Fixed #24521 -- Added support for serializing frozensets in migrations.

django/db/migrations/writer.py

@@ -302,7 +302,7 @@ class MigrationWriter(object):
             value = force_text(value)
 
         # Sequences
-        if isinstance(value, (list, set, tuple)):
+        if isinstance(value, (frozenset, list, set, tuple)):
             imports = set()
             strings = []
             for item in value:
@@ -312,6 +312,8 @@ class MigrationWriter(object):
             if isinstance(value, set):
                 # Don't use the literal "{%s}" as it doesn't support empty set
                 format = "set([%s])"
+            elif isinstance(value, frozenset):
+                format = "frozenset([%s])"
             elif isinstance(value, tuple):
                 # When len(value)==0, the empty tuple should be serialized as
                 # "()", not "(,)" because (,) is invalid Python syntax.

tests/migrations/test_writer.py

@@ -347,6 +347,10 @@ class WriterTests(TestCase):
         self.assertSerializedEqual(FoodManager('a', 'b'))
         self.assertSerializedEqual(FoodManager('x', 'y', c=3, d=4))
 
+    def test_serialize_frozensets(self):
+        self.assertSerializedEqual(frozenset())
+        self.assertSerializedEqual(frozenset("let it go"))
+
     def test_simple_migration(self):
         """
         Tests serializing a simple migration.