PCAP Object-Oriented Programming Practice Question
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?
⚠ Common exam trap
Python Institute often tests the misconception that inherited methods use the parent class's attribute values rather than the instance's own attributes, leading candidates to incorrectly expect an AttributeError or None.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
10
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 analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
None
Why it's wrong here
The method m references self.a, which is an instance attribute assigned in B.__init__. Since that constructor explicitly sets a to 10, the attribute is never None; Python's attribute lookup finds the integer 10 instead of a default None. None would only appear if the attribute were initialized to None or never assigned and a class-level None existed.
- ✗
AttributeError
Why it's wrong here
AttributeError is raised only when Python cannot find the requested attribute on the instance or its class hierarchy. Here, before any call to m, B.__init__ runs and creates self.a = 10 on the instance, so the attribute lookup succeeds. If B.__init__ omitted the assignment and A.__init__ did not run, only then would m raise AttributeError.
- ✓
10
Why this is correct
The method m, inherited from A, executes with self bound to a B instance. During instantiation, B.__init__ executes self.a = 10, placing a in the instance's __dict__. Consequently, the expression inside m that uses a evaluates to 10, matching the assigned value exactly.
- ✗
0
Why it's wrong here
The value 0 is not involved in this code path: B.__init__ assigns the literal 10 to self.a, and no subsequent operation decrements or reassigns the attribute. Unless m explicitly used getattr(self, 'a', 0), there is no mechanism that would produce a fallback default of 0, so the output cannot be 0.
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