Add mutation testing

Author: WillGibson

.gitignore

@@ -4,5 +4,7 @@ __pycache__
 coverage.xml
 junit.xml
 
+mutants
+
 # Ignore temporary experiements
 scratch*

README.md

@@ -17,3 +17,21 @@ poetry run pytest
 ## Metrics
 
 - [Codecov](https://app.codecov.io/gh/WillGibson/python-sandpit)
+-
+## Mutation testing
+
+Using [mutmut](https://github.com/boxed/mutmut) as a tool to help identify gaps in our unit test coverage.
+
+Every mutation that survives is a line of code that we can change without it being picked up by our unit tests.
+
+To run the mutation tests:
+
+```shell
+poetry run mutmut run
+```
+
+The to review the mutants that survived:
+
+```shell
+poetry run mutmut browse
+```

_starter_kit/src/model/__init__.py

@@ -1,6 +1,5 @@
 import pytest
-
-from _starter_kit.src.model.thing import Thing, ThingCantActException
+from _starter_kit.src.thing import Thing, ThingCantActException
 
 
 class TestThing:

_starter_kit/src/model/thing.py _starter_kit/src/thing.py_starter_kit/src/model/thing.py renamed to _starter_kit/src/thing.py

@@ -3,7 +3,9 @@
 
 
 def scattered_subset_adding_up_to(
-    full_set: list[int], target_sum: int, number_of_elements: int = 1
+    full_set: list[int],
+    target_sum: int,
+    number_of_elements: int = 1,
 ) -> Optional[list[int]]:
     for combination in combinations(full_set, number_of_elements):
         if sum(combination) == target_sum:

_starter_kit/tests/model/__init__.py

@@ -6,11 +6,24 @@ package-mode = false
 
 [tool.poetry.dependencies]
 python = "^3.10"
+typeguard = "^4.4.2"
 
 [tool.poetry.group.dev.dependencies]
 pytest = "^8.3.5"
 pytest-cov = "^6.1.1"
 pre-commit = "^4.2.0"
+mutmut = { git = "git@github.com:boxed/mutmut.git", branch = "main" }
 
 [tool.black]
 line-length = 100
+
+[tool.mutmut]
+paths_to_mutate = [
+    "_starter_kit/src/",
+    "fox_goose_corn/src/",
+    "list_things/src/",
+    "thing_rental/src/"
+]
+
+[tool.pytest.ini_options]
+addopts = "--ignore mutants"