Optional Multiple File Uploads Broken When Using Pydantic v2.3.0

[jbourquin]

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Example Code

from fastapi import FastAPI, UploadFile, File
from typing import Annotated, List

app = FastAPI()

@app.post("/")
async def upload_attachments(
    attachment_files: Annotated[List[UploadFile] | None, File(alias='attachments', description='Multiple attachments')] = []
):
  file_list = []
  if attachment_files:
    for f in attachment_files:
      file_list.append(f.filename)
  
  return {"file_list": file_list}

Description

FastAPI (v0.103.1) raises a list type error when multiple file uploads are specified as optional and Pydantic v2.3.0 is installed. With the Pydantic v2.3.0, the example code returns the following response:

Note: I am using HTTPie to generate the request but the error is the same for other clients like Postman

http -f POST localhost:8000 attachments@./document.pdf attachments@./image.jpg

{
    "detail": [
        {
            "input": {
                "file": {
                    "_TemporaryFileArgs": {
                        "buffering": -1,
                        "dir": null,
                        "encoding": null,
                        "errors": null,
                        "mode": "w+b",
                        "newline": null,
                        "prefix": null,
                        "suffix": null
                    },
                    "_file": {},
                    "_max_size": 1048576,
                    "_rolled": false
                },
                "filename": "image.jpg",
                "headers": {
                    "content-disposition": "form-data; name=\"attachments\"; filename=\"image.jpg\"",
                    "content-type": "image/jpeg"
                },
                "size": 57694
            },
            "loc": [
                "body",
                "attachments"
            ],
            "msg": "Input should be a valid list",
            "type": "list_type",
            "url": "https://errors.pydantic.dev/2.3/v/list_type"
        }
    ]
}

However, downgrading to Pydantic v1.10.12 returns the correct response:

http -f POST localhost:8000 attachments@./document.pdf attachments@./image.jpg

{
    "file_list": [
        "document.pdf",
        "image.jpg"
    ]
}

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.103.1

Pydantic Version

2.3.0

Python Version

3.10.9

Additional Context

No response

[pawel-kmiecik]

Looks like it's still bugged - confirmed multiple file uploads does not work with pydantic2

[JavierSanchezCastro]

Hi!

This works for me:

from fastapi import FastAPI, UploadFile

app = FastAPI()

@app.post("/")
async def upload_attachments(attachment_files: list[UploadFile] = []):
  file_list = []
  print(attachment_files)
  if attachment_files:
    for f in attachment_files:
      file_list.append(f.filename)
  
  return {"file_list": file_list}

fastapi==0.109.0 pydantic==2.5.2

[JavierSanchezCastro]

I just see some endpoints of my project are failing with Annotated typing. I'm going to check in which version fails

[jbourquin]

The bug is related to the multiple uploads as being optional, i.e. you can either pass a list of file objects or None. So in my use case I have form that has the option of attaching multiple files, but they are not required.

[progre55]

Still seems to be broken on pydantic==2.7

[pedroconceicao]

I think my problem is related to this discussion so I'm sharing it here:

I have an API that has an endpoint that accepts a variable amount of files. The endpoint is defined in the following way:

@router_my_router.post('/stream', response_model=PredictionResponseModel)
async def my_endpoint(image_files: Optional[List[UploadFile]] = File([], alias='imageFiles')):
    return my_endpoint.predict(image_files)

Using fastapi <= 0.104.0 the swagger looks like

however, using fastapi > 0.104.0 the file input field is broken

not allowing me to upload files and make a request.

In both cases the versions of pydantic and pydantic_core are 2.7.1 and 2.18.2, respectively. I'm not sure however if this is a fastapi issue
or an issue of one of its dependencies.

[msukmanowsky]

Despite the fact that these types look completely wrong, this does seem to work (using pydantic==2.6.4, fastapi==0.111.0, starlette==0.37.2):

@app.post(
    "/upload_files",
)
async def upload_files(
    files: list[UploadFile | str] = File(None),
):
    for file in files or []:
        print("file: %r %r", file.filename, file.content_type)
        contents = await file.read()
        print("contents: %r", contents[:100])
    return {"result": "ok"}

Tested for:

1. No files at all (files is None)
2. One file (files is a 1-length list)
3. Multiple files (files is an n-length list)

Would still appreciate someone having a look at this as it'd be nice to do:

files: Optional[list[UploadFile]] = None

[ArtemBalandin81]

Hi!
Unfortunately it doesn't work for me with suggested dependencies.
Any combinations besides list[UploadFile] crash the ability to upload files in swagger.
And list[UploadFile] = File(None) raise "Value error, Expected UploadFile, received: <class 'str'>"
if there is no file to upload.

[msukmanowsky]

I recall reading somewhere that Swagger has a known issue with file uploads, you may want to try CURL/Postman/Python directly.