Flask Value error view function did not return a response

Question

Error message:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1478, in full_dispatch_request
    response = self.make_response(rv)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1566, in make_response
    raise ValueError('View function did not return a response')
ValueError: View function did not return a response

And my code:

#Used for starting the flask server
from flask import Flask #Import flask mains
from flask import request #Import flask requests
from flask import render_template #Import flask render templates
import json, urllib #import api modules
import time #Imporing time in darlek voice
app = Flask(__name__)

@app.route('/')
def my_form():
    return render_template("my-form.html") #Set render template

@app.route('/', methods=['POST'])
def my_form_post():
    openwindow_at = float(request.form['open']) #When it reached ? open window
    if request.form['scale'] == "kelvin": #Change figures
        print("Do nothing") #Debug info
    elif request.form['scale'] == "celcius":
        openwindow_at = openwindow_at + int(273.15) #celcius to kelvin
    elif request.form['scale'] == "fah": #Fah to kelvin
        openwindow_at = (openwindow_at + 459.67) * 5 / 9  #F to kelvin
    text = request.form['text'] #Get info from First page
    url = "http://api.openweathermap.org/data/2.5/weather?q=" + text #Download the json
    response = urllib.urlopen(url) #Download Json
    data = json.loads(response.read()) #Parse json
    print("Current Weather in " + text + " " + data['weather'][0]['description']) #Debug infomation
    print("Temp: " + str(data['main']['temp'])) #Print temp 

    if data['weather'][0]['description'].find("rain") >= 0: #Check The weather
        print("Test")
        return "Shutting your window"
        Close the window(Tom's job)
    if float(data['main']['temp']) >= openwindow_at:
        return "Opening your window"
        #open the window (Tom's job)
if __name__ == '__main__':
    app.debug = True #Uncomment to enable debugging
    app.run() #Run the Server

and my-form.html is:

<!DOCTYPE html>
<html lang="en">
<body>
    <h1>Enter a place</h1>
    <form action="." method="POST">
        <input type="text" name="text">
        <input type="submit" name="my-form" value="Send">
    <h1>Options</h1>
    <p>Open window if temp is greater than<input type="textbox" name="open"></input> <input type="radio" name="scale" value="kelvin">Kelvin</input><input type="radio" name="scale" value="celcius">Celcius</input><input type="radio" name="scale" value="fah">FarenHeit</input></p>
    </form>
</body>
</html>

If I put a while loop there it loads for ever.

Then enter a temp higher than the current one the page loads forever. And if I use the current code listed above it gives the error.

This is not a duplicate, as here the request is a POST, for which it is unintuitive to return something, whereas in the "duplicate", it is a GET and it is fairly obvious that there is a mistake. IMHO this is a design issue in flask and I usually fix it with 'return ""' at the end of a post route — Iwan LD, Oct 25 '16 at 15:04
I ran into this issue when I explicitly defined the return value for when `request.method == 'GET'` and I called the route using `HEAD`. I fixed this to instead check for`request.method in ['HEAD', 'GET']`. — Steven C. Howell, Jan 02 '20 at 19:53

score 127 · Accepted Answer · answered Jul 30 '14 at 10:34

127

The following does not return a response:

You must return anything like return afunction() or return 'a string'.

This can solve the issue

answered Jul 30 '14 at 10:34

Avinash Babu

6,171
3
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Antti Haapala -- Слава Україні · Answer 2 · 2016-06-08T09:06:09.860

71

You are not returning a response object from your view my_form_post. The function ends with implicit return None, which Flask does not like.

Make the function my_form_post return an explicit response, for example

return 'OK'

at the end of the function.

edited Jun 08 '16 at 09:06

answered Jul 30 '14 at 10:28

Antti Haapala -- Слава Україні

129,958
22
279
321

1

+1 : *The function ends with implicit return None, which Flask does not like* was the point I was missing (a bare `return` is also the problem). `return ''` is fine so I am not sure if Flask does not explicitly like a `return None` or if there are other values as well (since `''` is fine it is not any falsey value) – WoJ Jan 20 '17 at 13:47
Exiting the function without `return` statement or `return` with no arguments returns `None` in Python and the caller cannot distinguish between them and `return None` at all. – Antti Haapala -- Слава Україні Apr 18 '19 at 04:46
Congratulations on 100k. – chux - Reinstate Monica Feb 11 '20 at 15:39
@chux-ReinstateMonica thanks – Antti Haapala -- Слава Україні Feb 11 '20 at 16:17

Flask Value error view function did not return a response

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