fix: forward generator return value through the trampoline

HISTORY.rst

@@ -4,6 +4,8 @@ Changelog
 Unreleased
 ~~~~~~~~~~
 
+* Fix the trampoline dropping a generator's return value, so ``yield from`` on a mutated generator no longer yields ``None`` instead of its result
+
 * Per-function source hashing for incremental cache invalidation — only re-test mutants in functions that changed
 
 * Cross-call dependency tracking — invalidate mutants in callers when a called function changes

src/mutmut/mutation/trampoline.py

@@ -93,7 +93,10 @@ def trampoline(*args: P.args, **kwargs: P.kwargs) -> R:
 
             @wraps(decorated_func)
             def _trampoline_wrapper(*args: P.args, **kwargs: P.kwargs) -> R:  # type: ignore
-                yield from trampoline(*args, **kwargs)  # type: ignore
+                # ``return`` the delegation result so a generator's StopIteration value
+                # (``return`` inside the generator) is forwarded to the caller's
+                # ``yield from``, matching the PEP 380 expansion.
+                return (yield from trampoline(*args, **kwargs))  # type: ignore
         elif inspect.iscoroutinefunction(decorated_func):
 
             @wraps(decorated_func)

tests/mutation/test_trampoline.py

@@ -13,6 +13,7 @@
 
 mutants_simple_func = {}
 mutants_generator_func = {}
+mutants_returning_generator_func = {}
 mutants_async_func = {}
 mutants_async_generator_func = {}
 mutants_cleanup_async_gen = {}
@@ -53,6 +54,25 @@ def generator_func_1(numbers: list[int]):
         yield n * 3
 
 
+@wrap_in_trampoline(mutants_returning_generator_func)
+def returning_generator_func(numbers: list[int]):
+    for n in numbers:
+        yield n * 2
+    return "done"
+
+
+def returning_generator_func_orig(numbers: list[int]):
+    for n in numbers:
+        yield n * 2
+    return "done"
+
+
+def returning_generator_func_1(numbers: list[int]):
+    for n in numbers:
+        yield n * 3
+    return "done"
+
+
 @wrap_in_trampoline(mutants_async_func)
 async def async_func(a: int, b: int):
     await asyncio.sleep(0)
@@ -199,6 +219,8 @@ def __init__(self, number: int):
 mutants_simple_func["simple_func__mutmut_1"] = simple_func_1
 mutants_generator_func["_mutmut_orig"] = generator_func_orig
 mutants_generator_func["generator_func__mutmut_1"] = generator_func_1
+mutants_returning_generator_func["_mutmut_orig"] = returning_generator_func_orig
+mutants_returning_generator_func["returning_generator_func__mutmut_1"] = returning_generator_func_1
 mutants_async_func["_mutmut_orig"] = async_func_orig
 mutants_async_func["async_func__mutmut_1"] = async_func_1
 mutants_async_generator_func["_mutmut_orig"] = async_generator_func_orig
@@ -240,6 +262,24 @@ def test_generator_func_mutated(self, monkeypatch):
         monkeypatch.setenv("MUTANT_UNDER_TEST", "test_trampoline.generator_func__mutmut_1")
         assert list(generator_func([1, 2, 3])) == [3, 6, 9], "Should call mutated func"
 
+    def test_generator_func_return_value_original(self, monkeypatch):
+        monkeypatch.setenv("MUTANT_UNDER_TEST", "")
+
+        def consume():
+            result = yield from returning_generator_func([1, 2, 3])
+            yield result
+
+        assert list(consume()) == [2, 4, 6, "done"], "Should forward the return value"
+
+    def test_generator_func_return_value_mutated(self, monkeypatch):
+        monkeypatch.setenv("MUTANT_UNDER_TEST", "test_trampoline.returning_generator_func__mutmut_1")
+
+        def consume():
+            result = yield from returning_generator_func([1, 2, 3])
+            yield result
+
+        assert list(consume()) == [3, 6, 9, "done"], "Should forward the return value"
+
     @pytest.mark.asyncio
     async def test_async_func_original(self, monkeypatch):
         monkeypatch.setenv("MUTANT_UNDER_TEST", "")