File upload using FastAPI returns error 422

Question

I am using the example from the official documentation: https://fastapi.tiangolo.com/tutorial/request-files/#import-file

Server code:

@app.post("/uploadfile")
async def create_upload_file(data: UploadFile = File(...)):
    print("> uploaded file:",data.filename)
    return {"filename": data.filename}

Client code:

files = {'upload_file': open('config.txt', 'rb')}
resp = requests.post(
        url = URL,
        files = files)
print(resp.json())

The problem is that the server always responds with error 422:

{'detail': [{'loc': ['body', 'data'], 'msg': 'field required', 'type': 'value_error.missing'}]}

I am using Python 3 on both server and client and the python-multipart package is already installed.

Can someone please tell me what I am doing wrong, what am I missing, how should I fix the code?

Any hints are much appreciated.

Answer 1

On client side you should use the same name given for the parameter on server side, when adding the file. In your case, that parameter is called data, as shown below:

@app.post('/uploadfile')
async def create_upload_file(data: UploadFile = File(...)):
                             ^^^^

Hence, you should replace upload_file with data on client side, as shown below:

url = 'http://127.0.0.1:8000/uploadfile'
files = {'data': open('config.txt', 'rb')}
r = requests.post(url=url, files=files)

More details and examples on how to upload file(s) using Python requests and FastAPI can be found in this answer, as well as here, here and here.

Answer 2

Well, I just realized my error (which is not immediately obvious for newbies like me :))

The parameter you pass on client side ('upload_file')

files = {'upload_file': open('config.txt', 'rb')}

must match the parameter on the server side ('data'):

async def create_upload_file(data: UploadFile = File(...)):

So in order to work I had to rename on client side 'upload_file' to 'data':

files = {'data': open('config.txt', 'rb')} # renamed 'upload_file' to 'data'

That's it. Hopefully this helps some others as well.