Using File to limit UploadFile's size should work but doesn't.

[Diegovsky]

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• I already checked if it is not related to FastAPI but to Swagger UI.
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Example Code

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

app = FastAPI()

@app.post("/")
async def echo(file: Annotated[UploadFile, File(max_length=1000)]):
    print(await file.read())

Description

According to this section in the official docs, File is used to tag additional information about UploadFile, however, it seems that setting max_length makes it not work, resulting in the following (reduced) error:

(snip)
File "/tmp/tmp.mWJ9k2fo2B/.direnv/python-3.12/lib/python3.12/site-packages/pydantic/_internal/_validators.py", line 278, in max_length_validator
    if len(x) > max_length:

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.111.0

Pydantic Version

2.7.4

Python Version

3.12.4

Additional Context

No response

[ceb10n]

When you import File, you are using a function that will return a File class, that inherits from Form. And you get generic validations that only applies to certain types.

For example, you can pass allow_inf_nan=True or decimal_places=2, but it is only applicable for numbers.

In your case, you are trying to use max_length, that according to the docs/source code, is only applicable to strings:

def File(  # noqa: N802
    default: Annotated[
        Any,
        Doc(
            """
            Default value if the parameter field is not set.
            """
        ),
    ] = Undefined,
    *,
   # other params
    max_length: Annotated[
        Optional[int],
        Doc(
            """
            Maximum length for strings.
            """
        ),

[Diegovsky]

Thanks, that does explain a lot :)

[chyok]

Hi @Diegovsky ,
you can try this:

@app.post("/")
async def echo(file: Annotated[UploadFile, File()],
               content_length: Annotated[int | None, Header(lt=1000*1024)] = None):

We can use the Content-Length request header to pre-validate the file size (the unit is Bytes). For example, the code you provided implemented a check to ensure the file size is less than 1000KB. Additionally, you may notice that in the Swagger UI, the browser will automatically calculate and populate this header for you. However, please note that for other clients, you need to ensure they properly send the Content-Length header.

By the way, you can directly use file.size to determine the file size (a property of the UploadFile). However, by the time the server receives the request, the file data has already been transmitted. If you want to save network bandwidth, the above approach that validate the Content-Length header before accepting the file, is more suitable for you.

[Diegovsky]

While this does solve my problem, it would be great if the docs mentioned this.

[chyok]

I think this is a general business solution, rather than something that should be covered in the FastAPI documentation.

[Diegovsky]

You're right about this, but the docs mention using File as the metadata for UploadFile, and this doesn't work like one would assume. I think there should be a section that warns against using File's validators