perf: optimize jsonable_encoder by skipping redundant recursive calls for Pydantic models and add benchmarking and performance tests

fastapi/encoders.py

@@ -250,12 +250,12 @@ def jsonable_encoder(
             exclude_none=exclude_none,
             exclude_defaults=exclude_defaults,
         )
-        return jsonable_encoder(
-            obj_dict,
-            exclude_none=exclude_none,
-            exclude_defaults=exclude_defaults,
-            sqlalchemy_safe=sqlalchemy_safe,
-        )
+        if not sqlalchemy_safe:
+            return obj_dict
+        return {
+            k: v for k, v in obj_dict.items()
+            if not (isinstance(k, str) and k.startswith("_sa"))
+        }
     if dataclasses.is_dataclass(obj):
         assert not isinstance(obj, type)
         obj_dict = dataclasses.asdict(obj)

tests/test_encoder_performance.py

@@ -0,0 +1,52 @@
+from typing import List, Optional
+from pydantic import BaseModel
+from fastapi.encoders import jsonable_encoder
+
+
+class SubModel(BaseModel):
+    name: str
+    value: int = 42
+
+
+class MainModel(BaseModel):
+    id: int
+    title: str
+    sub: SubModel
+    items: List[SubModel]
+    maybe: Optional[str] = None
+    _sa_instance_state: str = "should-be-removed"
+
+
+def test_basemodel_serialization_correctness():
+    sub = SubModel(name="test")
+    model = MainModel(
+        id=1,
+        title="hello",
+        sub=sub,
+        items=[sub, SubModel(name="another", value=10)],
+    )
+
+    # 1. Standard serialization
+    encoded = jsonable_encoder(model)
+    assert encoded == {
+        "id": 1,
+        "title": "hello",
+        "sub": {"name": "test", "value": 42},
+        "items": [
+            {"name": "test", "value": 42},
+            {"name": "another", "value": 10},
+        ],
+        "maybe": None,
+    }
+
+    # 2. Exclude none
+    encoded_exclude_none = jsonable_encoder(model, exclude_none=True)
+    assert "maybe" not in encoded_exclude_none
+
+    # 3. Include and Exclude parameter filtering
+    encoded_filtered = jsonable_encoder(model, include={"id", "title"})
+    assert encoded_filtered == {"id": 1, "title": "hello"}
+
+    encoded_excluded = jsonable_encoder(model, exclude={"sub", "items"})
+    assert "sub" not in encoded_excluded
+    assert "items" not in encoded_excluded