Sync fastapi docs from b5ca1324 on 2025-12-07

tests/test_datetime_custom_encoder.py

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+from datetime import datetime, timezone
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+from .utils import needs_pydanticv1, needs_pydanticv2
+
+
+@needs_pydanticv2
+def test_pydanticv2():
+    from pydantic import field_serializer
+
+    class ModelWithDatetimeField(BaseModel):
+        dt_field: datetime
+
+        @field_serializer("dt_field")
+        def serialize_datetime(self, dt_field: datetime):
+            return dt_field.replace(microsecond=0, tzinfo=timezone.utc).isoformat()
+
+    app = FastAPI()
+    model = ModelWithDatetimeField(dt_field=datetime(2019, 1, 1, 8))
+
+    @app.get("/model", response_model=ModelWithDatetimeField)
+    def get_model():
+        return model
+
+    client = TestClient(app)
+    with client:
+        response = client.get("/model")
+    assert response.json() == {"dt_field": "2019-01-01T08:00:00+00:00"}
+
+
+# TODO: remove when deprecating Pydantic v1
+@needs_pydanticv1
+def test_pydanticv1():
+    class ModelWithDatetimeField(BaseModel):
+        dt_field: datetime
+
+        class Config:
+            json_encoders = {
+                datetime: lambda dt: dt.replace(
+                    microsecond=0, tzinfo=timezone.utc
+                ).isoformat()
+            }
+
+    app = FastAPI()
+    model = ModelWithDatetimeField(dt_field=datetime(2019, 1, 1, 8))
+
+    @app.get("/model", response_model=ModelWithDatetimeField)
+    def get_model():
+        return model
+
+    client = TestClient(app)
+    with client:
+        response = client.get("/model")
+    assert response.json() == {"dt_field": "2019-01-01T08:00:00+00:00"}