__

Question 10

Which of the following describes the purpose of the __slots__ attribute in a class?

Accepted Answer

To improve performance and reduce memory usage.. __slots__ restricts the creation of new instance attributes and reduces memory footprint by preventing the creation of a per-instance __dict__.

Answer

To allow private attribute access.

Answer

To define the method resolution order.

Answer

To prevent inheritance.

Question 11

In the context of the Factory pattern in Python, what is a typical benefit of using a registry-based approach instead of a large if-else block?

Accepted Answer

It enables easy extension by adding new products without changing the factory.. Registry-based factories allow for decoupled code where new classes can be registered without modifying the factory's core logic.

Answer

It guarantees thread safety without locks.

Answer

It prevents the instantiation of abstract classes.

Answer

It significantly reduces memory usage.

Question 12

When overriding the __getattr__ method, what must you be careful to avoid?

Accepted Answer

Infinite recursion by accessing non-existent attributes.. Infinite recursion occurs if you try to access an attribute inside __getattr__ that does not exist, triggering __getattr__ again.

Answer

Raising AttributeError.

Answer

Accessing class attributes.

Answer

Calling super().__getattr__.

Question 13

Which magic method allows an object to behave like a function?

Accepted Answer

__call__. The __call__ method enables an object to be invoked with parentheses syntax.

Answer

__apply__

Answer

__invoke__

Answer

__func__

Question 14

What is the effect of setting __slots__ in a parent class on a child class that does not define its own __slots__?

Accepted Answer

The child class will have a __dict__ attribute, overriding the parent's optimization.. The child class will receive an instance __dict__ unless it also defines __slots__, effectively defeating the memory optimization of the parent.

Answer

The child class will raise an error at instantiation.

Answer

The child class will inherit the memory savings automatically.

Answer

The child class will be unable to add new attributes.

Question 15

If you define a class with a metaclass that has a custom __call__ method, when is that __call__ method executed?

Accepted Answer

When an instance of the class is created.. The metaclass's __call__ method is invoked whenever the class created by that metaclass is instantiated (i.e., when you call the class name like MyClass()).

Answer

Every time an instance method is called.

Answer

When the metaclass is first imported.

Answer

When the class itself is defined.