14811: Fix the order for the submitted form data

fastapi/dependencies/utils.py

@@ -893,17 +893,21 @@ async def _extract_form_body(
         ):
             # For types
             assert isinstance(value, sequence_types)
-            results: list[Union[bytes, str]] = []
+            results: list[Union[bytes, str, None]] = [None] * len(value)
 
             async def process_fn(
+                idx: int,
                 fn: Callable[[], Coroutine[Any, Any, Any]],
             ) -> None:
                 result = await fn()
-                results.append(result)  # noqa: B023
+                # Using index to preserve order
+                results[idx] = result  # noqa: B023
 
             async with anyio.create_task_group() as tg:
-                for sub_value in value:
+                for idx, sub_value in enumerate(value):
-                    tg.start_soon(process_fn, sub_value.read)
+                    tg.start_soon(process_fn, idx, sub_value.read)
+
+            assert all(item is not None for item in results)
             value = serialize_sequence_value(field=field, value=results)
         if value is not None:
             values[get_validation_alias(field)] = value

tests/test_list_bytes_file_order_preserved_issue_14811.py

@@ -0,0 +1,46 @@
+"""
+Regression test: preserve order when using list[bytes] + File()
+See https://github.com/fastapi/fastapi/discussions/14811
+Related: PR #3372
+"""
+
+from typing import Annotated
+
+import anyio
+import pytest
+from fastapi import FastAPI, File
+from fastapi.testclient import TestClient
+from starlette.datastructures import UploadFile as StarletteUploadFile
+
+
+def test_list_bytes_file_preserves_order_issue_14811(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    app = FastAPI()
+
+    @app.post("/upload")
+    async def upload(files: Annotated[list[bytes], File()]):
+        # return something that makes order obvious
+        return [b[0] for b in files]
+
+    original_read = StarletteUploadFile.read
+
+    async def patched_read(self: StarletteUploadFile, size: int = -1) -> bytes:
+        # Make the FIRST file slower *deterministically*
+        if self.filename == "slow.txt":
+            await anyio.sleep(0.05)
+        return await original_read(self, size)
+
+    monkeypatch.setattr(StarletteUploadFile, "read", patched_read)
+
+    client = TestClient(app)
+
+    files = [
+        ("files", ("slow.txt", b"A" * 10, "text/plain")),
+        ("files", ("fast.txt", b"B" * 10, "text/plain")),
+    ]
+    r = client.post("/upload", files=files)
+    assert r.status_code == 200, r.text
+
+    # Must preserve request order: slow first, fast second
+    assert r.json() == [ord("A"), ord("B")]