Sentry Answers>FastAPI>

FastAPI file upload returns error 422

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:

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

Here is the FastAPI endpoint code:

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

This is my client code:

Click to Copy
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:

Click to Copy
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.

  • Syntax.fmListen to the Syntax Podcast
  • Community SeriesIdentify, Trace, and Fix Endpoint Regression Issues
  • ResourcesBackend Error Monitoring 101
  • Syntax.fm logo
    Listen to the Syntax Podcast

    Tasty treats for web developers brought to you by Sentry. Get tips and tricks from Wes Bos and Scott Tolinski.

    SEE EPISODES

Considered “not bad” by 4 million developers and more than 100,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

learn more about sentryjoin our discord
© 2024 • Sentry is a registered Trademark of Functional Software, Inc.