The correct approach is to use `if len([c for c in s if c.isdigit()]) == 10:` because this list comprehension filters the string to keep only characters where `isdigit()` returns True, then checks that the resulting list has exactly ten elements. This method naturally handles spaces, dashes, and parentheses by ignoring them, focusing validation purely on the digit count. On the Certified Associate Python Programmer PCAP exam, this tests your understanding of string filtering with list comprehensions and the `isdigit()` method—a common trap is trying to strip or replace non-digit characters manually, which is less elegant and error-prone. The exam often presents this as a clean, Pythonic one-liner that demonstrates both comprehension syntax and string method usage. Memory tip: think of the list comprehension as a “digit sieve” that passes only numbers through, then count what comes out.
PCAP Strings Practice Question
This PCAP practice question tests your understanding of strings. 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 is tasked with validating user input that must be a 10-digit phone number. The input may contain spaces, dashes, and parentheses. Which approach best ensures the input contains exactly 10 digits?
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
✓
if len([c for c in s if c.isdigit()]) == 10:
Option A uses a list comprehension to filter only digit characters from the input string `s` and then checks if the count of those digits is exactly 10. This correctly handles any non-digit characters (spaces, dashes, parentheses) by ignoring them, ensuring the validation focuses solely on the presence of exactly ten digits.
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.
✓
if len([c for c in s if c.isdigit()]) == 10:
Why this is correct
This extracts all digits and checks if there are exactly 10, ignoring non-digits.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
if len(s) >= 10 and s.isdigit():
Why it's wrong here
This would reject strings with non-digit characters, even if they contain 10 digits.
✗
if s[:10].isdigit():
Why it's wrong here
This only checks the first 10 characters, ignoring the rest.
✗
if s.isdigit() and len(s) == 10:
Why it's wrong here
This requires the entire string to be digits, which fails for inputs with formatting.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the distinction between checking if a string *contains* a certain number of digits versus checking if the string *itself* is entirely composed of digits, leading candidates to mistakenly choose options that require the entire string to be numeric.
Detailed technical explanation
How to think about this question
The `str.isdigit()` method returns `True` only if all characters in the string are decimal digits (0-9) and the string is non-empty. In Python, list comprehensions are efficient for filtering; `[c for c in s if c.isdigit()]` creates a list of only digit characters, and `len()` then counts them. This approach is robust for phone number validation where formatting characters like parentheses, spaces, and hyphens are common but should be ignored when counting digits.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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.
Strings — This question tests Strings — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: if len([c for c in s if c.isdigit()]) == 10: — Option A uses a list comprehension to filter only digit characters from the input string `s` and then checks if the count of those digits is exactly 10. This correctly handles any non-digit characters (spaces, dashes, parentheses) by ignoring them, ensuring the validation focuses solely on the presence of exactly ten digits.
What should I do if I get this PCAP 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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This PCAP 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 PCAP exam.
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