test-driven-development

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files
• Pure refactoring (no behavior change)

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over. Implement fresh from tests.

Tempted to skip this? That impulse is the thing the rule is for.

Red-Green-Refactor

The TDD cycle with decision points:

digraph tdd_flow {
    rankdir=TB;
    compound=true;

    // Entry point
    run_all [label="Run All\nUnit Tests", shape=box, style="filled,rounded", fillcolor="#e8f5e9"];
    run_all_decision [label="", shape=diamond, width=0.4, height=0.4];

    // RED zone
    subgraph cluster_red {
        label="Red";
        style=filled;
        fillcolor="#ffcdd2";
        write_test [label="Write failing\nUnit Test", shape=box, style="filled,rounded", fillcolor="#ffebee"];
    }

    // GREEN zone
    subgraph cluster_green {
        label="Green";
        style=filled;
        fillcolor="#c8e6c9";
        change_behavior [label="Change\nBehavior", shape=box, style="filled,rounded", fillcolor="#e8f5e9"];
    }

    // REFACTOR zone
    subgraph cluster_refactor {
        label="Refactor";
        style=filled;
        fillcolor="#e0e0e0";
        change_structure [label="Change\nStructure", shape=box, style="filled,rounded", fillcolor="#fff9c4"];
    }

    run_unit [label="Run\nUnit Test", shape=box, style="filled,rounded", fillcolor="#fff9c4"];
    run_unit_decision [label="", shape=diamond, width=0.4, height=0.4];
    revert [label="Revert\n(test wrong!)", shape=box, style="filled,rounded", fillcolor="#ffccbc"];

    run_all -> run_all_decision;
    run_all_decision -> change_structure [label="Pass"];
    run_all_decision -> write_test [label="Fail"];
    write_test -> run_unit;
    run_unit -> run_unit_decision;
    run_unit_decision -> revert [label="Pass*\n(1st run)"];
    run_unit_decision -> change_behavior [label="Fail"];
    change_behavior -> run_unit;
    run_unit_decision -> change_structure [label="Pass"];
    change_structure -> run_all;
}