Just raise deprecation warning

doc/en/example/simple.rst

@@ -819,7 +819,7 @@ case we just write some information out to a ``failures`` file:
             with open("failures", mode, encoding="utf-8") as f:
                 # let's also access a fixture for the fun of it
                 if "tmp_path" in item.fixturenames:
-                    extra = " ({})".format(item.funcargs["tmp_path"])
+                    extra = " ({})".format(item._request.getfixturevalue("tmp_path"))
                 else:
                     extra = ""
 

src/_pytest/fixtures.py

@@ -683,11 +683,9 @@ class TopRequest(FixtureRequest):
     def _fillfixtures(self) -> None:
         item = self._pyfuncitem
         fixturenames = getattr(item, "fixturenames", self.fixturenames)
-        initialnames = item._fixtureinfo.initialnames
         for argname in fixturenames:
-            value = self.getfixturevalue(argname)
-            if argname not in item.funcargs and argname in initialnames:
-                item.funcargs[argname] = value
+            if argname not in item.funcargs:
+                item.funcargs[argname] = self.getfixturevalue(argname)
 
     def addfinalizer(self, finalizer: Callable[[], object]) -> None:
         self.node.addfinalizer(finalizer)

src/_pytest/python.py

@@ -1659,6 +1659,23 @@ def write_docstring(tw: TerminalWriter, doc: str, indent: str = "    ") -> None:
         tw.line(indent + line)
 
 
+class DeprecatingFuncArgs(Dict[str, object]):
+    def __init__(self, initialnames):
+        self.initialnames = initialnames
+        super().__init__()
+
+    def __getitem__(self, key: str) -> object:
+        if key not in self.initialnames:
+            warnings.warn(
+                "Accessing to names other than initialnames i.e., direct args,"
+                " the ones with `usefixture` or the ones with `autouse` through "
+                "`item.funcargs` is deprecated and will raise `KeyError` from "
+                "pytest 9. Please use `request.getfixturevalue` instead.",
+                DeprecationWarning,
+            )
+        return super().__getitem__(key)
+
+
 class Function(PyobjMixin, nodes.Item):
     """Item responsible for setting up and executing a Python test function.
 
@@ -1747,7 +1764,9 @@ class Function(PyobjMixin, nodes.Item):
         return super().from_parent(parent=parent, **kw)
 
     def _initrequest(self) -> None:
-        self.funcargs: Dict[str, object] = {}
+        self.funcargs: Dict[str, object] = DeprecatingFuncArgs(
+            self._fixtureinfo.initialnames
+        )
         self._request = fixtures.TopRequest(self, _ispytest=True)
 
     @property

testing/python/fixtures.py

@@ -9,6 +9,7 @@ from _pytest.config import ExitCode
 from _pytest.fixtures import deduplicate_names
 from _pytest.fixtures import TopRequest
 from _pytest.monkeypatch import MonkeyPatch
+from _pytest.pytester import get_public_names
 from _pytest.pytester import Pytester
 from _pytest.python import Function
 
@@ -121,15 +122,20 @@ class TestFillFixtures:
             ["*recursive dependency involving fixture 'fix1' detected*"]
         )
 
-    def test_funcarg_basic(self, pytester: Pytester) -> None:
+    def test_funcarg_basic(self, recwarn, pytester: Pytester) -> None:
         pytester.copy_example()
         item = pytester.getitem(Path("test_funcarg_basic.py"))
         assert isinstance(item, Function)
         # Execute's item's setup, which fills fixtures.
         item.session._setupstate.setup(item)
-        assert len(item.funcargs) == 2
+        assert len(recwarn) == 0
+        item.funcargs["request"]
+        assert len(recwarn) == 1 and recwarn[0].category is DeprecationWarning
+        del item.funcargs["request"]
+        assert len(get_public_names(item.funcargs)) == 2
         assert item.funcargs["some"] == "test_func"
         assert item.funcargs["other"] == 42
+        assert len(recwarn) == 1
 
     def test_funcarg_lookup_modulelevel(self, pytester: Pytester) -> None:
         pytester.copy_example()
@@ -839,7 +845,8 @@ class TestRequestBasic:
         val2 = req.getfixturevalue("other")  # see about caching
         assert val2 == 2
         assert item.funcargs["something"] == 1
-        assert len(item.funcargs) == 1
+        assert len(get_public_names(item.funcargs)) == 2
+        assert "request" in item.funcargs
 
     def test_request_addfinalizer(self, pytester: Pytester) -> None:
         item = pytester.getitem(