Allow callable defaults and verify serializability

marshmallow_jsonschema/base.py

@@ -1,5 +1,6 @@
 import datetime
 import decimal
+import json
 import uuid
 from enum import Enum
 from inspect import isclass
@@ -195,8 +196,13 @@ def _from_python_type(self, obj, field, pytype) -> typing.Dict[str, typing.Any]:
         if field.dump_only:
             json_schema["readOnly"] = True
 
-        if field.default is not missing and not callable(field.default):
-            json_schema["default"] = field.default
+        if field.default is not missing:
+            if (not callable(field.default) and self._is_serializable(field.default)):
+                json_schema["default"] = field.default
+            else:
+                default_value = field.default()
+                if self._is_serializable(default_value):
+                    json_schema["default"] = default_value
 
         if ALLOW_ENUMS and isinstance(field, EnumField):
             json_schema["enum"] = self._get_enum_values(field)
@@ -225,6 +231,13 @@ def _from_python_type(self, obj, field, pytype) -> typing.Dict[str, typing.Any]:
             )
         return json_schema
 
+    def _is_serializable(self, value):
+        try:
+            json.dumps(value)
+            return True
+        except TypeError:
+            return False
+
     def _get_enum_values(self, field) -> typing.List[str]:
         assert ALLOW_ENUMS and isinstance(field, EnumField)
 

tests/test_dump.py

@@ -42,6 +42,17 @@ class TestSchema(Schema):
     props = dumped["definitions"]["TestSchema"]["properties"]
     assert "default" not in props["uid"]
 
+def test_default_callable_serialized():
+    class TestSchema(Schema):
+        uid = fields.UUID(default=lambda: str(uuid.uuid4()))
+
+    schema = TestSchema()
+
+    dumped = validate_and_dump(schema)
+
+    props = dumped["definitions"]["TestSchema"]["properties"]
+    assert "default" in props["uid"]
+
 
 def test_uuid():
     schema = UserSchema()