PCPP1 Practice Exam 3 — 20 Questions

Question 1

When implementing secure coding practices in a web-facing Python application, how should you handle raw user input to prevent command injection?

Accepted Answer

Pass input arguments as a list to subprocess.run(args, shell=False).. Using the 'subprocess' module with 'shell=False' (the default) prevents the shell from interpreting user-provided strings as commands.

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Pass the entire input string to os.system().

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Concatenate the input string into a shell command for execution.

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Sanitize input using a custom regex to remove special characters.

Question 2

Which TWO of these identify a potential 'code smell' in Python exception handling?

Accepted Answer

Swallowing exceptions with an empty 'except' block.. Swallowing exceptions and catching BaseException are both poor practices.

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Using 'finally' for cleanup.

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Catching specific exceptions.

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Raising custom exceptions.

Question 3

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 4

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 5

What is the purpose of the 'type' function when called with three arguments?

Accepted Answer

To create a new class dynamically.. It is used for dynamic class creation: type(name, bases, dict).

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To register a class.

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To inherit from multiple classes.

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To check object type.

Question 6

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.

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To be called by print().

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User-friendly output.

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To store the object in memory.

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

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 9

Which method is used to customize the behavior of the 'in' operator?

Accepted Answer

__contains__. The __contains__ method is called when using the 'in' or 'not in' operators.

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__has__

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__find__

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__in__

Question 10

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 11

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 12

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 13

Which practice is recommended when dealing with 'bare' except clauses in Python?

Accepted Answer

Always specify the exception type you intend to catch.. Bare 'except:' clauses catch SystemExit and KeyboardInterrupt, which can prevent the user from stopping a program.

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Use bare 'except:' for performance optimization.

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Use 'except Exception:' as the standard catch-all.

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Use 'except:' but log the error immediately.

Question 14

What is the primary difference between @classmethod and @staticmethod?

Accepted Answer

@classmethod receives the class as the first argument; @staticmethod does not.. @classmethod receives the class as the first argument, while @staticmethod receives no implicit class or instance argument.

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@staticmethod can access instance variables.

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@classmethod is for private methods.

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@classmethod is slower.

Question 15

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 16

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 17

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 18

Why does the 'abc' module provide the 'ABCMeta' metaclass?

Accepted Answer

To enforce abstract method implementation at instantiation.. ABCMeta is the metaclass that implements the logic to prevent instantiation of classes with abstract methods.

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To allow private methods.

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To improve method resolution speed.

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To support multiple inheritance.

Question 19

What happens if you use a decorator on a class that returns a modified version of the class?

Accepted Answer

The original class object is replaced.. The class name, module, and other attributes might be replaced, which can confuse introspection tools; functools.wraps is usually not applicable to classes.

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It triggers a SyntaxError.

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The class cannot be instantiated.

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The metaclass is ignored.

Question 20

Which TWO of the following are true about magic methods?

Accepted Answer

They provide hooks into Python language operators.. Magic methods start and end with double underscores, and they allow objects to integrate with Python's built-in syntax.

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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.