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Capstone: A Small Data Project · Lesson 167

Add Tests

Tests encode important behaviour as executable checks so later refactoring cannot silently change a validated result.

ConceptWorked examplePracticeKnowledge check
Textbook walkthrough

Add Tests

Tests encode important behaviour as executable checks so later refactoring cannot silently change a validated result.

Learning goal: explain why Add Tests behaves this way, apply it to a small example, and verify the result independently. Begin by being able to justify this first step: Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state.

Deeper walkthrough

Read Add Tests as a mechanism, not a recipe

Treat this as a sequence of observable decisions rather than one opaque command. Stage 1: Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state. Stage 2: Keep the stage inside the same data/validation definitions used by the rest of the project. Stage 3: Save the evidence produced by this stage so the next stage can be audited.

Mechanism

Follow the transformation

Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state.

Keep the stage inside the same data/validation definitions used by the rest of the project.

Save the evidence produced by this stage so the next stage can be audited.

Evidence

Know what would convince you

  • Trace a tiny input by hand and compare the runtime result.
  • Inspect type, value/shape and any mutation/side effect explicitly.
Useful distinctionInput: Objects/values supplied to the operation.
Click a stage to inspect what happens, what changes, and what should be checked before moving on.
Stage 1

Test normal cases

Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state.

Verification focus: record the evidence you inspected and the condition that would make this stage fail.
How it works

Trace the mechanism step by step

  1. Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state.
  2. Keep the stage inside the same data/validation definitions used by the rest of the project.
  3. Save the evidence produced by this stage so the next stage can be audited.
Worked demonstration

Add Tests evidence

Add Tests evidence
Evidence: a deliberately introduced bug makes at least one relevant test fail.
Expected / illustrative result
The worked evidence makes the output of this project stage concrete and auditable.
Interpret the result.

For Add Tests, trace the specific input through the mechanism above and independently verify one returned value, state change or side effect.

Distinctions & related ideas

Place the concept correctly

InputObjects/values supplied to the operation.
StateNames or mutable objects that may change during execution.
OutputReturned value, side effect, file, plot or exception to inspect.
Use deliberately

When it is appropriate

Use Add Tests when it answers a defined question in Capstone: A Small Data Project and its inputs/assumptions match the current data or program state.

Boundary conditions

When to stop or reconsider

Reconsider Add Tests when the required information is unavailable, the operation would violate a validation/data boundary, or a simpler operation answers the question more transparently.

Common mistakes

Failure modes to recognise

  • Running the operation on the wrong object/type or in the wrong environment.
  • Inferring correctness from “no exception” without checking the produced value/state.
  • Hiding a boundary case instead of making its behaviour explicit.
Verification

How to check the result

  • Trace a tiny input by hand and compare the runtime result.
  • Inspect type, value/shape and any mutation/side effect explicitly.
  • Run an edge or invalid case and confirm the exception/behaviour is deliberate.
Hands-on practice

Demonstrate understanding

Try this:

Construct a tiny example of Add Tests. First test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state. Then keep the stage inside the same data/validation definitions used by the rest of the project. Predict the result before execution and explain one boundary or failure case.

List the stage inputs and expected artifact, rerun it from a clean state, and compare against a concrete acceptance check.
Knowledge check

Check reasoning, not memorisation

Which approach best demonstrates understanding of Add Tests?

Quick reference

Remember the logic

Step 1Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state.
Step 2Keep the stage inside the same data/validation definitions used by the rest of the project.
Step 3Save the evidence produced by this stage so the next stage can be audited.
Lesson summary

What to remember

  • Tests encode important behaviour as executable checks so later refactoring cannot silently change a validated result.
  • Test normal cases, edge cases and expected failures; keep calculation tests independent of unnecessary I/O; run the test suite from a clean project state.
  • Running the operation on the wrong object/type or in the wrong environment.
  • Trace a tiny input by hand and compare the runtime result.