🐛 Fix mypy strict mode compatibility

docker/Dockerfile.test

@@ -6,9 +6,10 @@ COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /bin/
 
 ENV UV_PROJECT_ENVIRONMENT=/opt/venv
 
-COPY . .
-
-RUN uv sync --group dev
+RUN --mount=type=cache,target=/root/.cache/uv \
+    --mount=type=bind,source=uv.lock,target=uv.lock \
+    --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
+    uv sync --locked --no-install-project
 
 ENTRYPOINT ["uv", "run", "pytest"]
 CMD ["--verbose"]

e2e_projects/type_checking_mypy/README.md

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

e2e_projects/type_checking_mypy/pyproject.toml

@@ -0,0 +1,25 @@
+[project]
+name = "type-checking-mypy"
+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 = [
+    "mypy>=1.0.0",
+    "pytest>=8.2.0",
+]
+
+[tool.mutmut]
+debug = true
+type_check_command = ["mypy", "--output", "json", "src"]
+
+[tool.mypy]
+strict = true

e2e_projects/type_checking_mypy/src/type_checking_mypy/__init__.py

@@ -0,0 +1,8 @@
+def hello() -> str:
+    greeting = "Hello from type-checking-mypy!"
+    return greeting
+
+def mutate_me() -> str:
+    # verify that hello() keeps the return type str
+    # (if not, this will type error and not be mutated)
+    return hello() + " Goodbye"

e2e_projects/type_checking_mypy/tests/test_type_checking_mypy.py

@@ -0,0 +1,4 @@
+from type_checking_mypy import mutate_me
+
+def test_mutate_me() -> None:
+    assert mutate_me() == "Hello from type-checking-mypy! Goodbye"

pyproject.toml

@@ -54,6 +54,7 @@ source-include = ["HISTORY.rst"]
 [dependency-groups]
 dev = [
     "inline-snapshot>=0.32.0",
+    "mypy>=1.0.0",
     "pre-commit>=4.5.1",
     "pyrefly>=0.53.0",
     "pytest-asyncio>=1.0.0",

src/mutmut/__main__.py

@@ -1778,7 +1778,7 @@ def get_mutant_name_from_selection(self) -> str | None:
             # noinspection PyTypeChecker
             mutants_table: DataTable[Any] = self.query_one("#mutants")  # type: ignore[assignment]
             if mutants_table.cursor_row is None or not mutants_table.is_valid_row_index(mutants_table.cursor_row):
-                return
+                return None
 
             return str(mutants_table.get_row_at(mutants_table.cursor_row)[0])
 

src/mutmut/file_mutation.py

@@ -323,26 +323,38 @@ def _get_local_name(func_name: str) -> cst.BaseExpression:
             cst.Arg(cst.Name("None" if class_name is None else "self")),
         ],
     )
-    # for non-async functions, simply return the value or generator
-    result_statement = cst.SimpleStatementLine([cst.Return(result)])
-
-    if function.asynchronous:
-        is_generator = _is_generator(function)
-        if is_generator:
-            # async for i in _mutmut_trampoline(...): yield i
-            result_statement = cst.For(  # type: ignore[assignment]
-                target=cst.Name("i"),
-                iter=result,
-                body=cst.IndentedBlock([cst.SimpleStatementLine([cst.Expr(cst.Yield(cst.Name("i")))])]),
-                asynchronous=cst.Asynchronous(),
-            )
-        else:
-            # return await _mutmut_trampoline(...)
-            result_statement = cst.SimpleStatementLine([cst.Return(cst.Await(result))])
 
     type_ignore_whitespace = cst.TrailingWhitespace(comment=cst.Comment("# type: ignore"))
 
-    function.whitespace_after_type_parameters
+    result_statement: cst.SimpleStatementLine | cst.For
+    if not function.asynchronous:
+        # for non-async functions, simply return the value or generator
+        result_statement = cst.SimpleStatementLine(
+            [cst.Return(result)],
+            trailing_whitespace=type_ignore_whitespace,
+        )
+    elif not _is_generator(function):
+        # return await _mutmut_trampoline(...)
+        result_statement = cst.SimpleStatementLine(
+            [cst.Return(cst.Await(result))],
+            trailing_whitespace=type_ignore_whitespace,
+        )
+    else:
+        # async for i in _mutmut_trampoline(...): yield i
+        result_statement = cst.For(
+            target=cst.Name("i"),
+            iter=result,
+            body=cst.IndentedBlock(
+                [
+                    cst.SimpleStatementLine(
+                        [cst.Expr(cst.Yield(cst.Name("i")))],
+                        trailing_whitespace=type_ignore_whitespace,
+                    )
+                ]
+            ),
+            asynchronous=cst.Asynchronous(),
+        )
+
     return function.with_changes(
         body=cst.IndentedBlock(
             [

src/mutmut/trampoline_templates.py

@@ -37,27 +37,27 @@ def mangle_function_name(*, name: str, class_name: str | None) -> str:
 
 def _mutmut_trampoline(orig, mutants, call_args, call_kwargs, self_arg = None): # type: ignore
     \"""Forward call to original or mutated function, depending on the environment\"""
-    import os # type: ignore
-    mutant_under_test = os.environ['MUTANT_UNDER_TEST'] # type: ignore
-    if mutant_under_test == 'fail': # type: ignore
+    import os
+    mutant_under_test = os.environ['MUTANT_UNDER_TEST']
+    if mutant_under_test == 'fail':
         from mutmut.__main__ import MutmutProgrammaticFailException # type: ignore
-        raise MutmutProgrammaticFailException('Failed programmatically')       # type: ignore
-    elif mutant_under_test == 'stats': # type: ignore
-        from mutmut.__main__ import record_trampoline_hit # type: ignore
-        record_trampoline_hit(orig.__module__ + '.' + orig.__name__) # type: ignore
+        raise MutmutProgrammaticFailException('Failed programmatically')
+    elif mutant_under_test == 'stats':
+        from mutmut.__main__ import record_trampoline_hit
+        record_trampoline_hit(orig.__module__ + '.' + orig.__name__)
         # (for class methods, orig is bound and thus does not need the explicit self argument)
-        result = orig(*call_args, **call_kwargs) # type: ignore
-        return result # type: ignore
-    prefix = orig.__module__ + '.' + orig.__name__ + '__mutmut_' # type: ignore
-    if not mutant_under_test.startswith(prefix): # type: ignore
-        result = orig(*call_args, **call_kwargs) # type: ignore
-        return result # type: ignore
-    mutant_name = mutant_under_test.rpartition('.')[-1] # type: ignore
-    if self_arg is not None: # type: ignore
+        result = orig(*call_args, **call_kwargs)
+        return result
+    prefix = orig.__module__ + '.' + orig.__name__ + '__mutmut_'
+    if not mutant_under_test.startswith(prefix):
+        result = orig(*call_args, **call_kwargs)
+        return result
+    mutant_name = mutant_under_test.rpartition('.')[-1]
+    if self_arg is not None:
         # call to a class method where self is not bound
-        result = mutants[mutant_name](self_arg, *call_args, **call_kwargs) # type: ignore
+        result = mutants[mutant_name](self_arg, *call_args, **call_kwargs)
     else:
-        result = mutants[mutant_name](*call_args, **call_kwargs) # type: ignore
-    return result # type: ignore
+        result = mutants[mutant_name](*call_args, **call_kwargs)
+    return result
 
 """

tests/e2e/test_e2e_type_checking_mypy.py

@@ -0,0 +1,20 @@
+from inline_snapshot import snapshot
+
+from tests.e2e.e2e_utils import run_mutmut_on_project
+
+
+def test_type_checking_mypy_result_snapshot():
+    assert run_mutmut_on_project("type_checking_mypy") == snapshot(
+        {
+            "mutants/src/type_checking_mypy/__init__.py.meta": {
+                "type_checking_mypy.x_hello__mutmut_1": 37,
+                "type_checking_mypy.x_hello__mutmut_2": 1,
+                "type_checking_mypy.x_hello__mutmut_3": 1,
+                "type_checking_mypy.x_hello__mutmut_4": 1,
+                "type_checking_mypy.x_mutate_me__mutmut_1": 37,
+                "type_checking_mypy.x_mutate_me__mutmut_2": 1,
+                "type_checking_mypy.x_mutate_me__mutmut_3": 1,
+                "type_checking_mypy.x_mutate_me__mutmut_4": 1,
+            }
+        }
+    )

tests/test_mutation regression.py

@@ -18,7 +18,7 @@ def test_create_trampoline_wrapper_async_method():
 async def foo(a: str, b, *args, **kwargs) -> dict[str, int]:
     args = [a, b, *args]# type: ignore
     kwargs = {**kwargs}# type: ignore
-    return await _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None)\
+    return await _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None)# type: ignore\
 """)
 
 
@@ -34,7 +34,7 @@ async def foo():
     args = []# type: ignore
     kwargs = {}# type: ignore
     async for i in _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None):
-        yield i\
+        yield i# type: ignore\
 """)
 
 
@@ -45,7 +45,7 @@ def test_create_trampoline_wrapper_with_positionals_only_args():
 def foo(p1, p2=None, /, p_or_kw=None, *, kw):
     args = [p1, p2, p_or_kw]# type: ignore
     kwargs = {'kw': kw}# type: ignore
-    return _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None)\
+    return _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None)# type: ignore\
 """)
 
 
@@ -56,7 +56,7 @@ def test_create_trampoline_wrapper_for_class_method():
 def foo(self, a, b):
     args = [a, b]# type: ignore
     kwargs = {}# type: ignore
-    return _mutmut_trampoline(object.__getattribute__(self, 'x_foo__mutmut_orig'), object.__getattribute__(self, 'x_foo__mutmut_mutants'), args, kwargs, self)\
+    return _mutmut_trampoline(object.__getattribute__(self, 'x_foo__mutmut_orig'), object.__getattribute__(self, 'x_foo__mutmut_mutants'), args, kwargs, self)# type: ignore\
 """)
 
 
@@ -99,33 +99,33 @@ def add(self, value):
 
 def _mutmut_trampoline(orig, mutants, call_args, call_kwargs, self_arg = None): # type: ignore
     """Forward call to original or mutated function, depending on the environment"""
-    import os # type: ignore
-    mutant_under_test = os.environ['MUTANT_UNDER_TEST'] # type: ignore
-    if mutant_under_test == 'fail': # type: ignore
+    import os
+    mutant_under_test = os.environ['MUTANT_UNDER_TEST']
+    if mutant_under_test == 'fail':
         from mutmut.__main__ import MutmutProgrammaticFailException # type: ignore
-        raise MutmutProgrammaticFailException('Failed programmatically')       # type: ignore
-    elif mutant_under_test == 'stats': # type: ignore
-        from mutmut.__main__ import record_trampoline_hit # type: ignore
-        record_trampoline_hit(orig.__module__ + '.' + orig.__name__) # type: ignore
+        raise MutmutProgrammaticFailException('Failed programmatically')
+    elif mutant_under_test == 'stats':
+        from mutmut.__main__ import record_trampoline_hit
+        record_trampoline_hit(orig.__module__ + '.' + orig.__name__)
         # (for class methods, orig is bound and thus does not need the explicit self argument)
-        result = orig(*call_args, **call_kwargs) # type: ignore
-        return result # type: ignore
-    prefix = orig.__module__ + '.' + orig.__name__ + '__mutmut_' # type: ignore
-    if not mutant_under_test.startswith(prefix): # type: ignore
-        result = orig(*call_args, **call_kwargs) # type: ignore
-        return result # type: ignore
-    mutant_name = mutant_under_test.rpartition('.')[-1] # type: ignore
-    if self_arg is not None: # type: ignore
+        result = orig(*call_args, **call_kwargs)
+        return result
+    prefix = orig.__module__ + '.' + orig.__name__ + '__mutmut_'
+    if not mutant_under_test.startswith(prefix):
+        result = orig(*call_args, **call_kwargs)
+        return result
+    mutant_name = mutant_under_test.rpartition('.')[-1]
+    if self_arg is not None:
         # call to a class method where self is not bound
-        result = mutants[mutant_name](self_arg, *call_args, **call_kwargs) # type: ignore
+        result = mutants[mutant_name](self_arg, *call_args, **call_kwargs)
     else:
-        result = mutants[mutant_name](*call_args, **call_kwargs) # type: ignore
-    return result # type: ignore
+        result = mutants[mutant_name](*call_args, **call_kwargs)
+    return result
 
 def foo(a: list[int], b):
     args = [a, b]# type: ignore
     kwargs = {}# type: ignore
-    return _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None)
+    return _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, args, kwargs, None)# type: ignore
 
 def x_foo__mutmut_orig(a: list[int], b):
     return a[0] > b
@@ -145,7 +145,7 @@ def x_foo__mutmut_2(a: list[int], b):
 def bar():
     args = []# type: ignore
     kwargs = {}# type: ignore
-    return _mutmut_trampoline(x_bar__mutmut_orig, x_bar__mutmut_mutants, args, kwargs, None)
+    return _mutmut_trampoline(x_bar__mutmut_orig, x_bar__mutmut_mutants, args, kwargs, None)# type: ignore
 
 def x_bar__mutmut_orig():
     yield 1
@@ -162,7 +162,7 @@ class Adder:
     def __init__(self, amount):
         args = [amount]# type: ignore
         kwargs = {}# type: ignore
-        return _mutmut_trampoline(object.__getattribute__(self, 'xǁAdderǁ__init____mutmut_orig'), object.__getattribute__(self, 'xǁAdderǁ__init____mutmut_mutants'), args, kwargs, self)
+        return _mutmut_trampoline(object.__getattribute__(self, 'xǁAdderǁ__init____mutmut_orig'), object.__getattribute__(self, 'xǁAdderǁ__init____mutmut_mutants'), args, kwargs, self)# type: ignore
     def xǁAdderǁ__init____mutmut_orig(self, amount):
         self.amount = amount
     def xǁAdderǁ__init____mutmut_1(self, amount):
@@ -176,7 +176,7 @@ def xǁAdderǁ__init____mutmut_1(self, amount):
     def add(self, value):
         args = [value]# type: ignore
         kwargs = {}# type: ignore
-        return _mutmut_trampoline(object.__getattribute__(self, 'xǁAdderǁadd__mutmut_orig'), object.__getattribute__(self, 'xǁAdderǁadd__mutmut_mutants'), args, kwargs, self)
+        return _mutmut_trampoline(object.__getattribute__(self, 'xǁAdderǁadd__mutmut_orig'), object.__getattribute__(self, 'xǁAdderǁadd__mutmut_mutants'), args, kwargs, self)# type: ignore
 
     def xǁAdderǁadd__mutmut_orig(self, value):
         return self.amount + value