Flask Webhook Endpoint Returns 500 Error When Receiving POST Requests

[elizabeths]

I’m having trouble with my Flask application when it tries to handle incoming POST requests from an automation service. Every time I test the endpoint, I get a 500 Internal Server Error.

@app.route("/api/webhook", methods=['POST'])
def handle_incoming_data(req):
    print(req.json)
    return req.json

The error message shows:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/dist-packages/flask/app.py", line 2073, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.9/dist-packages/flask/app.py", line 1518, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.9/dist-packages/flask/app.py", line 1516, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.9/dist-packages/flask/app.py", line 1502, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
TypeError: handle_incoming_data() missing 1 required positional argument: 'req'

I’m confused because I thought the external service would automatically pass the request data as a parameter to my function. Am I missing something about how Flask handles incoming POST data? My setup is running on Ubuntu with Nginx and Gunicorn.

[CreativePainter33]

Flask route handlers don’t take parameters like that. When you define def handle_incoming_data(req):, Flask tries to call it without arguments but your function expects one. Remove the req parameter and import request from Flask instead:

from flask import request

@app.route("/api/webhook", methods=['POST'])
def handle_incoming_data():
    print(request.json)
    return request.json

I made this same mistake when I started with Flask webhooks. The request object is a global context variable Flask provides, not a parameter. This’ll fix your 500 error.

[amelial]

ah i see the issue - your function signature is wrong. flask doesn’t pass request as parameter automatically, you need to import it. change to def handle_incoming_data(): and add from flask import request at top, then use request.json inside the function.

[Samuel87]

This error got me too when I first started using Flask webhooks. The traceback tells you exactly what’s wrong - Flask tries to call your function without arguments, but you defined it to expect a req parameter. That’s not how Flask works. Your function should be def handle_incoming_data(): with no parameters. Then you grab the request data through Flask’s global request object that you import separately. Hit this same TypeError setting up payment webhooks last year. Remove that parameter and use the proper import - your webhook will work fine with Nginx/Gunicorn.