Fixed #22095 -- Enabled backward migrations for RunPython operations

Added reversible property to RunPython so that migrations will not
refuse to reverse migrations including RunPython operations, so long as
reverse_code is set in the RunPython constructor. Included tests to
check the reversible property on RunPython and the similar RunSQL.

django/db/migrations/operations/special.py

@@ -103,7 +103,6 @@ class RunPython(Operation):
     """
 
     reduces_to_sql = False
-    reversible = False
 
     def __init__(self, code, reverse_code=None):
         # Forwards code
@@ -118,6 +117,10 @@ class RunPython(Operation):
                 raise ValueError("RunPython must be supplied with callable arguments")
             self.reverse_code = reverse_code
 
+    @property
+    def reversible(self):
+        return self.reverse_code is not None
+
     def state_forwards(self, app_label, state):
         # RunPython objects have no state effect. To add some, combine this
         # with SeparateDatabaseAndState.

tests/migrations/test_operations.py

@@ -468,6 +468,7 @@ class OperationTests(MigrationTestBase):
             operation.database_forwards("test_runsql", editor, project_state, new_state)
         self.assertTableExists("i_love_ponies")
         # And test reversal
+        self.assertTrue(operation.reversible)
         with connection.schema_editor() as editor:
             operation.database_backwards("test_runsql", editor, new_state, project_state)
         self.assertTableNotExists("i_love_ponies")
@@ -484,7 +485,11 @@ class OperationTests(MigrationTestBase):
             Pony = models.get_model("test_runpython", "Pony")
             Pony.objects.create(pink=1, weight=3.55)
             Pony.objects.create(weight=5)
-        operation = migrations.RunPython(inner_method)
+        def inner_method_reverse(models, schema_editor):
+            Pony = models.get_model("test_runpython", "Pony")
+            Pony.objects.filter(pink=1, weight=3.55).delete()
+            Pony.objects.filter(weight=5).delete()
+        operation = migrations.RunPython(inner_method, reverse_code=inner_method_reverse)
         # Test the state alteration does nothing
         new_state = project_state.clone()
         operation.state_forwards("test_runpython", new_state)
@@ -494,13 +499,24 @@ class OperationTests(MigrationTestBase):
         with connection.schema_editor() as editor:
             operation.database_forwards("test_runpython", editor, project_state, new_state)
         self.assertEqual(project_state.render().get_model("test_runpython", "Pony").objects.count(), 2)
-        # And test reversal fails
-        with self.assertRaises(NotImplementedError):
-            operation.database_backwards("test_runpython", None, new_state, project_state)
+        # Now test reversal
+        self.assertTrue(operation.reversible)
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_runpython", editor, project_state, new_state)
+        self.assertEqual(project_state.render().get_model("test_runpython", "Pony").objects.count(), 0)
         # Now test we can't use a string
         with self.assertRaises(ValueError):
             operation = migrations.RunPython("print 'ahahaha'")
 
+        # Also test reversal fails, with an operation identical to above but without reverse_code set
+        no_reverse_operation = migrations.RunPython(inner_method)
+        self.assertFalse(no_reverse_operation.reversible)
+        with connection.schema_editor() as editor:
+            no_reverse_operation.database_forwards("test_runpython", editor, project_state, new_state)
+            with self.assertRaises(NotImplementedError):
+                no_reverse_operation.database_backwards("test_runpython", editor, new_state, project_state)
+
+
 
 class MigrateNothingRouter(object):
     """