Write the behavior contract before refactoring
See this course’s project requirements
A before/after fictional fee calculator split into pure functions with an executable equivalence suite, clearer names and documented intentional nonchanges.
Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.
What to include
- Produce a complete valid/rejected behavior contract and intentional-change boundary.
- Retain full finite valid-space coverage and explicitly scoped invalid-case comparisons.
- Create checked validation/subtotal helpers and preserve the public entry contract.
- Produce a domain-name/reference review with unchanged discount and delivery semantics.
- Retain a passing baseline and an expected failing price mutation without altering final pricing.
- Deliver an independently reasoned, repeatable refactor with finite coverage and selected-invalid limits.
Review and revise
Use these criteria to check your work and choose what to improve.
- The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
- Normal, boundary and rejected cases are recorded with expected versus observed results.
- The changed case has a correct explanation and does not rely on hidden state.
- The project includes source, outputs, instructions and honest tool, privacy and verification limits.
Want more background?
The idea in everyday life
Fictional workshop kits: quantity 1–20, $3 per kit, a 20% member discount on kit subtotal, and $2.50 only for post. Calculations return integer cents. legacy_fee.py and refactored_fee.py must agree on permitted and rejected requests.
Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups.
Write the behavior contract before refactoring — original worked practice
Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups.
Fictional workshop kits: quantity 1–20, $3 per kit, a 20% member discount on kit subtotal, and $2.50 only for post. Calculations return integer cents. legacy_fee.py and refactored_fee.py must agree on permitted and rejected requests.
1. Step 1
Read legacy_fee.py and write its input types/ranges, cents unit, discount, and delivery rule.
You should see: The current contract is explicit.
Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups.
2. Step 2
Compute 2 member kits by post: 600×80/100+250.
You should see: The expected fee is 730 cents.
Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups.
3. Step 3
Record rejected quantity 0, quantity 21, Boolean quantity, non-Boolean member, and unknown delivery.
You should see: Rejection is part of the behavior to preserve.
Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups.
4. Step 4
List permitted structural changes separately from forbidden contract changes.
You should see: A behavior-preservation brief.
Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups.
What to finish
Produce a complete valid/rejected behavior contract and intentional-change boundary.
Watch for this
Changing a price, accepted type, or error contract while calling the edit only a refactor.
How can I tell I understand it?
Demonstrate the rule: Refactoring changes structure while preserving the declared observable behavior. New prices or accepted inputs are separate product changes, not silent cleanups. Use the supplied variant to distinguish a changed result from a broken implementation.
Quick knowledge check
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Source & access notes · checked 2026-10-01
Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.
Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.
Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.
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