Assertions and Behavior Verification Questions

Choosing what to assert and how to verify behavior meaningfully. Covers assertion strategy, verifying observable behavior over implementation detail, avoiding weak or over-specified assertions, and using assertion libraries effectively. Emphasizes assertions that fail for the right reasons and give a clear diagnosis.

MediumTechnical

84 practiced

Write a pytest unit test (high-level pseudocode is fine) asserting that a web endpoint returns HTTP 422 for validation errors and that an error log line includes the tag "validation_failure" and the request_id. Describe how you'd structure the test setup and assertions.

Sample Answer

Approach: use pytest with a test client (Flask/FastAPI/starlette) and capture logs (caplog fixture or a logger mock). Send an invalid request that triggers validation, include a unique request_id header, then assert response status is 422 and that a log record contains both "validation_failure" and the same request_id. pythondef test_endpoint_returns_422_and_logs_validation_failure(client, caplog): # Arrange invalid_payload = {"bad": "data"} # causes validation error request_id = "test-req-1234" headers = {"X-Request-ID": request_id, "Content-Type": "application/json"} # capture application logs at INFO/ERROR level caplog.set_level("ERROR", logger="myapp") # or root logger name used by app # Act response = client.post("/items", json=invalid_payload, headers=headers) # Assert - HTTP status assert response.status_code == 422 # Assert - log contains validation tag and request_id matching = [ rec for rec in caplog.records if "validation_failure" in rec.getMessage() and request_id in rec.getMessage() ] assert len(matching) >= 1, "Expected at least one log with validation_failure and request_id" # Optional: check structured log fields if using JSON logs # for rec in caplog.records: # data = json.loads(rec.getMessage()) # assert data.get("tag") == "validation_failure" # assert data.get("request_id") == request_id Key notes: Use caplog for simple capture; for structured logs mock the logger to inspect emitted dict. Ensure test injects request_id header or context so logs include it. Edge cases: middleware might log at different level or use different logger name — adjust caplog.set_level and logger name.

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