52 questions · Advanced Object Oriented Programming · All types, answers revealed
Which of the following describes the purpose of the __slots__ attribute in a class?
This is the primary design goal of __slots__.
Why this answer
__slots__ restricts the creation of new instance attributes and reduces memory footprint by preventing the creation of a per-instance __dict__.
What happens if you use a decorator on a class that returns a modified version of the class?
The variable name points to the returned object, not the original class definition.
Why this answer
The class name, module, and other attributes might be replaced, which can confuse introspection tools; functools.wraps is usually not applicable to classes.
Which magic method is responsible for providing the informal string representation of an object, typically used for user-facing output?
__str__ is the standard method for informal string conversion.
Why this answer
The __str__ method is used for informal, readable string representations, whereas __repr__ is for formal, unambiguous representations.
Which magic method allows an object to behave like a function?
This is the correct magic method for function-like objects.
Why this answer
The __call__ method enables an object to be invoked with parentheses syntax.
In the context of the Descriptor Protocol, what is the primary difference between a data descriptor and a non-data descriptor?
This is the strict definition of the descriptor protocol's precedence.
Why this answer
A data descriptor defines both __get__ and __set__ (or __delete__), while a non-data descriptor only defines __get__.
In the Factory pattern, what should you do if the requested type is unknown?
Explicitly signaling an error is best practice.
Why this answer
Raising a ValueError or a custom 'UnknownTypeException' is the standard way to handle invalid inputs in a factory.
What is the difference between a class-level variable and an instance-level variable?
Modifying a class variable affects all instances.
Why this answer
A class-level variable is shared across all instances of the class, whereas an instance-level variable is specific to each object.
When overriding the __getattr__ method, what must you be careful to avoid?
Always use super() or direct dictionary access to avoid the cycle.
Why this answer
Infinite recursion occurs if you try to access an attribute inside __getattr__ that does not exist, triggering __getattr__ again.
When using a metaclass, how can you access the class attributes during the class definition?
The dictionary is the source of all definitions.
Why this answer
The class dictionary passed to the metaclass's __new__ method contains all the attributes defined within the class body.
Which TWO of the following are true regarding multiple inheritance in Python?
This allows all classes in the chain to execute.
Why this answer
Python uses C3 MRO to resolve calls, and super() is used for cooperative calls.
Which THREE of the following statements are correct about Python metaclasses?
All classes are types.
Why this answer
Metaclasses create classes, they can be inherited, and they can be used for automatic registration.
What is the primary risk of using 'getattr' without a default value?
This is the standard behavior when the attribute is missing.
Why this answer
If the attribute is not found, it raises an AttributeError, potentially crashing the application if not handled.
When using the __call__ method to implement a decorator, what does the decorator receive as an argument?
The decorator captures the function object.
Why this answer
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.
Why does the __init__ method not return anything?
The instance is already created by __new__.
Why this answer
The __init__ method is for initialization only; the object creation is handled by __new__.
What is the purpose of the 'weakref' module in relation to the Observer pattern?
Weak references do not increment the reference count, allowing collection.
Why this answer
It allows the subject to maintain references to observers without preventing their collection by the garbage collector.
How do you define a property in a class to make it read-only?
This creates a read-only attribute interface.
Why this answer
By defining the property with only a getter (using @property) and omitting the setter method.
Which magic method is used to control how an instance is displayed when using the print() function?
This is the primary method for string conversion.
Why this answer
print() implicitly calls str(obj), which in turn calls __str__.
Which THREE of the following are true about the Python descriptor protocol?
Descriptors are class attributes.
Why this answer
Descriptors allow you to manage attribute access, they have specific lookup order, and they are used by property/classmethod internally.
In the context of the Observer pattern, what is the benefit of an event-driven system over a direct-method-call approach?
The subject maintains a generic interface for observers.
Why this answer
Decoupling is the primary benefit; the subject does not need to know the implementation details of the observers.
What is the purpose of the __set_name__ method in descriptors?
This is the purpose of the method.
Why this answer
__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.
Why should you use super() instead of calling the parent class method directly by name?
This is the correct architectural use of super().
Why this answer
super() respects the MRO and allows for cooperative multiple inheritance, whereas hard-coding the class name breaks if the hierarchy is modified.
What is the purpose of the 'type' function when called with three arguments?
The three-argument form is the type constructor for classes.
Why this answer
It is used for dynamic class creation: type(name, bases, dict).
What does the @abstractmethod decorator do when applied to a method inside an ABC?
This is the core requirement of ABCs.
Why this answer
It marks the method as requiring an override in any non-abstract concrete subclass.
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?
This is the core behavior of ABCs; they act as templates that must be fully realized.
Why this answer
Python will raise a TypeError when you attempt to instantiate the class, preventing the creation of an incomplete object.
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?
Using a metaclass or a decorator ensures that the instance returned during deserialization is the same one already created, maintaining the Singleton property.
Why this answer
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.
What is the consequence of not calling super().__init__() in a class with multiple inheritance?
Cooperative multiple inheritance depends on every class calling super().
Why this answer
The MRO chain will break, and sibling classes may not be initialized properly, potentially causing missing attributes or logic errors.
Which magic method is triggered when an object is deleted using the 'del' keyword?
This is the finalizer method.
Why this answer
The __del__ method is called when the object's reference count reaches zero.
What is the result of applying the @staticmethod decorator to a method inside a class that also defines a metaclass?
The descriptor logic for @staticmethod operates independently of the class-level metaclass definition.
Why this answer
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.
What is the effect of setting __slots__ in a parent class on a child class that does not define its own __slots__?
By default, subclasses receive a __dict__.
Why this answer
The child class will receive an instance __dict__ unless it also defines __slots__, effectively defeating the memory optimization of the parent.
Which THREE of the following are potential pitfalls of the Singleton pattern in Python?
Singletons are global by definition.
Why this answer
Singletons can be hard to test, cause hidden global state, and create difficulties in multi-threaded environments.
How do you implement the 'Strategy' pattern in Python?
Dependency injection is a key part of the Strategy pattern.
Why this answer
By injecting different strategy objects into a context class, which then delegates operations to the injected object.
When designing a class hierarchy, what is the best way to prevent a method from being overridden by subclasses?
The @final decorator is the standard way to indicate that a method should not be overridden.
Why this answer
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.
How do you correctly call a method from a sibling class in a diamond inheritance structure using super()?
super() ensures that each class in the MRO is initialized exactly once.
Why this answer
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.
Which TWO of the following are correct regarding the 'with' statement in Python?
These are the mandatory methods.
Why this answer
The 'with' statement handles setup and teardown, and it requires both __enter__ and __exit__.
In the context of the Factory pattern, what does a concrete factory implement?
This is the essence of a concrete factory.
Why this answer
A concrete factory implements the interface defined by the abstract factory to produce specific products.
When implementing the Singleton pattern, what is the advantage of using a module-level instance over a class-based Singleton?
Modules are cached in sys.modules, making them thread-safe and singular by design.
Why this answer
Python modules are naturally Singletons because they are initialized only once upon the first import.
What is the purpose of the __repr__ method?
This is the intended goal of __repr__.
Why this answer
It provides a formal, unambiguous string representation of an object, ideally suitable for re-creating the object.
Why does the 'abc' module provide the 'ABCMeta' metaclass?
This is the core purpose of ABCMeta.
Why this answer
ABCMeta is the metaclass that implements the logic to prevent instantiation of classes with abstract methods.
Which THREE of the following represent common issues when using __slots__?
Otherwise, the child gets a dictionary.
Why this answer
Slots can break pickling in some cases, make multiple inheritance tricky, and prevent adding attributes dynamically.
You are implementing the Observer pattern. Why should you avoid using a strong reference to the observers in the subject's list?
Weak references allow the garbage collector to reclaim the observer even if it's in the subject's list.
Why this answer
Strong references prevent garbage collection of the observers, leading to memory leaks if observers are meant to be temporary.
What is the primary difference between @classmethod and @staticmethod?
This is the fundamental distinction.
Why this answer
@classmethod receives the class as the first argument, while @staticmethod receives no implicit class or instance argument.
Which magic method enables index access (e.g., obj[i])?
This is for retrieval.
Why this answer
The __getitem__ method allows an object to support indexing.
Which TWO of the following are true about magic methods?
They are the standard hooks for operators like +, -, etc.
Why this answer
Magic methods start and end with double underscores, and they allow objects to integrate with Python's built-in syntax.
Which TWO of the following are valid ways to create a singleton in Python?
Can be used, though metaclasses are cleaner.
Why this answer
Modules are natural singletons, and metaclasses can be used to control instance creation.
What happens when you add two objects of a class that implements __add__?
This is how operator overloading works.
Why this answer
The __add__ method is automatically called by the interpreter when the '+' operator is used.
If you define a class with a metaclass that has a custom __call__ method, when is that __call__ method executed?
Metaclass __call__ intercepts the instantiation of its class objects.
Why this answer
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()).
Which method is used to customize the behavior of the 'in' operator?
This is the correct magic method.
Why this answer
The __contains__ method is called when using the 'in' or 'not in' operators.
Which magic method should you implement to make your objects support the 'with' statement context manager?
These are the mandatory methods for the context manager protocol.
Why this answer
The __enter__ and __exit__ methods are required to implement the context manager protocol.
Which function is used to check if an object is an instance of a class?
This handles inheritance correctly.
Why this answer
isinstance(obj, Class) is the standard way to check inheritance.
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?
This is the open/closed principle in action.
Why this answer
Registry-based factories allow for decoupled code where new classes can be registered without modifying the factory's core logic.
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?
The __init__ method is invoked after the class creation, allowing you to filter out the base class based on attributes or names.
Why this answer
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.
Why might you use the __init_subclass__ hook instead of a metaclass?
Metaclass conflicts arise when multiple classes have different metaclasses; __init_subclass__ avoids this.
Why this answer
__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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