Answers by Sentry

FastAPI file upload returns error 422

David Y.

The Problem

I’ve written a FastAPI endpoint that takes an uploaded file and a Python client application that should upload a file to this endpoint. However, when I run the client, my server produces an error with the code 422 and the following details:

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

Here is the FastAPI endpoint code:

@app.post("/upload")
async def upload_file(data: UploadFile = File(...)):
    # ... process file ...
    return { "filename": data.filename }

This is my client code:

import requests

response = requests.post(
            url = "http://localhost:8000/upload",
            files = { 'file': open('example.txt', 'rb') })

print(response.json())

Is the problem on the client or server side and how can I fix it?

The Solution

The client code causes the server to return an error message because the name of the POST parameter containing the file data in the request does not match what the server expects. The /upload endpoint takes a UploadFile parameter named data, but the client uploads the file using a parameter named file. As the endpoint does not receive the data parameter it expects, it returns value_error.missing.

We can fix this by making the following change to the client code:

import requests

response = requests.post(
            url = "http://localhost:8000/upload",
            files = { 'data': open('example.txt', 'rb') }) # changed 'file' to 'data'

print(response.json())

The client should now be able to upload the file successfully to the FastAPI endpoint.

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