hardMultiple ChoiceObjective-mapped
How to Debug an IndexError in Python
A software developer is using an integrated development environment (IDE) to write a Python script that processes a CSV file containing customer data. The script uses a for loop to iterate over rows and calculates a total purchase amount. When the developer runs the script, it throws an error: "IndexError: list index out of range". The developer suspects the error occurs when the script tries to access a column that doesn't exist in some rows. Which debugging strategy should the developer use first to isolate the issue?
Quick Answer
The correct debugging strategy is to place a breakpoint inside the loop and step through each row in debug mode. This approach directly isolates the IndexError by allowing you to inspect the actual list structure of each row as the loop iterates, revealing exactly which row lacks the expected column index. On the CompTIA ITF+ FC0-U61 exam, this tests your understanding of systematic debugging techniques and strategies—specifically, that a debugger provides runtime visibility into data flow, whereas a try-except block would only mask the symptom without identifying the root cause. A common trap is to immediately check the CSV file manually, but that doesn’t reveal how the script interprets the data at runtime. Remember the mnemonic: “Break before you patch”—always use a breakpoint to see the data before adding error handlers.
⚠ Common exam trap
Many exam-takers choose Option B (try-except) because it seems like a quick fix, but the question asks for the first step to isolate the issue, not to patch it—CompTIA tests the understanding that debugging should identify the root cause before applying a solution.
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
✓
Place a breakpoint inside the loop and step through each row in debug mode.
Placing a breakpoint inside the loop and stepping through each row in debug mode allows the developer to inspect the actual data in each row and identify exactly which row causes the IndexError. This is the most direct and efficient first step to isolate the issue, as it lets the developer see the state of the list and the index being accessed at the moment of failure.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Rewrite the script to use a dictionary instead of a list.
Why it's wrong here
This changes data structure but does not fix the index issue; it may introduce new bugs.
- ✗
Add a try-except block to catch the exception and continue processing.
Why it's wrong here
This hides the bug and may cause incorrect results; it does not help isolate the cause.
- ✗
Examine the CSV file to ensure all rows have the same number of columns.
Why it's wrong here
This could be useful but is not a debugging technique for the script execution; the error may still occur due to logic issues.
- ✓
Place a breakpoint inside the loop and step through each row in debug mode.
Why this is correct
Debugging allows step-by-step execution to identify which row and column access causes the IndexError.
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Same concept, more angles
1 more way this is tested on FC0-U71
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A developer is debugging a program that is producing incorrect output. Which of the following are common debugging techniques? (Choose three.)
hard- ✓ A.Using a debugger
- ✓ B.Code review
- ✓ C.Adding print statements
- D.Reinstalling the operating system
- E.Changing the program requirements
Why A: Common debugging techniques include adding print statements, using a debugger, and performing code reviews. Changing requirements or reinstalling the OS are not debugging techniques.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This FC0-U71 practice question is part of Courseiva's free CompTIA 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 FC0-U71 exam.