[1.7.x] Fixed #22679 -- Fixed empty tuple serialization in MigrationWriter.

Thanks rockallite.wulf for the report.

Backport of ad994a3c5b from master.

django/db/migrations/writer.py

@@ -206,7 +206,9 @@ class MigrationWriter(object):
             if isinstance(value, set):
                 format = "set([%s])"
             elif isinstance(value, tuple):
-                format = "(%s)" if len(value) > 1 else "(%s,)"
+                # When len(value)==0, the empty tuple should be serialized as
+                # "()", not "(,)" because (,) is invalid Python syntax.
+                format = "(%s)" if len(value) != 1 else "(%s,)"
             else:
                 format = "[%s]"
             return format % (", ".join(strings)), imports
@@ -296,7 +298,9 @@ class MigrationWriter(object):
                 item_string, item_imports = cls.serialize(item)
                 imports.update(item_imports)
                 strings.append(item_string)
-            format = "(%s)" if len(strings) > 1 else "(%s,)"
+            # When len(strings)==0, the empty iterable should be serialized as
+            # "()", not "(,)" because (,) is invalid Python syntax.
+            format = "(%s)" if len(strings) != 1 else "(%s,)"
             return format % (", ".join(strings)), imports
         # Uh oh.
         else:

tests/migrations/test_writer.py

@@ -125,6 +125,18 @@ class WriterTests(TestCase):
             )
         )
 
+    def test_serialize_empty_nonempty_tuple(self):
+        """
+        Ticket #22679: makemigrations generates invalid code for (an empty
+        tuple) default_permissions = ()
+        """
+        empty_tuple = ()
+        one_item_tuple = ('a')
+        many_items_tuple = ('a', 'b', 'c')
+        self.assertSerializedEqual(empty_tuple)
+        self.assertSerializedEqual(one_item_tuple)
+        self.assertSerializedEqual(many_items_tuple)
+
     def test_simple_migration(self):
         """
         Tests serializing a simple migration.