8000 🐛 Fix optional sequence handling in `serialize sequence value` with Pydantic V2 by YuriiMotov · Pull Request #14297 · fastapi/fastapi · GitHub 8000
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🐛 Fix optional sequence handling in serialize sequence value with Pydantic V2 #14297

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tiangolo merged 4 commits into from
Dec 2, 2025
Merged

🐛 Fix optional sequence handling in serialize sequence value with Pydantic V2 #14297

tiangolo merged 4 commits into from
Dec 2, 2025
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@YuriiMotov YuriiMotov commented Nov 5, 2025

Currently the following test code leads to TypeError: issubclass() arg 1 must be a class (with Python 3.14 error is different, but the root cause is the same):

from typing import List, Optional

from fastapi import FastAPI, File
from fastapi.testclient import TestClient

app = FastAPI()


@app.post("/files")
async def upload_files(files: Optional[List[bytes]] = File(None)):
    if files is None:
        return {"files_count": 0}
    return {"files_count": len(files), "sizes": [len(f) for f in files]}


def test_optional_bytes_list():
    client = TestClient(app)
    response = client.post(
        "/files",
        files=[("files", b"content1"), ("files", b"content2")],
    )
    assert response.status_code == 200
    assert response.json() == {"files_count": 2, "sizes": [8, 8]}

This is because for optional sequence the origin_type in the following line is Union:

fastapi/fastapi/_compat/v2.py

Line 374 in 972a967

assert issubclass(origin_type, shared.sequence_types) # type: ignore[arg-type]

So, we need to unwrap that Uinion, take non-None type from its arguments and use it as origin_type

@YuriiMotov YuriiMotov added the bug Something isn't working label Nov 5, 2025
@YuriiMotov YuriiMotov mentioned this pull request Nov 5, 2025
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@YuriiMotov YuriiMotov marked this pull request as ready for review November 5, 2025 13:17
@YuriiMotov YuriiMotov mentioned this pull request Nov 18, 2025
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tiangolo
tiangolo approved these changes Dec 2, 2025
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Nice, thank you @YuriiMotov! 🙌

This will be available in FastAPI 0.123.3, released in the next hours. 🚀

@tiangolo tiangolo merged commit 0f613d9 into fastapi:master Dec 2, 2025
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@Viicos Viicos mentioned this pull request Dec 2, 2025
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