Quick Answer
The answer is RuntimeError: dictionary changed size during iteration. This error occurs because Python dictionaries are dynamic structures, and the interpreter forbids adding or deleting keys while actively iterating over the dictionary’s keys, values, or items—doing so corrupts the internal iteration state. In the given code, the loop deletes keys as it iterates, which changes the dictionary’s size and triggers the exception. On the Certified Entry-Level Python Programmer PCEP exam, this question tests your understanding of dictionary mutation rules and common iteration pitfalls; it often appears as a trap where candidates expect a partial deletion or a silent skip. A reliable memory tip is to think of a dictionary as a snapshot: once iteration begins, you cannot alter the frame. If you need to modify a dictionary during a loop, iterate over a copy of its keys using list(dict.keys()) instead.
This PCEP practice question tests your understanding of functions, tuples, dictionaries and exceptions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Refer to the exhibit.
data = {'a': 1, 'b': 2, 'c': 3}
for key, value in data.items():
if value % 2 == 0:
data.pop(key)
print(data)What is the output of this code?
Refer to the exhibit.
data = {'a': 1, 'b': 2, 'c': 3}
for key, value in data.items():
if value % 2 == 0:
data.pop(key)
print(data)RuntimeError: dictionary changed size during iteration
Modifying dict while iterating over its items raises RuntimeError.
{}
Why wrong: Not the output.
{'b': 2}
Why wrong: Not the output.
{'a': 1, 'c': 3}
Why wrong: Modification during iteration raises error.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
RuntimeError: dictionary changed size during iteration
Option A is correct because modifying a dictionary's size (adding or deleting keys) during iteration over its keys, values, or items raises a RuntimeError. In this code, the loop iterates over the dictionary's keys while deleting them, which changes the dictionary's size and triggers the exception.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
RuntimeError: dictionary changed size during iteration
Why this is correct
Modifying dict while iterating over its items raises RuntimeError.
Related concept
Read the scenario before looking for a memorised answer.
{}
Why it's wrong here
Not the output.
{'b': 2}
Why it's wrong here
Not the output.
{'a': 1, 'c': 3}
Why it's wrong here
Modification during iteration raises error.
Common exam traps
Python Institute often tests the misconception that deleting keys during iteration will silently skip or partially modify the dictionary, but Python explicitly forbids size changes during iteration to enforce safe iteration contracts.
Trap categories for this question
Command / output trap
Not the output.
Detailed technical explanation
Under the hood, Python dictionaries maintain an internal hash table and a view object for iteration. When a key is deleted, the dictionary's size changes, and the iterator detects this modification via a version counter, raising RuntimeError to prevent undefined behavior. This is a safeguard to avoid unpredictable results like skipped keys or infinite loops, as specified in Python's data model documentation.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the PCEP exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Functions, Tuples, Dictionaries and Exceptions — This question tests Functions, Tuples, Dictionaries and Exceptions — Read the scenario before looking for a memorised answer..
The correct answer is: RuntimeError: dictionary changed size during iteration — Option A is correct because modifying a dictionary's size (adding or deleting keys) during iteration over its keys, values, or items raises a RuntimeError. In this code, the loop iterates over the dictionary's keys while deleting them, which changes the dictionary's size and triggers the exception.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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.
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