PCPP1 Advanced Object Oriented Programming Practice Test 4 — 15 Questions

Question 1

Which TWO of the following are valid ways to create a singleton in Python?

Accepted Answer

Using a base class with a custom __new__ method.. Option C is correct because overriding __new__ in a base class lets you intercept instance creation and return the same cached instance every time, which is the canonical Python idiom for enforcing a single instance. Option E is correct because Python modules are imported only once and cached in sys.modules, so module-level state and functions naturally behave as a singleton. Option A is wrong because an uninitialized global variable provides no instance at all and no control over creation. Option B is wrong because returning a new instance on every call is the opposite of the singleton pattern. Option D is wrong because a plain class with no special methods creates a fresh object on each instantiation.

Answer

Using a global variable that is not initialized.

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Using a function that returns a new instance every call.

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Using a standard class without any special methods.

Question 2

Which TWO of the following are true regarding multiple inheritance in Python?

Accepted Answer

super() is used for cooperative method calls.. Option B is correct because in Python's multiple inheritance, super() does not simply call the parent class but follows the Method Resolution Order to invoke the next class in the chain, enabling cooperative multiple inheritance where each class's method calls super() to pass control along the MRO. Option D is correct because Python resolves the method resolution order for multiple inheritance using the C3 linearization algorithm, which produces a consistent, monotonic ordering that respects local precedence and subclass relationships. Option A is incorrect because Python explicitly supports multiple inheritance, allowing a class to inherit from several base classes. Option C is incorrect because the MRO is computed at class creation time (when the class is defined), not dynamically at runtime for each method call. Option E is incorrect because composition is often preferred over multiple inheritance for maintainability and avoiding the diamond problem; multiple inheritance is not always preferred.

Answer

It is not supported.

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The MRO is determined at runtime.

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Multiple inheritance is always preferred over composition.

Question 3

Which TWO of the following are correct regarding the 'with' statement in Python?

Accepted Answer

It requires the class to define __enter__ and __exit__.. Option D is correct because the context manager protocol requires the object used in a 'with' statement to implement the special methods __enter__() (which runs on entry and may return a value bound via 'as') and __exit__() (which runs on exit and receives exception type, value, and traceback). Option E is correct because the 'with' statement is semantically equivalent to a try/finally block, so __exit__() is invoked whether the block completes normally or raises an exception, ensuring cleanup such as closing files or releasing locks. Option A is wrong because 'with' does not rely on __init__; initialization happens when the object is constructed, not when the context is entered. Option B is wrong because 'with' is not limited to file objects; any object implementing the context manager protocol works, including locks, sockets, and database connections. Option C is wrong as stated because not every object supports the protocol — only those defining __enter__ and __exit__ can be used directly in a 'with' statement.

Answer

It works by implicit __init__ calling.

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It only works with file objects.

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It can be used with any object.

Question 4

Which TWO of the following are true about magic methods?

Accepted Answer

They provide hooks into Python language operators.. Option B is correct because magic methods (also called dunder methods) act as hooks that let user-defined classes integrate with Python's built-in syntax and operators — for example, __add__ is invoked by the + operator, __len__ by len(), and __iter__ by iteration. Option C is correct because magic methods are conventionally and syntactically named with a leading and trailing double underscore, such as __init__, __str__, or __eq__. Option A is wrong because magic methods are not limited to built-in types; user-defined classes can define and override them. Option D is wrong because magic methods can be overridden in subclasses or custom classes to customize behavior. Option E is wrong because magic methods are meant to be invoked implicitly by the interpreter through operators and built-in functions, not called directly by the user.

Answer

They are only available for built-in types.

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They cannot be overridden.

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They are intended for direct calling by the user.

Question 5

What is the purpose of the __repr__ method?

Accepted Answer

Unambiguous representation for debugging.. It provides a formal, unambiguous string representation of an object, ideally suitable for re-creating the object.

Answer

To be called by print().

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User-friendly output.

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To store the object in memory.

Question 6

When using the __call__ method to implement a decorator, what does the decorator receive as an argument?

Accepted Answer

The function to be decorated.. The decorator receives the function object that is being decorated as the argument to the constructor (if the decorator is a class instance) or directly if it is a function.

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The arguments to the function.

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The instance of the class.

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The class object.

Question 7

Which THREE of the following are potential pitfalls of the Singleton pattern in Python?

Accepted Answer

They introduce global state.. Option B is correct because a Singleton stores its single instance in a class-level or module-level location, effectively creating global mutable state that any part of the program can read or modify, which increases coupling and makes behavior harder to reason about. Option C is correct because Singleton clients typically call a hard-coded accessor such as getInstance() or reference a module-level instance directly, so you cannot easily substitute an alternative implementation through constructor injection or a DI container. Option D is correct because that same hard-coded access and shared state persist across tests, so tests can leak state into one another and cannot easily isolate the class under test with a mock or fake. Option A is wrong because Singletons can participate in inheritance (subclasses can inherit or override the instance accessor), even if doing so is awkward. Option E is wrong because a naive Singleton implementation is not automatically thread-safe; without a lock or other synchronization, two threads can race and create multiple instances.

Answer

They cannot be used with inheritance.

Answer

They are always thread-safe.

Question 8

In the context of the Factory pattern, what does a concrete factory implement?

Accepted Answer

The creation interface for specific product variants.. A concrete factory implements the interface defined by the abstract factory to produce specific products.

Answer

The registration logic for all factories.

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The logic for the singleton pattern.

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The product logic.

Question 9

What is the primary risk of using 'getattr' without a default value?

Accepted Answer

It raises an AttributeError.. If the attribute is not found, it raises an AttributeError, potentially crashing the application if not handled.

Answer

It returns None.

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It enters an infinite loop.

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It creates the attribute.

Question 10

Which function is used to check if an object is an instance of a class?

Accepted Answer

isinstance(). isinstance(obj, Class) is the standard way to check inheritance.

Answer

hasattr()

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issubclass()

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type()

Question 11

Which THREE of the following represent common issues when using __slots__?

Accepted Answer

Subclasses must define their own __slots__ to avoid creating a __dict__.. Option A is correct because if a subclass does not declare its own __slots__, Python automatically gives it a __dict__, defeating the memory-saving purpose of __slots__ in the parent class. Option B is correct because __slots__ restricts instances to only the named attributes, so assigning any new attribute not listed raises an AttributeError. Option E is correct because defining __slots__ removes the per-instance __dict__ by default, which is the primary mechanism for saving memory. Option C is wrong because __slots__ works fine with most decorators such as @property, @staticmethod, and @classmethod. Option D is wrong because __slots__ does not prevent multiple inheritance; it only imposes constraints when multiple bases have non-empty __slots__ with conflicting slot names.

Answer

They are incompatible with all decorators.

Answer

They prevent the use of multiple inheritance.

Question 12

Why should you use super() instead of calling the parent class method directly by name?

Accepted Answer

It correctly resolves the next method in the MRO.. super() is preferred over calling the parent class method directly because it correctly resolves the next method in the Method Resolution Order (MRO). This ensures that in multiple inheritance scenarios, all appropriate parent classes are called in the correct order, avoiding duplication or skipping.

Answer

It is required for private methods.

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It is faster.

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It makes the code more verbose.

Question 13

What happens when you add two objects of a class that implements __add__?

Accepted Answer

The __add__ method is called.. The __add__ method is automatically called by the interpreter when the '+' operator is used.

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It does nothing.

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It concatenates the objects.

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It raises a TypeError.

Question 14

Why does the __init__ method not return anything?

Accepted Answer

Because it is designed to initialize an existing instance.. The __init__ method is for initialization only; the object creation is handled by __new__.

Answer

Because of internal Python constraints.

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To allow for multiple initialization.

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Because it's a generator.

Question 15

What is the purpose of the __set_name__ method in descriptors?

Accepted Answer

To allow the descriptor to know its name in the owner class.. __set_name__ is called at class creation time to inform the descriptor of its name in the owner class, allowing it to store values in a private attribute without needing to know the name beforehand.

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To set the name of the class.

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To delete the attribute.

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To rename the attribute.