How to prevent Flask (python) from emitting html?

April 11, 2022

It appears that Flask assumes that the server is returning html to the client (browser).

Here’s a simple example;

import json
from flask import Flask

app = Flask(__name__)

@app.route("/")
def home():
  msg = ['Hello, world!']
  return json.dumps(msg) + '\n'

This code works as expected and returns the desired json;

$ curl -s http://localhost:5000/
["Hello, world!"]

But if I introduce an error;

import json
from flask import Flask

app = Flask(__name__)

@app.route("/")
def home():
  msg = ['Hello, world!']
  return json.dumps(XXmsg) + '\n'

Then Flask emits the error wrapped in several pages worth of html, starting like;

$ curl -s http://localhost:5000/
<!DOCTYPE html>
<html>
  <head>
    <title>NameError: name 'XXmsg' is not defined
 // Werkzeug Debugger</title>
    <link rel="stylesheet" href="?__debugger__=yes&amp;cmd=resource&amp;f=style.css">
    <link rel="shortcut icon"
        href="?__debugger__=yes&amp;cmd=resource&amp;f=console.png">
    <script src="?__debugger__=yes&amp;cmd=resource&amp;f=debugger.js"></script>
    <script>
      var CONSOLE_MODE = false,
          EVALEX = true,
          EVALEX_TRUSTED = false,
          SECRET = "Mq5TSy6QE4OuOHUfvk8b";
    </script>
  </head>
  <body style="background-color: #fff">
    <div class="debugger">

Emitting html makes sense if you’re creating a page load app. But I’m creating an api that only returns json.

Is there anyway to prevent Flask from emitting html at all?

Thanks
Mike

>Solution :

Have a look at the section Returning API Errors as JSON of the Flask docs.

Basically, you have to replace the default error handler with a function that returns the error as json. A very basic example:

@app.errorhandler(HTTPException)
def handle_exception(exception):
    response = exception.get_response()
    response.content_type = "application/json"
    response.data = json.dumps({"code": exception.code})
    return response