PCPP-32-101 1.8 Apply Subclassing of Built-In Classes PCPP-32-1 Practice Questions (Page 3)

Questions 11–15

1. 11application · hard

   A developer creates a subclass of str that overrides __eq__ to compare strings case-insensitively. They write: class CaseInsensitiveStr(str): def __eq__(self, other): return self.lower() == other.lower() They test CaseInsensitiveStr('Hello') == 'hello' and it returns True. However, when they test 'hello' == CaseInsensitiveStr('Hello'), it returns False. Why?

   • AThe comparison raises TypeError because the two types are incompatible.
   • BThe CaseInsensitiveStr.__eq__ is not called because str.__eq__ is implemented in C and takes precedence.
   • CThe str.__eq__ method is called first and returns NotImplemented, so Python falls back to the subclass's __eq__.
   • DThe left operand's __eq__ is called first; since 'hello' is a plain str, its __eq__ compares directly and returns False.
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2. 12application · medium

   A developer creates a subclass of list that overrides __setitem__ to validate that the value is an integer. They also override append() to use the same validation. When they use the .insert() method with a non-integer value, no error is raised. Why?

   • AThe .insert() method calls append() internally, but since append() is overridden, the validation should run; the issue is that the override is not inherited.
   • BThe .insert() method works correctly, but the validation only applies to __setitem__ and append(), so insert() is expected to bypass it.
   • CThe .insert() method calls __setitem__ but only for the shifted elements, not for the inserted value.
   • DThe .insert() method is implemented in C and does not call __setitem__, so the validation is bypassed.
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4. 13application · hard

   A developer needs a string type that automatically strips whitespace from both ends when created. They write: class CleanString(str): def __new__(cls, value): return super().__new__(cls, value.strip()) They test CleanString(' hello ') and it works. However, when they call CleanString(' hello ').upper(), the result is 'HELLO ' with trailing spaces. Why?

   • AThe upper() method returns a plain str, not a CleanString, so the result is the original unstripped string.
   • BThe upper() method is not overridden, so it operates on the original value before stripping; the developer must override upper() to strip again.
   • CThe __new__ method strips the value, so the instance is 'hello'; upper() should return 'HELLO'. The reported trailing spaces indicate a bug in the test, not in the code.
   • DThe upper() method is implemented in C and bypasses the __new__ override, so it uses the raw input string.
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5. 14expert · hard

   A developer creates a subclass of list that overrides __getitem__ to return a default value when the index is out of range. They write: class SafeList(list): def __getitem__(self, index): try: return super().__getitem__(index) except IndexError: return None They test it with SafeList([1,2,3])[5] and it returns None. However, when they iterate over the list with a for loop, it raises IndexError. Why?

   • AThe for loop works correctly, but the override only applies to direct indexing, not to iteration.
   • BThe for loop uses the iterator protocol, which calls __iter__ and then __next__, not __getitem__, so the override is bypassed.
   • CThe for loop calls __getitem__ with a slice object, which the override does not handle.
   • DThe for loop calls __getitem__ with negative indices, which the override does not handle.
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6. 15application · medium

   A developer needs a list that rejects duplicate elements, similar to a set but preserving insertion order. They subclass list and override append() to check for duplicates before calling super().append(). They test it and it works. However, when they use the .extend() method with a list containing duplicates, duplicates are added. Why?

   • AThe .extend() method requires the developer to override __init__ as well, because extend() is only called during initialization.
   • BThe .extend() method calls append() for each element, but the override only checks the first element.
   • CThe .extend() method works correctly, but the duplicate check is only applied to the first element of the iterable.
   • DThe .extend() method is implemented in C and does not call append(), so the duplicate check is bypassed.
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