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Level 7 / Project 04 - Source Field Mapper

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— This project — — Flashcards — —

Focus

  • explicit source-to-target mapping contracts

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-7/04-source-field-mapper
python project.py --input data/sample_input.txt --output data/output_summary.json
pytest -q

Expected terminal output

... output_summary.json written ...
2 passed

Expected artifacts

  • data/output_summary.json
  • Passing tests
  • Updated notes.md

Alter it (required)

  1. Add a "datetime" cast type that parses ISO-format strings into Unix timestamps.
  2. Add a drop_unmapped option that removes source fields not in the mapping rules.
  3. Re-run script and tests — verify new cast and drop behavior work correctly.

Break it (required)

  1. Map a field with cast: "int" but provide a non-numeric string (e.g. "abc").
  2. Reference a source field that does not exist in the record and has no default.
  3. Observe the ValueError or KeyError in the output.

Fix it (required)

  1. Wrap cast operations in try/except and log a warning instead of crashing.
  2. Skip missing source fields when no default is configured, adding them to a skipped_fields report.
  3. Add a test for bad cast values and missing source fields.

Explain it (teach-back)

  1. Why are explicit mapping rules better than renaming fields in-place?
  2. What happened when a string could not be cast to int?
  3. How did the try/except prevent a pipeline crash on bad data?
  4. Where would you use field mapping in a real ETL pipeline?

Mastery check

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

Mastery Check

  • Can you explain the architectural trade-offs in your solution?
  • Could you refactor this for a completely different use case?
  • Can you identify at least two alternative approaches and explain why you chose yours?
  • Could you debug this without print statements, using only breakpoint()?
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