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Level 3 / Project 06 - Structured Error Handler

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— This project — — Flashcards — Browser

Estimated time: 40 minutes

Focus

  • typed errors and safe propagation

Why this project exists

This project gives you level-appropriate practice in a realistic operations context. Goal: run the baseline, alter behavior, break one assumption, recover safely, and explain the fix.

Run (copy/paste)

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-3/06-structured-error-handler
python project.py records.json --schema schema.json
python project.py records.json --schema schema.json --json
pytest -q

Expected terminal output

Processed 4 records: 2 passed, 2 failed
Error breakdown:
  REQUIRED: 2
  INVALID_FORMAT: 1
16 passed

Expected artifacts

  • Validation results on stdout
  • Passing tests
  • Updated notes.md

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension

  1. Add a range rule that validates numeric values (e.g., age 0-150).
  2. Add an --exit-code flag that returns non-zero if any record fails.
  3. Add error severity levels to the schema (warning vs error).

Break it (required) — Core

  1. Pass a schema file that doesn't exist — what error appears?
  2. Pass records with a field not in the schema — is it validated or ignored?
  3. Trigger an unexpected exception inside a validator — does safe_process catch it?

Fix it (required) — Core

  1. Add a friendly error message when schema or records file is missing.
  2. Validate the schema itself before processing (are rule names valid?).
  3. Ensure capture_error works for all built-in exception types.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)

  1. Why create a custom exception hierarchy (AppError, ValidationError, etc.)?
  2. What is the "Result pattern" and how does OperationResult implement it?
  3. How does safe_process differ from scattering try/except everywhere?
  4. What does traceback.format_exception return and when would you use it?

Mastery check

You can move on when you can: - design a custom exception hierarchy, - use the Result pattern instead of exceptions for flow control, - accumulate errors across a batch without crashing, - write structured error reports with codes and context.

Stuck? Ask AI

If you are stuck after trying for 20 minutes, use one of these prompts:

  • "I am working on Structured Error Handler. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to create custom exception classes. Can you show me how to define a custom exception with extra attributes, using a different domain than mine?"
  • "Can you explain when to raise an exception vs when to return an error value?"
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