PCAP Hard Questions — 25 Difficult Questions

Question 1

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.

Answer

Interface Segregation Principle

Answer

Single Responsibility Principle

Answer

Dependency Inversion Principle

Question 2

A developer notices that a custom package 'mypackage' is not being found when importing, even though it is installed in the site-packages directory. The developer suspects a conflict with another package of the same name. Which command should the developer run to diagnose the location from which Python is importing the package?

Accepted Answer

print(mypackage.__file__). Option C is correct because `mypackage.__file__` returns the filesystem path from which the module was loaded, allowing the developer to see exactly which `mypackage` Python is using. This directly reveals if the wrong package (e.g., from a different location or a conflicting installation) is being imported instead of the intended one.

Answer

print(mypackage)

Answer

print(__file__)

Answer

import os; print(os.getcwd())

Question 3

Which THREE methods return a boolean value?

Accepted Answer

str.startswith(). B is correct because str.startswith() returns True if the string starts with the specified prefix, otherwise False. It is a boolean-returning method, as required by the question.

Answer

str.upper()

Answer

str.find()

Question 4

Refer to the exhibit. What is the output?

Accepted Answer

1 2. Class variable count increments each time a new object is created. First object gets id=1, second gets id=2.

Answer

1 1

Answer

0 1

Answer

2 2

Question 5

Refer to the exhibit. What does the 'from None' clause do in the second raise statement?

Accepted Answer

It prevents chaining of exceptions, so only the final exception is displayed.. Using 'from None' suppresses the chained exception context, so only the second exception is shown.

Answer

It causes the original exception to be ignored.

Answer

It causes both exceptions to be raised simultaneously.

Answer

It replaces the original exception with a new one.

Question 6

Refer to the exhibit. Which THREE statements about the class hierarchy are correct?

Accepted Answer

Calling D().method() returns 'B'. Option B is correct because when `D().method()` is called, Python's MRO (Method Resolution Order) for class D is `['D', 'B', 'C', 'A', 'object']`. Since class D does not define `method`, Python looks up the MRO and finds `method` first in class B. The `return 'B'` in B's `method` is executed, so the call returns 'B'.

Answer

D does not have a method named 'method'

Answer

The super() call in B would call C.method

Question 7

What is the output of the Python code after reading the config.txt file?

Accepted Answer

8080. The code reads the config.txt file and splits its content by newlines. The first line contains 'port=8080', and after splitting by '=', the second element is '8080'. The int() function converts this string to the integer 8080, which is then printed. Option C is correct because the output is the integer 8080, not a string or quoted form.

Answer

8080 (as string)

Answer

An exception is raised.

Answer

'8080'

Question 8

A developer is tasked with validating user input that must be a 10-digit phone number. The input may contain spaces, dashes, and parentheses. Which approach best ensures the input contains exactly 10 digits?

Accepted Answer

if len([c for c in s if c.isdigit()]) == 10:. Option A uses a list comprehension to filter only digit characters from the input string `s` and then checks if the count of those digits is exactly 10. This correctly handles any non-digit characters (spaces, dashes, parentheses) by ignoring them, ensuring the validation focuses solely on the presence of exactly ten digits.

Answer

if len(s) >= 10 and s.isdigit():

Answer

if s[:10].isdigit():

Answer

if s.isdigit() and len(s) == 10:

Question 9

Refer to the exhibit. What is the output?

Accepted Answer

HI, WORLD!. The correct answer is D because the code uses the `upper()` method on the string `'Hi, World!'`, which converts all lowercase letters to uppercase. The output is `'HI, WORLD!'`. The `upper()` method does not modify the original string but returns a new string with all characters in uppercase.

Answer

Hi, World!

Answer

Hello, World!

Answer

HELLO, WORLD!

Question 10

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

Answer

The file handle is stored as a private attribute __file

Answer

The subclass overrides the 'log' method

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__.

Answer

None

Answer

AttributeError

Answer

0

Question 12

A developer is implementing a custom context manager to manage database connections. The context manager should automatically roll back the transaction if an exception occurs, but commit if no exception occurs. Which pattern ensures this behavior correctly?

Accepted Answer

def __exit__(self, exc_type, exc_val, exc_tb):
    if exc_type:
        self.connection.rollback()
    else:
        self.connection.commit(). Option B is correct because the `__exit__` method receives the exception type (`exc_type`), value (`exc_val`), and traceback (`exc_tb`) as arguments. By checking `if exc_type:` (which is `None` when no exception occurs, and a class object otherwise), the developer can conditionally roll back the transaction on an exception or commit if the block completes normally. This pattern ensures atomicity and proper resource management for database connections.

Answer

def __exit__(self, exc_type, exc_val, exc_tb): self.connection.commit()

Answer

def __exit__(self, exc_type, exc_val, exc_tb): self.connection.rollback()

Answer

def __exit__(self, exc_type, exc_val, exc_tb):
    if exc_val:
        self.connection.rollback()
    else:
        self.connection.commit()

Question 13

Which THREE of the following are true about Python's exception hierarchy?

Accepted Answer

KeyboardInterrupt inherits from BaseException.. Option A is correct because `KeyboardInterrupt` inherits directly from `BaseException`, not from `Exception`. This design ensures that `KeyboardInterrupt` (raised by Ctrl+C) is not caught by a generic `except Exception:` clause, allowing the program to be interrupted even when broad exception handling is in place.

Answer

IOError is a separate class from OSError.

Answer

GeneratorExit inherits from Exception.

Question 14

You are a developer for a data science team. The team uses a shared module 'utilities' located at /team/shared/utilities.py. This module is not part of any package, and they want to import it from various project scripts without copying the file. Some projects are in /home/user/proj_A/ and others in /var/data/proj_B/. Currently, each script manually adds /team/shared/ to sys.path using sys.path.insert(0, '/team/shared/'). This works but is repetitive. The team wants a cleaner solution that also works when the script is run from different working directories. They consider creating a package 'utilities' by adding an __init__.py to the directory and using relative imports. However, the module currently uses absolute imports for some external libraries. What is the best course of action to allow clean imports of utilities from any location while minimizing changes to the module itself?

Accepted Answer

Set the PYTHONPATH environment variable to include /team/shared/ in the shell profile.. Option C is correct. PYTHONPATH is an environment variable that Python checks for additional module search paths. Setting it once system-wide or per user avoids modifying sys.path in each script. Option A is wrong because relative imports would require restructuring the module into a package, which changes import behavior and may break existing absolute imports. Option B is wrong because __init__.py alone does not make the module discoverable from any path; it still requires the directory to be on sys.path. Option D is wrong because installing a .pth file is system-wide and may not be allowed; also it modifies the search path globally, which is less controlled than PYTHONPATH.

Answer

Create an empty __init__.py in /team/shared/ to make it a namespace package.

Answer

Place a .pth file in the site-packages directory that points to /team/shared/.

Answer

Convert utilities.py into a package by adding __init__.py and using relative imports inside.

Question 15

Which THREE methods must be implemented to create an immutable object that supports iteration and membership testing?

Accepted Answer

__getitem__. Option B is correct because implementing __getitem__ allows indexing and slicing, which is essential for iteration in Python when __iter__ is not defined; Python falls back to __getitem__ for iteration. This method also supports membership testing via the 'in' operator when __contains__ is absent, as Python will iterate through the object to check membership.

Answer

__init__

Answer

__contains__

Question 16

Which of the following is a correct use of the @property decorator to create a getter and setter for an attribute named 'score' that ensures score stays between 0 and 100?

Accepted Answer

@property
def score(self): return self._score
@score.setter
def score(self, value): if 0 <= value <= 100: self._score = value. The @property decorator creates a getter method. The @score.setter decorator creates a setter that validates the value. Inside the setter, you should assign to a private attribute like _score to avoid recursion.

Answer

@property
def _score(self): return self.score
@_score.setter
def _score(self, value): self.score = value

Answer

@property
def score(self): return self.score
@score.setter
def score(self, value): self.score = value

Answer

def get_score(self): return self._score
def set_score(self, value): self._score = value
score = property(get_score, set_score)

Question 17

Refer to the exhibit. What is the output?

Accepted Answer

DBCA. The MRO for class D using C3 linearization is D -> B -> C -> A. Therefore, D calls super() to B, B calls super() to C, C calls super() to A. Output: D B C A (as a concatenated string).

Answer

DBACA

Answer

DCBA

Answer

DBAC

Answer

Error

Question 18

Which of the following best describes the use of the @abstractmethod decorator in Python?

Accepted Answer

It indicates that a method must be overridden in any non-abstract subclass.. The @abstractmethod decorator is used in abstract base classes to declare methods that must be implemented by concrete subclasses. It cannot be instantiated directly if any abstract methods are present.

Answer

It automatically provides a default implementation for a method.

Answer

It creates a method that can be called without an instance.

Answer

It raises AttributeError if the method is called.

Question 19

Refer to the exhibit. What is the root cause of the error?

Accepted Answer

The attribute 'age' is not defined in the class or instance.. Option B is correct because the attribute 'age' is not defined anywhere in the class or instance. Option A is incorrect because there is no private attribute involved. Option C is incorrect because __init__ was called. Option D is incorrect because 'age' is not a class attribute.

Answer

The __init__ method was not called.

Answer

The attribute 'age' is private.

Answer

The attribute 'age' is a class attribute but not initialized.

Question 20

Which THREE of the following escape sequences are valid in a Python string and represent a single character? (Select exactly three.)

Accepted Answer

\'. Option B (\'), Option C (\), and Option E (
) are valid escape sequences. Option A (\q) is not a standard escape sequence. Option D (\x) is invalid without two hex digits; \x1F is valid.

Answer

\x

Answer

\q