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TEST001-test-ratio — Low Test-to-Source Ratio

Dimension: TestSmell Default severity: Low Languages: All Last reviewed: 2026-05-05

What it detects​

Flags directories where the ratio of test files to source files is below a configurable threshold. One finding is emitted per under-tested directory, anchored at the lexicographically-first source file in that directory.

Why it matters​

A low test ratio is an early indicator of technical debt accumulation:

  • Bugs introduced by refactoring go undetected.
  • New contributors have no safety net when modifying code they don't know.
  • Confidence in releases decreases as the codebase grows untested.

The per-directory grouping surfaces hot spots: a single directory with many source files and no tests is more actionable than a project-level summary.

Algorithm​

1. File classification​

Each parsed file is classified as Test or Source. A file is Test when any of the following hold (checked in order):

HeuristicExamples
Path component is a known test directorytests/, __tests__/, spec/, test_*/
Filename stem starts with test_test_utils.py, test_models.rs
Filename stem ends with _testfoo_test.rs
Full filename contains .test. or .spec.foo.test.ts, bar.spec.tsx
SemanticIndex has ≥1 function with is_test = truefunctions annotated #[test] (Rust) or named test_* (Python/JS)

All other files are classified as Source.

2. Directory grouping​

Files are grouped by their immediate parent directory (file_path.parent()).

3. Threshold check​

For each directory that:

  • Contains at least MIN_SOURCE_FILES = 3 source files (hard-coded, not configurable in v1 — see below), and
  • Has (test_count * 100) / source_count < threshold,

one finding is emitted.

Minimum source files​

The constant MIN_SOURCE_FILES = 3 prevents noise from small directories: a single-file utility module with no tests would otherwise always trigger at the default threshold. This value is intentionally not exposed as a configuration knob in v1 to keep the rule simple. Future versions may add a min_source_files config key.

Configuration​

ParameterTypeDefaultNotes
thresholdu3210Percentage; a directory with fewer than this percent test files is flagged.

Example zuit.toml:

[rules.TEST001-test-ratio]
threshold = 20   # require 20% test coverage by file count

A threshold of 0 disables the ratio check (every directory with enough source files would pass).

Example — flagged​

Fixture: fixtures/python/no_tests/

Contains 5 source files (alpha.py … epsilon.py) and 0 test files. Ratio = 0% < 10% threshold.

TEST001-test-ratio [Low]
fixtures/python/no_tests/alpha.py:1:1
directory 'fixtures/python/no_tests' has a low test:source ratio (0%, target ≥10%)

Example — not flagged​

A directory with 3 source files and 1 test file has a 33% ratio — well above the default 10% threshold. No finding is emitted.

A directory with only 2 source files is also not flagged because it is below MIN_SOURCE_FILES = 3.

Fix guidance​

  • Add unit-test files named test_<module>.py (Python), <module>.test.ts (JS), or a #[cfg(test)] mod tests { … } block (Rust) for each source module.
  • Start with the most critical modules (authentication, data validation, core business logic) and expand coverage incrementally.
  • Consider a per-PR policy that requires at least one test file for every new source file added.

Implementation​

  • Source: crates/zuit-analyzers/src/test_ratio.rs
  • Classification helper: classify_file (public; testable in isolation).
  • MIN_SOURCE_FILES = 3 (constant; not configurable in v1).
  • Fixtures: fixtures/{python,js,rust}/no_tests/.

References​