The correct answer is to check that all items are numeric before calculation and raise a TypeError otherwise. This approach is best because it follows Python’s principle of explicit error handling—validating input upfront ensures the function’s contract is clear and prevents silent failures that lead to subtle bugs. On the Certified Entry-Level Python Programmer PCEP exam, this question tests your understanding of defensive programming and type validation, a common topic in the “Data Aggregations” and “Exceptions” sections. A frequent trap is choosing to return None or ignore non-numeric values, which can mask errors and produce misleading results. The key insight is that averaging is mathematically defined only for numbers, so the function should enforce that contract rather than guessing the user’s intent. Memory tip: “Validate before you calculate—raise early, fail clearly.”
PCEP Practice Question: Functions, Tuples, Dictionaries and Exceptions
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.
A developer writes a function to calculate the average of a list of numbers, but the function sometimes returns a wrong result when the list contains non-numeric values. What is the best way to handle this?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check that all items are numeric before calculation, and raise TypeError otherwise.
Option D is correct because it explicitly validates that all items are numeric before performing the calculation, raising a TypeError if any non-numeric value is found. This follows Python's principle of explicit error handling and ensures the function's contract is clear: it only works with numeric data. Returning None (A) or silently ignoring values (B) can lead to subtle bugs, while converting to strings (C) would produce a concatenated string, not an average.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Return None if any non-numeric value is encountered.
Why it's wrong here
Returning None may cause silent errors downstream.
✗
Use try-except to ignore non-numeric values and proceed with the remaining numbers.
Why it's wrong here
Ignoring values may produce inaccurate results.
✗
Convert all values to string and concatenate them.
Why it's wrong here
Concatenating strings does not give an average.
✓
Check that all items are numeric before calculation, and raise TypeError otherwise.
Why this is correct
Raising an exception is the standard way to handle invalid input.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the distinction between silently handling errors (e.g., returning None or ignoring bad data) and explicitly raising exceptions, where candidates may mistakenly choose a 'graceful' option like ignoring non-numeric values, not realizing that it can lead to incorrect results without any warning.
Detailed technical explanation
How to think about this question
Under the hood, Python's TypeError is raised by built-in operations like sum() when non-numeric types are encountered; explicit type checking before the calculation allows the developer to provide a clear, predictable error message. In real-world scenarios, such as processing sensor data logs where occasional non-numeric entries may appear, raising a TypeError forces the caller to handle the data quality issue explicitly, rather than silently producing a skewed average. This approach aligns with the 'fail fast' principle, making debugging easier and preventing data corruption from propagating.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Functions, Tuples, Dictionaries and Exceptions — This question tests Functions, Tuples, Dictionaries and Exceptions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check that all items are numeric before calculation, and raise TypeError otherwise. — Option D is correct because it explicitly validates that all items are numeric before performing the calculation, raising a TypeError if any non-numeric value is found. This follows Python's principle of explicit error handling and ensures the function's contract is clear: it only works with numeric data. Returning None (A) or silently ignoring values (B) can lead to subtle bugs, while converting to strings (C) would produce a concatenated string, not an average.
What should I do if I get this PCEP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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