Handle JSON encoding Pydantic UndefinedType

fastapi/encoders.py

@@ -23,7 +23,7 @@ from pydantic.color import Color
 from pydantic.networks import NameEmail
 from pydantic.types import SecretBytes, SecretStr
 
-from ._compat import PYDANTIC_V2, MultiHostUrl, Url, _model_dump
+from ._compat import PYDANTIC_V2, MultiHostUrl, UndefinedType, Url, _model_dump
 
 
 # Taken from Pydantic v1 as is
@@ -167,6 +167,8 @@ def jsonable_encoder(
         return str(obj)
     if isinstance(obj, (str, int, float, type(None))):
         return obj
+    if isinstance(obj, UndefinedType):
+        return None
     if isinstance(obj, dict):
         encoded_dict = {}
         allowed_keys = set(obj.keys())

tests/test_jsonable_encoder.py

@@ -7,7 +7,7 @@ from pathlib import PurePath, PurePosixPath, PureWindowsPath
 from typing import Optional
 
 import pytest
-from fastapi._compat import PYDANTIC_V2
+from fastapi._compat import PYDANTIC_V2, Undefined
 from fastapi.encoders import jsonable_encoder
 from pydantic import BaseModel, Field, ValidationError
 
@@ -309,3 +309,9 @@ def test_encode_deque_encodes_child_models():
     dq = deque([Model(test="test")])
 
     assert jsonable_encoder(dq)[0]["test"] == "test"
+
+
+@needs_pydanticv2
+def test_encode_pydantic_undefined():
+    data = {"value": Undefined}
+    assert jsonable_encoder(data) == {"value": None}