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# Elisp Testing Rules
Applies to: `**/tests/*.el`
Implements the core principles from `testing.md`. All rules there apply here —
this file covers Elisp-specific patterns.
## Framework: ERT
Use `ert-deftest` for all tests. One test = one scenario.
## File Layout
- `tests/test-<module>.el` — tests for `<module>.el`
- `tests/test-<module>--<helper>.el` — tests for a specific private helper (matches `<module>--<helper>` function naming)
- `tests/testutil-<module>.el` — fixtures and mocks scoped to one module
- `tests/testutil-*.el` — cross-module helpers (shared fixtures, generic mocks, filesystem helpers); name them for what they help with
Tests must `(require 'module-name)` before the testutil file that stubs its internals, unless documented otherwise. Order matters — a testutil that defines a stub can be shadowed by a later `require` of the real module.
## Test Naming
```elisp
(ert-deftest test-<module>-<function>-<scenario> ()
"Normal/Boundary/Error: brief description."
...)
```
Put the category (Normal, Boundary, Error) in the docstring so the category is grep-able.
## Required Coverage
Every non-trivial function needs at least:
- One **Normal** case (happy path)
- One **Boundary** case (empty, nil, min, max, unicode, long string)
- One **Error** case (invalid input, missing resource, failure mode)
Missing a category is a test gap. If three cases look near-identical, parametrize with a loop or `dolist` rather than copy-pasting.
## TDD Workflow
Write the failing test first. A failing test proves you understand the change. Assume the bug is in production code until the test proves otherwise — never fix the test before proving the test is wrong.
For untested code, write a **characterization test** that captures current behavior before you change anything. It becomes the safety net for the refactor.
## Mocking
Mock at boundaries:
- Shell: `cl-letf` on `shell-command`, `shell-command-to-string`, `call-process`
- File I/O when tests shouldn't touch disk
- Network: URL retrievers, HTTP clients
- Time: `cl-letf` on `current-time`, `format-time-string`
Never mock:
- The code under test
- Core Emacs primitives (buffer ops, string ops, lists)
- Your own domain logic — restructure it to be testable instead
## Idioms
- `cl-letf` for scoped overrides (self-cleaning)
- `with-temp-buffer` for buffer manipulation tests
- `make-temp-file` with `.el` suffix for on-disk fixtures
- Tests must run in any order; no shared mutable state
## Running Tests
```bash
make test # All
make test-file FILE=tests/test-foo.el # One file
make test-name TEST=pattern # Match by test name pattern
```
A PostToolUse hook runs matching tests automatically after edits to a module, when the match count is small enough to be fast.
## Anti-Patterns
- Hardcoded timestamps — generate relative to `current-time` or mock
- Testing implementation details (private storage structure) instead of behavior
- Mocking the thing you're testing
- Skipping a failing test without an issue to track it
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