Use type checking to detect invalid mutants

README.rst

@@ -167,6 +167,26 @@ If you only want to mutate lines that are called (according to coverage.py), you
     mutate_only_covered_lines=true
 
 
+Filter generated mutants with type checker
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When your project is type checked, you can also use it to filter out invalid mutants.
+For instance, mutmut mutates `x: str = 'foo'` to `x: str = None` which can easily caught by type checkers.
+
+To enable this filtering, configure the `type_check_command` to output json results as follows:
+
+.. code-block::
+
+    # for pyrefly
+    type_check_command = ['pyrefly', 'check', '--output-format=json']
+    # for mypy
+    type_check_command = ['mypy', 'traces_parser', '--output', 'json']
+
+Currently, only `pyrefly` and `mypy` are supported.
+With `pyright` and `ty`, mutating a class method `Foo.bar()` can break the types of all methods of `Foo`,
+and therefore mutmut cannot match the type error with the mutant that caused the type error.
+
+
 Enable debug output (increase verbosity)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

e2e_projects/type_checking/README.md

@@ -0,0 +1 @@
+This project uses type checking to detect invalid mutants.

e2e_projects/type_checking/pyproject.toml

@@ -0,0 +1,32 @@
+[project]
+name = "type-checking"
+version = "0.1.0"
+description = "Add your description here"
+readme = "README.md"
+authors = []
+requires-python = ">=3.10"
+dependencies = []
+
+[build-system]
+requires = ["uv_build>=0.9.18,<0.10.0"]
+build-backend = "uv_build"
+
+[dependency-groups]
+dev = [
+    "pyrefly>=0.52.0",
+    "pyright>=1.1.408",
+    "pytest>=8.2.0",
+]
+
+[tool.mutmut]
+debug = true
+type_check_command = ["pyrefly", "check", "--output-format=json"]
+
+[tool.pyrefly]
+project-includes = [
+    "**/*.py*",
+    "**/*.ipynb",
+]
+
+[tool.pyright]
+typeCheckingMode = "strict"

e2e_projects/type_checking/src/type_checking/__init__.py

@@ -0,0 +1,23 @@
+def hello() -> str:
+    greeting: str = "Hello from type-checking!"
+    return greeting
+
+def a_hello_wrapper() -> str:
+    # verify that hello() keeps the return type str
+    # (if not, this will type error and not be mutated)
+    return hello() + "2"
+
+class Person:
+    def set_name(self, name: str):
+        self.name = name
+
+    def get_name(self):
+        # return type should be inferred as "str"
+        return self.name
+
+def mutate_me():
+    p = Person()
+    p.set_name('charlie')
+    # Verify that p.get_name keeps the return type str
+    name: str = p.get_name()
+    return name

e2e_projects/type_checking/tests/test_type_checking.py

@@ -0,0 +1,10 @@
+from type_checking import *
+
+def test_hello():
+    assert hello() == "Hello from type-checking!"
+
+def test_a_hello_wrapper():
+    assert isinstance(a_hello_wrapper(), str)
+
+def test_mutate_me():
+    assert mutate_me() == "charlie"

pyproject.toml

@@ -54,6 +54,7 @@ source-include = ["HISTORY.rst"]
 [dependency-groups]
 dev = [
     "inline-snapshot>=0.32.0",
+    "pyrefly>=0.53.0",
     "pytest-asyncio>=1.0.0",
     "ruff>=0.15.1",
 ]