Don't delete FixtureDef.cached_result, set it to None instead

Previously `cached_result` was either set or deleted. Type annotations
cannot handle this, so use `None` for the non-set state instead.

changelog/TODO.breaking.rst

@@ -0,0 +1,7 @@
+The ``cached_result`` attribute of ``FixtureDef`` is now set to ``None`` when
+the result is unavailable, instead of being deleted.
+
+If your plugin perform checks like ``hasattr(fixturedef, 'cached_result')``,
+for example in a ``pytest_fixture_post_finalizer`` hook implementation, replace
+it with ``fixturedef.cached_result is not None``. If you ``del`` the attribute,
+set it to ``None`` instead.

src/_pytest/fixtures.py

@@ -855,6 +855,7 @@ def __init__(
         self.argnames = getfuncargnames(func, name=argname, is_method=unittest)
         self.unittest = unittest
         self.ids = ids
+        self.cached_result = None
         self._finalizers = []
 
     def addfinalizer(self, finalizer):
@@ -881,8 +882,7 @@ def finish(self, request):
             # the cached fixture value and remove
             # all finalizers because they may be bound methods which will
             # keep instances alive
-            if hasattr(self, "cached_result"):
-                del self.cached_result
+            self.cached_result = None
             self._finalizers = []
 
     def execute(self, request):
@@ -894,9 +894,8 @@ def execute(self, request):
                 fixturedef.addfinalizer(functools.partial(self.finish, request=request))
 
         my_cache_key = self.cache_key(request)
-        cached_result = getattr(self, "cached_result", None)
-        if cached_result is not None:
-            result, cache_key, err = cached_result
+        if self.cached_result is not None:
+            result, cache_key, err = self.cached_result
             # note: comparison with `==` can fail (or be expensive) for e.g.
             # numpy arrays (#6497)
             if my_cache_key is cache_key:
@@ -908,7 +907,7 @@ def execute(self, request):
             # we have a previous but differently parametrized fixture instance
             # so we need to tear it down before creating a new one
             self.finish(request)
-            assert not hasattr(self, "cached_result")
+            assert self.cached_result is None
 
         hook = self._fixturemanager.session.gethookproxy(request.node.fspath)
         return hook.pytest_fixture_setup(fixturedef=self, request=request)
@@ -953,6 +952,7 @@ def pytest_fixture_setup(fixturedef, request):
     kwargs = {}
     for argname in fixturedef.argnames:
         fixdef = request._get_active_fixturedef(argname)
+        assert fixdef.cached_result is not None
         result, arg_cache_key, exc = fixdef.cached_result
         request._check_scope(argname, request.scope, fixdef.scope)
         kwargs[argname] = result

src/_pytest/hookspec.py

@@ -423,9 +423,9 @@ def pytest_fixture_setup(fixturedef, request):
 
 
 def pytest_fixture_post_finalizer(fixturedef, request):
-    """ called after fixture teardown, but before the cache is cleared so
-    the fixture result cache ``fixturedef.cached_result`` can
-    still be accessed."""
+    """Called after fixture teardown, but before the cache is cleared, so
+    the fixture result ``fixturedef.cached_result`` is still available (not
+    ``None``)."""
 
 
 # -------------------------------------------------------------------------

src/_pytest/setuponly.py

@@ -34,8 +34,8 @@ def pytest_fixture_setup(fixturedef, request):
         _show_fixture_action(fixturedef, "SETUP")
 
 
-def pytest_fixture_post_finalizer(fixturedef):
-    if hasattr(fixturedef, "cached_result"):
+def pytest_fixture_post_finalizer(fixturedef) -> None:
+    if fixturedef.cached_result is not None:
         config = fixturedef._fixturemanager.config
         if config.option.setupshow:
             _show_fixture_action(fixturedef, "TEARDOWN")