PCEP Computer Programming and Python Fundamentals Practice Question
A dictionary: d = {1: 'a', 2: 'b', 3: 'c'}. Which code will cause a KeyError?
⚠ Common exam trap
The PCEP exam often tests the distinction between safe dictionary access methods (get, setdefault, in) and the direct indexing operator ([]), expecting candidates to know that only [] raises a KeyError for missing keys.
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
✓
d[4]
Accessing a dictionary key that does not exist using square bracket notation (d[4]) raises a KeyError. Since the dictionary d has keys 1, 2, and 3, the key 4 is absent, so d[4] triggers the error.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
d.get(4)
Why it's wrong here
dict.get returns None for a missing key instead of raising, so no KeyError occurs. It is tempting because it queries an absent key, but get is precisely the lookup method designed to avoid KeyError; d[4] would raise one.
- ✗
if 4 in d: d[4]
Why it's wrong here
The membership test returns False for the absent key 4, so the body never executes and no KeyError occurs. It is tempting because direct indexing with d[4] would raise KeyError, making this guarded form look risky when it is actually safe.
- ✓
d[4]
Why this is correct
Accessing d[4] raises KeyError because the dictionary only maps keys 1, 2 and 3; no entry exists for key 4. Unlike get(), which returns None for absent keys, subscript access demands the key be present, so the missing key triggers the exception directly.
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d.setdefault(4, 'd')
Why it's wrong here
setdefault inserts key 4 with value 'd' when absent and returns it, raising nothing. It is tempting because it names a missing key, yet its whole purpose is supplying a default; only direct indexing d[4] raises KeyError.
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Written by Johnson Ajibi, MSc IT Security
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This PCEP practice question is part of Courseiva's free Python Institute certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCEP exam.