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Course 285 · Digital course 48

Refactor Code Without Changing Behavior

Build a before/after fictional fee calculator split into pure functions with an executable equivalence suite, clearer names and documented intentional nonchanges. Work from supplied original examples and check changed cases.

Before you start

Complete or review Programming with Python before starting.

Programming with Python See everything you’ll need

Free download · Original practice data · No external account required. Download the starter pack

What you’ll leave with

A before/after fictional fee calculator split into pure functions with an executable equivalence suite, clearer names and documented intentional nonchanges.

Start course Start without an account. Sign in to save lesson progress across devices.

Your project

Start with the practice files. Build your project one lesson at a time.

Practice files are free without an account. Sign in to save your project record in My Proof: a description, checklist, and optional work link. Keep the original files in your own app or notebook; saving a project record does not upload them.

  1. Get your materialsDownload practice filesSupplied practice cases, a blank working record, complete lesson instructions, and annotated worked examples.
  2. Make your projectView the project checklist Work in your own notebook, document, or software. Use the supplied practice examples when you prefer.
  3. Keep a private recordSave a project record Sign in to save a description and checklist, then return to update them.
Course handbook

Know the route before you begin.

Here’s what you’ll need and what you’ll make. You can return to this guide during any lesson.

Who this is for

Learners ages 14+ and adults ready for the named prerequisite; no employment or private records required.

Prepare before lesson one
  • Complete or review Programming with Python before starting.
  • Use a local text editor and a free Python 3.10+ interpreter for Python files, or a current browser for HTML files. Supplied code uses no paid services or external packages.
  • Create a fresh practice folder and use only the supplied fictional inputs. A paper trace is labeled a trace, not an executed test.
Your final project

Finish with proof you can use.

A before/after fictional fee calculator split into pure functions with an executable equivalence suite, clearer names and documented intentional nonchanges.

Save a project record in My Proof
See project checklist and review criteria

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.
Three ideas to help you along the way
Three useful ideas

The rules behind the steps.

01

State the contract

Name inputs, permitted states and observable outputs before editing the implementation.

02

Preserve the evidence

Keep the actual source, changed input and expected/observed record so someone can repeat the work.

03

Test the boundary

Check a rejection or changed case and distinguish local practice from real service or native-tool behavior.

What do these words mean?
Key words

Key terms worth knowing.

refactor
Change code structure while preserving the declared observable behavior.
pure function
A calculation using explicit inputs with no hidden side effects.
characterization
Recording existing behavior for comparison.
equivalence
Agreement of observable results under the chosen contract.
mutation
A deliberate small wrong change used to test detection.
Your route

6 lessons.
One useful project.

Work in order the first time. Each lesson makes something the next lesson can use.

  1. 1
    Up next

    Write the behavior contract before refactoring

    Produce a complete valid/rejected behavior contract and intentional-change boundary.

  2. 2
    22 min

    Capture equivalence across the input space

    Retain full finite valid-space coverage and explicitly scoped invalid-case comparisons.

  3. 3
    24 min

    Extract pure helpers with one responsibility

    Create checked validation/subtotal helpers and preserve the public entry contract.

  4. 4
    19 min

    Rename for meaning without altering semantics

    Produce a domain-name/reference review with unchanged discount and delivery semantics.

  5. 5
    26 min

    Prove the suite detects a real wrong change

    Retain a passing baseline and an expected failing price mutation without altering final pricing.

  6. 6
    29 min

    Deliver the refactor with precise coverage limits

    Deliver an independently reasoned, repeatable refactor with finite coverage and selected-invalid limits.

Use the tools

Tools to help you practice.

See all tools
ZIP · practice files

Refactor Code Without Changing Behavior practice files

Supplied practice cases, a blank working record, complete lesson instructions, and annotated worked examples.

Text worksheet

Your course project worksheet

A downloadable text worksheet with each lesson’s working output, space for your answers, and the shared final-project checklist. Save your own copy.

Source transparency

Sources and further study.

Explore the references behind the lessons and take your learning further. Source notes explain access and reuse.

docs.python.org — unittest.html
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.

docs.python.org — controlflow.html
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.

docs.python.org — modules.html
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.

Ready when you are

Start with the first useful move.

You’ll always know what to do, what to make, and where to go next.

Start lesson one