Request Files

You can define files to be uploaded by the client using File.

Import File

Import File from fastapi:

from fastapi import FastAPI, File

app = FastAPI()


@app.post("/files/")
async def create_file(*, file: bytes = File(...)):
    return {"file_size": len(file)}

Define File parameters

Create file parameters the same way you would for Body or Form:

from fastapi import FastAPI, File

app = FastAPI()


@app.post("/files/")
async def create_file(*, file: bytes = File(...)):
    return {"file_size": len(file)}

The files will be uploaded as form data and you will receive the contents as bytes.

Info

File is a class that inherits directly from Form.

Info

To declare File bodies, you need to use File, because otherwise the parameters would be interpreted as query parameters or body (JSON) parameters.

"Form Data"?

The way HTML forms (<form></form>) sends the data to the server normally uses a "special" encoding for that data, it's different from JSON.

FastAPI will make sure to read that data from the right place instead of JSON.

Technical Details

Data from forms is normally encoded using the "media type" application/x-www-form-urlencoded when it doesn't include files.

But when the form includes files, it is encoded as multipart/form-data. If you use File, FastAPI will know it has to get the files from the correct part of the body.

If you want to read more about these encondings and form fields, head to the MDN web docs for POST.

Warning

You can declare multiple File and Form parameters in a path operation, but you can't also declare Body fields that you expect to receive as JSON, as the request will have the body encoded using multipart/form-data instead of application/json.

This is not a limitation of FastAPI, it's part of the HTTP protocol.

Recap

Use File to declare files to be uploaded as input parameters (as form data).