PCAP Object-Oriented Programming — Timed 15 Questions

Question 1

A developer creates a Python class with a method that is intended to be overridden in subclasses. Which approach best ensures that the method is not accidentally called on the base class?

Accepted Answer

Raise NotImplementedError inside the method body. Raising NotImplementedError inside the base class method is the standard Python idiom for defining an abstract-like method that must be overridden in subclasses. If a subclass fails to override the method and it is called, Python will raise an explicit error at runtime, preventing accidental use of the base implementation. This approach enforces the contract that the method is intended only for subclasses, without requiring the `abc` module.

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Use 'pass' as the method body

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Delete the method from the base class using 'del'

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Add a comment '# override in subclass' inside the method body

Question 2

A developer wants to ensure that a class attribute is shared among all instances but cannot be modified from outside the class. Which approach is most appropriate?

Accepted Answer

Define a private class attribute (e.g., __shared) and provide a class method to access it. Option D is correct because defining a private class attribute with name mangling (e.g., `__shared`) prevents direct external modification, and providing a class method (using `@classmethod`) allows read-only access to the attribute. This ensures the attribute is shared among all instances (since it belongs to the class, not instances) while enforcing encapsulation.

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Use a property decorator on a class method

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Define a public class attribute and document it as read-only

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Define an instance attribute inside __init__

Question 3

A Python class 'BankAccount' has a method 'withdraw(amount)' that deducts 'amount' from 'self.balance'. A developer writes a subclass 'SavingsAccount' that overrides 'withdraw' to add a penalty if balance drops below minimum. Which design pattern is being used?

Accepted Answer

Method overriding. Option C is correct because method overriding is the mechanism where a subclass provides a specific implementation of a method that is already defined in its superclass. In this scenario, SavingsAccount overrides the withdraw method from BankAccount to add penalty logic, which is the defining characteristic of method overriding in Python.

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Composition

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Aggregation

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Inheritance

Question 4

A team is developing a system that must handle different types of documents (PDF, Word, etc.). Each document type has a unique parsing method. To avoid massive conditional logic, which OOP concept should be applied?

Accepted Answer

Polymorphism. Polymorphism allows different document types (PDF, Word, etc.) to be treated uniformly through a common interface (e.g., a `parse()` method) while each class implements its own parsing logic. This eliminates the need for conditional statements (like `if type == 'PDF'`) because the correct method is resolved at runtime via dynamic dispatch, which is exactly what the team needs to avoid massive conditional logic.

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Encapsulation

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Inheritance

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Abstraction

Question 5

A developer writes a class 'Logger' with a class method 'log(msg)' that writes to a file. Another class 'AppLogger' inherits from 'Logger'. The developer expects both classes to share the same file handle. However, after creating an instance of 'AppLogger', the file handle is different. What is the most likely cause?

Accepted Answer

The file handle is opened in the __init__ method of the base class. Option B is correct because if the file handle is opened in the `__init__` method of the base class, each time a new instance is created (including when an `AppLogger` instance is created), a new file handle is opened. This means the `Logger` class and the `AppLogger` class do not share the same file handle; instead, each instance gets its own handle. To share a single file handle across all instances, the file handle should be opened as a class attribute or in a class method, not in `__init__`.

Answer

The 'log' method is defined as a class method using @classmethod

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The file handle is stored as a private attribute __file

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The subclass overrides the 'log' method

Question 6

A class 'MyClass' has a method 'do_something' that uses 'self.__private'. A subclass 'MySubClass' tries to access 'self.__private' and gets an AttributeError. Why?

Accepted Answer

Because name mangling renames the attribute to _MyClass__private, and the subclass implicitly accesses _MySubClass__private. Option C is correct because Python's name mangling mechanism renames any attribute prefixed with double underscores (like `__private`) in a class definition to `_ClassName__private`. When `MySubClass` tries to access `self.__private`, Python looks for `_MySubClass__private`, which does not exist, causing an AttributeError. The attribute `_MyClass__private` is still accessible from the subclass, but only via its mangled name.

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Because the attribute is defined as a class attribute, not instance attribute

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Because the subclass overrides the method that uses the attribute

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Because the attribute is private and not inherited

Question 7

A Python class 'Shape' defines an abstract method 'area'. Subclasses 'Circle' and 'Square' implement 'area'. A function 'calculate_area(shape)' expects a 'Shape' instance. Which principle ensures that the function works correctly without knowing the specific subclass?

Accepted Answer

Liskov Substitution Principle. The Liskov Substitution Principle (LSP) states that objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program. In this scenario, 'calculate_area(shape)' accepts a 'Shape' instance, and because both 'Circle' and 'Square' are proper subtypes that honor the contract of the 'area' method, the function works correctly regardless of which subclass is passed. This is the core of LSP: substitutability without side effects.

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Interface Segregation Principle

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Single Responsibility Principle

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Dependency Inversion Principle

Question 8

A developer defines a class 'Car' with an attribute 'wheels = 4'. They create two instances: car1 = Car() and car2 = Car(). Then they set car1.wheels = 3. What is the value of car2.wheels?

Accepted Answer

4. Option A is correct because in Python, setting an attribute directly on an instance (car1.wheels = 3) creates an instance attribute that shadows the class attribute for that specific instance only. The class attribute 'wheels = 4' remains unchanged and is still accessible via car2.wheels, which has not been overridden. Therefore, car2.wheels retains the class-level value of 4.

Answer

3

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AttributeError

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None

Question 9

A class 'Point' is defined with __slots__ = ['x', 'y']. A developer creates an instance p = Point() and tries to set p.z = 10. What happens?

Accepted Answer

AttributeError is raised. When a class defines `__slots__`, it restricts attribute assignment to only those names listed in the tuple. Attempting to assign an attribute not in `__slots__` raises an `AttributeError`. This is a deliberate memory optimization that prevents the creation of a per-instance `__dict__`, so dynamic attributes are disallowed.

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The assignment is silently ignored

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Python issues a warning and adds the attribute

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The attribute 'z' is added dynamically

Question 10

A class 'Employee' has a method 'work()'. A subclass 'Manager' overrides 'work()' but also wants to call the parent's 'work()'. Which syntax correctly achieves this?

Accepted Answer

super().work(). Option B is correct because `super().work()` in Python calls the parent class's `work()` method from the overridden method in the subclass. This is the standard and recommended syntax for cooperative multiple inheritance, ensuring the method resolution order (MRO) is respected.

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self.work()

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super(Manager, self).work()

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Employee.work(self)

Question 11

A developer defines a class 'A' with a method 'm' that uses 'self.a'. Class 'B' inherits from 'A' and defines __init__ that sets 'self.a = 10'. An instance of B is created and method m is called. What is the output?

Accepted Answer

10. Option C is correct because when an instance of class B is created, its __init__ method sets self.a = 10. When method m (inherited from A) is called on that instance, self refers to the B instance, so self.a resolves to 10. Python's attribute lookup follows the instance's __dict__ first, finding the attribute set by B's __init__.

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None

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AttributeError

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0

Question 12

A developer is designing a class hierarchy for a library system. They want to ensure that a method 'borrow' in the base class 'Item' can be overridden by subclasses like 'Book' and 'DVD', but the base implementation should not be callable directly. Which approach best achieves this?

Accepted Answer

Define borrow with raise NotImplementedError and override in subclasses. Option A is correct because raising `NotImplementedError` in the base class `Item.borrow` makes the method abstract in practice: it cannot be called directly without causing an error, forcing subclasses like `Book` and `DVD` to provide their own override. This pattern enforces that the base implementation is never invoked accidentally, while still allowing polymorphic dispatch through inheritance.

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Define borrow as a static method and override in subclasses

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Define borrow as a class method and override in subclasses

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Define borrow with pass and let subclasses override

Question 13

A Python program uses multiple inheritance. Class A defines method 'foo', class B also defines 'foo', and class C(A, B) inherits from both. The developer expects that calling 'foo' on an instance of C should invoke the method from class A. However, it invokes the method from class B. What is the most likely cause?

Accepted Answer

The method resolution order (MRO) is not as expected due to the order of base classes. Option A is correct because Python's Method Resolution Order (MRO) uses the C3 linearization algorithm, which respects the order of base classes as listed in the class definition. When class C(A, B) is defined, the MRO is C → A → B, so 'foo' from A should be found first. However, if the developer mistakenly wrote C(B, A), the MRO becomes C → B → A, causing B's 'foo' to be invoked. The question states the developer expected A's method but got B's, indicating the base class order was reversed.

Answer

Python always calls the method from the last base class

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The 'foo' method in class B is defined after class A's 'foo' in the same file

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Python uses depth-first, left-to-right resolution

Question 14

Which TWO of the following are valid ways to define a class attribute that is shared by all instances?

Accepted Answer

class MyClass: pass
MyClass.attr = 0. Option C is correct because assigning an attribute directly to the class after its definition (MyClass.attr = 0) creates a class-level attribute that is shared by all instances. Option D is correct because defining an attribute directly inside the class body (attr = 0) also creates a class-level attribute, accessible to all instances unless shadowed by an instance attribute.

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class MyClass: attr: int = 0

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class MyClass: def set_attr(self): MyClass.attr = 0

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class MyClass: def __init__(self): self.attr = 0

Question 15

What is the output of the code?

Accepted Answer

20. Option A is correct because the `Derived` class inherits the `get_x` method from the `Base` class, which returns `self._x`. When `obj.get_x()` is called, `self` refers to the `Derived` instance, and `self._x` accesses the `_x` attribute set in `Derived.__init__` (value 20). The `_x` attribute in `Derived` shadows the one in `Base`, so the output is 20.

Answer

AttributeError: 'Derived' object has no attribute 'get_x'

Answer

10

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AttributeError: 'Derived' object has no attribute '_x'