Merge pull request #5768 from robholt/fixture-class-instance

Fix self reference in function scoped fixtures

src/_pytest/fixtures.py

@@ -900,6 +900,12 @@ def resolve_fixture_function(fixturedef, request):
         # request.instance so that code working with "fixturedef" behaves
         # as expected.
         if request.instance is not None:
+            # handle the case where fixture is defined not in a test class, but some other class
+            # (for example a plugin class with a fixture), see #2270
+            if hasattr(fixturefunc, "__self__") and not isinstance(
+                request.instance, fixturefunc.__self__.__class__
+            ):
+                return fixturefunc
             fixturefunc = getimfunc(fixturedef.func)
             if fixturefunc != fixturedef.func:
                 fixturefunc = fixturefunc.__get__(request.instance)