PCPP1 Advanced Object Oriented Programming — 30 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__.

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Non-data descriptors can override instance dictionary lookup.

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Data descriptors only function with class attributes.

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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.

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Call the grandparent class directly.

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Use the __bases__ attribute to iterate through parents.

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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.

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__format__

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__unicode__

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__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.

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

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It raises a TypeError at class definition time.

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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.

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The __new__ method cannot accept variable arguments in Python 3.

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The __new__ method is reserved for static methods only.

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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.

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It causes circular dependency issues.

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It makes the notify method thread-unsafe.

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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.

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

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

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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.

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

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

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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.

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__context__

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__open__ and __close__

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__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__.

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To allow private attribute access.

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To define the method resolution order.

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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.

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It guarantees thread safety without locks.

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It prevents the instantiation of abstract classes.

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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.

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Raising AttributeError.

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Accessing class attributes.

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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.

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__apply__

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__invoke__

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__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.

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The child class will raise an error at instantiation.

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The child class will inherit the memory savings automatically.

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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()).

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Every time an instance method is called.

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When the metaclass is first imported.

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When the class itself is defined.

Question 16

When implementing the Singleton pattern, what is the advantage of using a module-level instance over a class-based Singleton?

Accepted Answer

It is simpler and leverages Python's built-in module caching mechanism.. Python modules are naturally Singletons because they are initialized only once upon the first import.

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It is more secure.

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It allows multiple instances if needed.

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It supports lazy loading.

Question 17

Which magic method is used to control how an instance is displayed when using the print() function?

Accepted Answer

__str__. print() implicitly calls str(obj), which in turn calls __str__.

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__display__

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__repr__

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__print__

Question 18

Why might you use the __init_subclass__ hook instead of a metaclass?

Accepted Answer

It is easier to implement and avoids metaclass conflicts.. __init_subclass__ provides a simpler way to perform logic when a subclass is created, without the complexity and inheritance issues associated with custom metaclasses.

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Metaclasses are deprecated.

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Metaclasses cannot be used for registration.

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It is faster at runtime.

Question 19

When designing a class hierarchy, what is the best way to prevent a method from being overridden by subclasses?

Accepted Answer

Using the @final decorator from typing.. Python does not have a native 'final' keyword, but naming conventions (e.g., __method) or raising errors in the subclass can be used, though it is often discouraged in favor of clear documentation.

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Defining it as a private method.

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Defining it as a static method.

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Using the 'final' keyword.

Question 20

In the context of the Observer pattern, what is the benefit of an event-driven system over a direct-method-call approach?

Accepted Answer

Decoupling between the subject and observers.. Decoupling is the primary benefit; the subject does not need to know the implementation details of the observers.

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Reduced memory usage.

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Increased performance.

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Automatic thread synchronization.