testability

name	testability
description	Testability assessment criteria for code review. Apply when writing new code that will be tested or evaluating code for dependency injection, global state, pure functions, and test seams.
user-invocable	false
layers	{"rules":"rules.md","examples":"examples.md"}

Testability Assessment

Evaluate whether code can be effectively unit tested in isolation.

Layers:

• rules.md - Quick reference and severity guidance
• examples.md - Detailed code examples

Quick Reference

Factor	Question	Severity if problematic
Dependency injection	Are deps passed in?	High
Global state	Is shared state avoided?	High
Pure functions	Is logic separated from I/O?	Medium
Time/randomness	Are these injectable?	Medium
File system	Can it be abstracted?	Medium
Seams	Can behavior be substituted?	Medium-High
Observability	Can you assert on outputs?	Medium
Mock avoidance	Does the design eliminate the need for mocking?	High

Quick Heuristic

If testing a function requires:

• 0 mocks: Excellent testability (pure function)
• 1-2 mocks: Acceptable (clear external boundaries only)
• 3-5 mocks: Redesign required (too many responsibilities or mocking internal code)
• 6+ mocks: Design failure (refactor before writing tests)

If any mock or monkeypatch.setattr targets internal functions, methods, or classes rather than external boundaries, the count is irrelevant -- redesign for testability instead.

For rules: see rules.md For examples: see examples.md