The answer is that the app.yaml file incorrectly specifies `runtime: python27` while the code uses Python 3, causing the deployment to fail with an `ImportError: No module named requests`. This happens because the App Engine standard Python 2.7 runtime does not automatically install dependencies from a `requirements.txt` file—it relies on the `libraries` section in app.yaml instead. In contrast, the Python 3 runtime automatically reads `requirements.txt` and installs all listed packages during deployment. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how App Engine runtimes handle dependency management, a common trap where candidates assume all Python runtimes behave identically. The key distinction is that Python 2.7 requires explicit library declarations, while Python 3 expects a `requirements.txt` file. A quick memory tip: Python 3 is "self-serve" from requirements.txt, while Python 2.7 needs a "menu" in app.yaml.
PCD Building and testing applications Practice Question
This PCD practice question tests your understanding of building and testing applications. 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.
You are a Cloud Developer working for an e-commerce company. The company uses Cloud Build to build and deploy a Python application to App Engine standard environment. The application uses Cloud SQL for its database. The team recently updated the application code and added a new dependency. The build succeeds, but the deployment fails with 'Error Response: [9] Application startup error!' You check the logs and see 'ImportError: No module named requests'. The team uses a requirements.txt file. However, the requirements.txt file has been configured correctly in the past. The team also uses a custom runtime config in app.yaml with a 'entrypoint' field. They did not change any configuration files. What is the most likely cause of this error?
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.
The app.yaml file incorrectly specifies 'runtime: python27' but the code uses Python 3, and python27 does not automatically install dependencies from requirements.txt.
Python 2.7 runtime requires manually specifying libraries in app.yaml.
B
The custom entrypoint in app.yaml bypasses the automatic installation of dependencies.
Why wrong: Entrypoint only specifies command, not dependency installation.
C
The app.yaml file specifies 'runtime: python39' which is not supported by App Engine standard.
Why wrong: python39 is a supported runtime.
D
The requirements.txt file is missing from the deployment directory.
Why wrong: The team says it was configured correctly and not changed.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The app.yaml file incorrectly specifies 'runtime: python27' but the code uses Python 3, and python27 does not automatically install dependencies from requirements.txt.
Option D is correct because App Engine standard environment uses the Python 3 runtime and does not install dependencies from requirements.txt automatically unless the runtime is Python 3 and the file is present. If the runtime is Python 2.7, it uses app.yaml to specify libraries. Given the error, likely the wrong runtime. Option A is wrong because the file is present and hasn't changed. Option B is wrong because the entrypoint field doesn't affect dependency installation. Option C is wrong because there is no 'runtime: python39' field; it's 'python39' as a runtime value.
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 app.yaml file incorrectly specifies 'runtime: python27' but the code uses Python 3, and python27 does not automatically install dependencies from requirements.txt.
Why this is correct
Python 2.7 runtime requires manually specifying libraries in app.yaml.
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 custom entrypoint in app.yaml bypasses the automatic installation of dependencies.
Why it's wrong here
Entrypoint only specifies command, not dependency installation.
✗
The app.yaml file specifies 'runtime: python39' which is not supported by App Engine standard.
Why it's wrong here
python39 is a supported runtime.
✗
The requirements.txt file is missing from the deployment directory.
Why it's wrong here
The team says it was configured correctly and not changed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
Entrypoint only specifies command, not dependency installation.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Building and testing applications — This question tests Building and testing applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The app.yaml file incorrectly specifies 'runtime: python27' but the code uses Python 3, and python27 does not automatically install dependencies from requirements.txt. — Option D is correct because App Engine standard environment uses the Python 3 runtime and does not install dependencies from requirements.txt automatically unless the runtime is Python 3 and the file is present. If the runtime is Python 2.7, it uses app.yaml to specify libraries. Given the error, likely the wrong runtime. Option A is wrong because the file is present and hasn't changed. Option B is wrong because the entrypoint field doesn't affect dependency installation. Option C is wrong because there is no 'runtime: python39' field; it's 'python39' as a runtime value.
What should I do if I get this PCD question wrong?
Identify which PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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Question Discussion
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