Fixed #24098 -- Added no-op attributes to RunPython and RunSQL

Thanks Loïc Bistuer and Tim Graham for the discussion and review.

django/db/migrations/operations/special.py

@@ -61,6 +61,7 @@ class RunSQL(Operation):
     Also accepts a list of operations that represent the state change effected
     by this SQL change, in case it's custom column/table creation/deletion.
     """
+    noop = ''
 
     def __init__(self, sql, reverse_sql=None, state_operations=None):
         self.sql = sql
@@ -100,9 +101,9 @@ def database_backwards(self, app_label, schema_editor, from_state, to_state):
     def describe(self):
         return "Raw SQL operation"
 
-    def _run_sql(self, schema_editor, sql):
-        if isinstance(sql, (list, tuple)):
-            for sql in sql:
+    def _run_sql(self, schema_editor, sqls):
+        if isinstance(sqls, (list, tuple)):
+            for sql in sqls:
                 params = None
                 if isinstance(sql, (list, tuple)):
                     elements = len(sql)
@@ -111,8 +112,8 @@ def _run_sql(self, schema_editor, sql):
                     else:
                         raise ValueError("Expected a 2-tuple but got %d" % elements)
                 schema_editor.execute(sql, params=params)
-        else:
-            statements = schema_editor.connection.ops.prepare_sql_script(sql)
+        elif sqls != RunSQL.noop:
+            statements = schema_editor.connection.ops.prepare_sql_script(sqls)
             for statement in statements:
                 schema_editor.execute(statement, params=None)
 
@@ -175,3 +176,7 @@ def database_backwards(self, app_label, schema_editor, from_state, to_state):
 
     def describe(self):
         return "Raw Python operation"
+
+    @staticmethod
+    def noop(apps, schema_editor):
+        return None

docs/ref/migration-operations.txt

@@ -245,6 +245,14 @@ operation that adds that field and so will try to run it again).
     The ability to pass parameters to the ``sql`` and ``reverse_sql`` queries
     was added.
 
+.. attribute:: RunSQL.noop
+
+    .. versionadded:: 1.8
+
+    Pass the ``RunSQL.noop`` attribute to ``sql`` or ``reverse_sql`` when you
+    want the operation not to do anything in the given direction. This is
+    especially useful in making the operation reversible.
+
 .. _sqlparse: https://pypi.python.org/pypi/sqlparse
 
 RunPython
@@ -321,6 +329,14 @@ set ``atomic=False``.
     ``schema_editor.connection.alias``, where ``schema_editor`` is the second
     argument to your function).
 
+.. staticmethod:: RunPython.noop
+
+    .. versionadded:: 1.8
+
+    Pass the ``RunPython.noop`` method to ``code`` or ``reverse_code`` when
+    you want the operation not to do anything in the given direction. This is
+    especially useful in making the operation reversible.
+
 SeparateDatabaseAndState
 ------------------------
 

docs/releases/1.8.txt

@@ -457,6 +457,11 @@ Migrations
 * A :ref:`generic mechanism to handle the deprecation of model fields
   <migrations-removing-model-fields>` was added.
 
+* The :attr:`RunPython.noop <django.db.migrations.operations.RunPython.noop>`
+  and :meth:`RunSQL.noop() <django.db.migrations.operations.RunSQL.noop>` class
+  attribute/method were added to ease in making ``RunPython`` and ``RunSQL``
+  operations reversible.
+
 Models
 ^^^^^^
 

tests/migrations/test_migrations_squashed_complex/1_auto.py

@@ -7,5 +7,5 @@
 class Migration(migrations.Migration):
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/2_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "1_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/3_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "2_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/3_squashed_5.py

@@ -15,5 +15,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "2_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/4_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "3_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/5_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "4_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/6_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "5_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex/7_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "6_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app1/1_auto.py

@@ -7,5 +7,5 @@
 class Migration(migrations.Migration):
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app1/2_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("app1", "1_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app1/2_squashed_3.py

@@ -14,5 +14,5 @@ class Migration(migrations.Migration):
     dependencies = [("app1", "1_auto"), ("app2", "2_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app1/3_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("app1", "2_auto"), ("app2", "2_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app1/4_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("app1", "3_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app2/1_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("app1", "1_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app2/1_squashed_2.py

@@ -14,5 +14,5 @@ class Migration(migrations.Migration):
     dependencies = [("app1", "1_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_complex_multi_apps/app2/2_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("app2", "1_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_erroneous/1_auto.py

@@ -7,5 +7,5 @@
 class Migration(migrations.Migration):
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_erroneous/2_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "1_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_erroneous/3_squashed_5.py

@@ -15,5 +15,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "2_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_erroneous/6_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "5_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_migrations_squashed_erroneous/7_auto.py

@@ -9,5 +9,5 @@ class Migration(migrations.Migration):
     dependencies = [("migrations", "6_auto")]
 
     operations = [
-        migrations.RunPython(lambda apps, schema_editor: None)
+        migrations.RunPython(migrations.RunPython.noop)
     ]

tests/migrations/test_operations.py

@@ -1494,6 +1494,15 @@ def test_run_sql_params_invalid(self):
                 operation.database_backwards,
                 "test_runsql", editor, new_state, project_state)
 
+    def test_run_sql_noop(self):
+        """
+        #24098 - Tests no-op RunSQL operations.
+        """
+        operation = migrations.RunSQL(migrations.RunSQL.noop, migrations.RunSQL.noop)
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_runsql", editor, None, None)
+            operation.database_backwards("test_runsql", editor, None, None)
+
     def test_run_python(self):
         """
         Tests the RunPython operation
@@ -1620,6 +1629,17 @@ def inner_method(models, schema_editor):
         self.assertEqual(definition[1], [])
         self.assertEqual(sorted(definition[2]), ["atomic", "code"])
 
+    def test_run_python_noop(self):
+        """
+        #24098 - Tests no-op RunPython operations.
+        """
+        project_state = ProjectState()
+        new_state = project_state.clone()
+        operation = migrations.RunPython(migrations.RunPython.noop, migrations.RunPython.noop)
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_runpython", editor, project_state, new_state)
+            operation.database_backwards("test_runpython", editor, new_state, project_state)
+
     @unittest.skipIf(sqlparse is None and connection.features.requires_sqlparse_for_splitting, "Missing sqlparse")
     def test_separate_database_and_state(self):
         """