Extract pure helpers with one responsibility
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.
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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.
A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects.
Extract pure helpers with one responsibility — original worked practice
A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects.
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 validate in refactored_fee.py and compare its condition and error message with the legacy check.
You should see: Validation behavior is unchanged.
A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects.
2. Step 2
Read subtotal(quantity,member) and compute 2 member kits without delivery.
You should see: The helper returns 480 cents.
A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects.
3. Step 3
Read fee preserving the public q,m,d signature while composing validation, subtotal, and post charge.
You should see: The public entry remains compatible.
A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects.
4. Step 4
Run the equivalence suite after extraction and retain source differences.
You should see: A checked responsibility-based extraction record.
A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects.
What to finish
Create checked validation/subtotal helpers and preserve the public entry contract.
Watch for this
Extracting a helper that mixes calculation with hidden I/O or changes the public call interface.
How can I tell I understand it?
Demonstrate the rule: A helper should separate a real responsibility, such as validation or subtotal calculation. It should use explicit inputs and return a result without hidden file or global-state effects. Use the supplied variant to distinguish a changed result from a broken implementation.
Quick knowledge check
Answer all 3 questions. Get at least 2 right and the lesson completes automatically.
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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.
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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