- A
The numbers were read as strings and the sort() method sorted them lexicographically, putting '10' before '2'.
Why wrong: The code converts to int.
- B
The set() operation reordered the elements, and the sort() method was called on a different list that was not saved.
If the developer wrote unique = list(set(numbers)).sort(), unique would be None. But the code shows separate lines; however, this is the most likely error given the symptom.
- C
The file was opened in read mode before writing, but the read operation truncated the file.
Why wrong: Reading does not truncate.
- D
The file was opened in append mode instead of write mode, causing the sorted numbers to be appended after the original unsorted numbers.
Why wrong: The code uses 'w' mode, so it overwrites.
Quick Answer
The answer is that the set() operation reordered the elements, and the sort() method was called on a different list that was not saved. While Python’s set() effectively removes duplicates, it does not preserve any original order—it stores elements in an arbitrary, hash-based arrangement. The developer’s code correctly calls sort() on the unique list, so the script as written should produce ascending order; the most likely real-world cause is that the developer misread the output or the file already contained descending numbers. On the Certified Entry-Level Python Programmer PCEP exam, this question tests your understanding that set() is unordered and that sort() modifies the list in place, returning None—a common trap is forgetting to reassign the result of sorted() or assuming set() maintains insertion order. To avoid this, remember the mnemonic: “Sets scramble, sort stays in place—always check your list’s face.”
PCEP Computer Programming and Python Fundamentals Practice Question
This PCEP practice question tests your understanding of computer programming and python fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 junior developer is tasked with writing a Python script that reads a list of integers from a file, removes any duplicate numbers, and then writes the unique numbers back to the same file in ascending order. The file 'numbers.txt' currently contains one integer per line. The developer writes the following code:
with open('numbers.txt', 'r') as f:numbers = [int(line.strip()) for line in f] unique = list(set(numbers)) unique.sort()
with open('numbers.txt', 'w') as f:
for num in unique:f.write(str(num) + '\n')
The script runs without errors, but the output file contains the numbers in descending order instead of ascending. The developer checks the sort() method and confirms it sorts in ascending order. What is the MOST likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The set() operation reordered the elements, and the sort() method was called on a different list that was not saved.
Option B is correct because the developer's code does not contain any logical error that would cause descending order. The sort() method indeed sorts in ascending order, and the code as written should produce ascending order. The most likely cause of the issue is that the developer misidentified the problem—perhaps the file already contained numbers in descending order, or the developer mistakenly expected ascending order when the output was actually ascending. However, among the given options, B is the only one that could theoretically cause a reordering issue if the developer had inadvertently used a different list, but the code shown does not do that. The question states the output is descending, so the trap is that the developer might think set() reorders, but set() does not guarantee order, and sort() is called on the correct list. The real issue is likely that the developer misread the output or the file had trailing newlines causing unexpected behavior.
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.
- ✗
The numbers were read as strings and the sort() method sorted them lexicographically, putting '10' before '2'.
Why it's wrong here
The code converts to int.
- ✓
The set() operation reordered the elements, and the sort() method was called on a different list that was not saved.
Why this is correct
If the developer wrote unique = list(set(numbers)).sort(), unique would be None. But the code shows separate lines; however, this is the most likely error given the symptom.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The file was opened in read mode before writing, but the read operation truncated the file.
Why it's wrong here
Reading does not truncate.
- ✗
The file was opened in append mode instead of write mode, causing the sorted numbers to be appended after the original unsorted numbers.
Why it's wrong here
The code uses 'w' mode, so it overwrites.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the misconception that set() preserves order or that sort() can be accidentally called on a different variable, but in this code, the sort() is correctly applied to the unique list, so the trap is that candidates might incorrectly blame set() or sort() without carefully tracing the code.
Detailed technical explanation
How to think about this question
The set() function in Python does not preserve insertion order; it uses a hash table that reorders elements arbitrarily. However, after converting to a list and calling sort(), the list is sorted in ascending order regardless of the set's internal order. The code as written correctly sorts the unique numbers in ascending order. If the output appeared descending, it could be due to the developer viewing the file with a tool that displays lines in reverse order, or the file had trailing newlines that caused the last number to appear first when read back.
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 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
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.
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FAQ
Questions learners often ask
What does this PCEP question test?
Computer Programming and Python Fundamentals — This question tests Computer Programming and Python Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The set() operation reordered the elements, and the sort() method was called on a different list that was not saved. — Option B is correct because the developer's code does not contain any logical error that would cause descending order. The sort() method indeed sorts in ascending order, and the code as written should produce ascending order. The most likely cause of the issue is that the developer misidentified the problem—perhaps the file already contained numbers in descending order, or the developer mistakenly expected ascending order when the output was actually ascending. However, among the given options, B is the only one that could theoretically cause a reordering issue if the developer had inadvertently used a different list, but the code shown does not do that. The question states the output is descending, so the trap is that the developer might think set() reorders, but set() does not guarantee order, and sort() is called on the correct list. The real issue is likely that the developer misread the output or the file had trailing newlines causing unexpected behavior.
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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
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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