FastAPI StreamingResponse start download while waiting for data

[valeriodeblasio-1]

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Example Code

async def async_read_file():
    # this code simulate the activate waiting for the file to be created
    count = 0
    while count < 5:
        await asyncio.sleep(2)
        count += 1

    file = open('file.zip', 'rb')
    while True:
        data = file.read(64 * 1024)
        if not data:
            break
        yield data


@app.get('/download')
async def download():
    async def iterate_file_():
        async for chunk in async_read_file():
            yield chunk

    response = StreamingResponse(iterate_file_(), headers={
        "Content-Disposition": "attachment; filename*=utf-8''{}".format(quote('file.zip'))})

    return response

Description

I want to send a file to a user, however the file is generated asynchronously via a thread which can take a long time (even 30 seconds).

I would like the browser to show the file being downloaded right away (with 0 bytes and unknown size), tried via cUrl and it works but with a browser (Chrome or Firefox) this doesn't happen, the download only starts after the first bytes have been sent.

The only way I've been able to find is to send empty characters (\u200B) while waiting for the file to be generated (as suggested in this similar problem Chrome doesn't show file as being downloaded until 8 bytes are sent (Firefox does)), but this way the resulting file contains these characters at the beginning, which then gives problems when loading on other systems.

Operating System

Linux, macOS

Operating System Details

No response

FastAPI Version

0.109.2

Pydantic Version

2.1.1

Python Version

3.11.2

Additional Context

No response

[patrick91]

@valeriodeblasio-1 you can do this with the application/force-download content type 😊

@app.get("/download")
async def download():
    async def iterate_file_():
        async for chunk in async_read_file():
            yield chunk

    response = StreamingResponse(
        iterate_file_(),
        headers={
            "Content-Type": "application/force-download", # 👈
            "Content-Disposition": "attachment; filename*=utf-8''{}".format("file.zip"),
        },
    )

    return response

[valeriodeblasio-1]

Thanks man, it seems that “force-download” only works for Chrome, but not for Firefox (I've also read that it's not a standard MIME, so I don't know if it would break anything).