diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index e380af3ccc..ae096b9fce 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -27,10 +27,7 @@ from typing_extensions import Annotated, Doc  # type: ignore [attr-defined]
 
 from ._compat import PYDANTIC_V2, Url, _model_dump
 
-
-# Taken from Pydantic v1 as is
-def isoformat(o: Union[datetime.date, datetime.time]) -> str:
-    return o.isoformat()
+isoformat: Callable[[Union[datetime.date, datetime.time]], str] = methodcaller("isoformat")
 
 
 # Taken from Pydantic v1 as is