Fix self reference in function scoped fixtures

AUTHORS

@@ -209,6 +209,7 @@ Raphael Castaneda
 Raphael Pierzina
 Raquel Alegre
 Ravi Chandra
+Robert Holt
 Roberto Polli
 Roland Puntaier
 Romain Dorgueil

changelog/2270.bugfix.rst

@@ -0,0 +1 @@
+Fix ``self`` reference in function scoped fixtures that are in a plugin class

src/_pytest/fixtures.py

@@ -897,6 +897,10 @@ def resolve_fixture_function(fixturedef, request):
         # request.instance so that code working with "fixturedef" behaves
         # as expected.
         if request.instance is not None:
+            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)

testing/python/fixtures.py

@@ -3945,6 +3945,35 @@ class TestScopeOrdering:
         reprec = testdir.inline_run()
         reprec.assertoutcome(passed=2)
 
+    def test_class_fixture_self_instance(self, testdir):
+        testdir.makeconftest(
+            """
+            import pytest
+
+            def pytest_configure(config):
+                config.pluginmanager.register(MyPlugin())
+
+            class MyPlugin():
+                def __init__(self):
+                    self.arg = 1
+
+                @pytest.fixture(scope='function')
+                def myfix(self):
+                    assert isinstance(self, MyPlugin)
+                    return self.arg
+        """
+        )
+
+        testdir.makepyfile(
+            """
+            class TestClass(object):
+                def test_1(self, myfix):
+                    assert myfix == 1
+        """
+        )
+        reprec = testdir.inline_run()
+        reprec.assertoutcome(passed=1)
+
 
 def test_call_fixture_function_error():
     """Check if an error is raised if a fixture function is called directly (#4545)"""