How to fix Python bottle Unsupported response type: <class 'dict'>

Problem:

You are running a Python HTTP server using bottle. When you access your HTTP server endpoint, you see a HTTP 500 error message like

Unsupported response type: <class 'dict'>

Solution

This occurs because you are trying to return a Python list of dictionaries, for example in

from bottle import route, run, template, response

@route('/')
def index():
    # We expect bottle to return a JSON here
    # but that doesn't happen!
    return [{"a": "b"}]

run(host='localhost', port=8080)

In order to work around this behaviour, you need to set response.content_type and explicitly use json.dumps() to convert your JSON into a string:

from bottle import route, run, template, response
import json

@route('/')
def index():
    response.content_type = 'application/json'
    return json.dumps([{"a": "b"}])

run(host='localhost', port=8080)