Flask projects on heroku returns 500 internal server error

Question

I have a basic flask project and i deploy it to heroku. And page returns 500 internal server error. I m not using database. Here is my logs :

2015-01-10T23:40:03.161880+00:00 heroku[web.1]: Starting process with command `gunicorn print:app --log-file=-`
2015-01-10T23:40:03.827603+00:00 app[web.1]: [2015-01-10 23:40:03 +0000] [2] [INFO] Using worker: sync
2015-01-10T23:40:03.827516+00:00 app[web.1]: [2015-01-10 23:40:03 +0000] [2] [INFO] Listening at: http://0.0.0.0:5144 (2)
2015-01-10T23:40:03.835308+00:00 app[web.1]: [2015-01-10 23:40:03 +0000] [8] [INFO] Booting worker with pid: 8
2015-01-10T23:40:03.887218+00:00 app[web.1]: [2015-01-10 23:40:03 +0000] [9] [INFO] Booting worker with pid: 9
2015-01-10T23:40:03.826883+00:00 app[web.1]: [2015-01-10 23:40:03 +0000] [2] [INFO] Starting gunicorn 19.1.1
2015-01-10T23:40:04.268774+00:00 heroku[web.1]: State changed from starting to up
2015-01-10T23:41:19.847544+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=a2450ab9-3183-473f-aa0e-8e970b266b28 fwd="88.238.99.0" dyno=web.1 connect=1ms service=26ms status=500 bytes=456
2015-01-10T23:41:20.206409+00:00 heroku[router]: at=info method=GET path="/favicon.ico" host=topstreaks.herokuapp.com request_id=1c969a6b-a8e7-4fcd-b84a-81b55cec18a6 fwd="88.238.99.0" dyno=web.1 connect=1ms service=2ms status=404 bytes=527
2015-01-10T23:41:24.949004+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=7f90b786-3ec9-49ab-b51a-a2cd942bc83d fwd="88.238.99.0" dyno=web.1 connect=1ms service=13ms status=500 bytes=456
2015-01-10T23:44:05.320256+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=8c0a51bd-459c-438e-8184-911557410c95 fwd="88.238.99.0" dyno=web.1 connect=3ms service=3ms status=500 bytes=456
2015-01-10T23:44:05.697401+00:00 heroku[router]: at=info method=GET path="/favicon.ico" host=topstreaks.herokuapp.com request_id=59f43b94-fb30-414a-9a28-6ae7c195bd4b fwd="88.238.99.0" dyno=web.1 connect=0ms service=2ms status=404 bytes=527
2015-01-10T23:48:29.595955+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=13941ed3-2496-4d0e-aacc-5fb6a7a122e9 fwd="88.238.99.0" dyno=web.1 connect=3ms service=2ms status=500 bytes=456
2015-01-10T23:48:47.739233+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=6c370ced-99fe-429a-829b-0ea55b4aa508 fwd="88.238.99.0" dyno=web.1 connect=0ms service=2ms status=500 bytes=456
2015-01-10T23:52:15.563473+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=a0ceaf25-359e-4bc1-9247-5d6f478d1dd2 fwd="88.238.99.0" dyno=web.1 connect=0ms service=1ms status=500 bytes=456
2015-01-10T23:52:20.050874+00:00 heroku[router]: at=info method=GET path="/" host=topstreaks.herokuapp.com request_id=1086dffb-e144-4873-b49b-5e4ac44477f3 fwd="88.238.99.0" dyno=web.1 connect=0ms service=2ms sta
tus=500 bytes=456

The server looks up but i cant find internal error on logs or anywhere. It runs fine on local. How can i find error on heroku because debug mod not working although i write debug = True on flask.

And flask code :

from flask import Flask, render_template
app = Flask(__name__)

@app.route('/')


def home():
    return render_template('topstreaks.html')

@app.route('/script')
def script():
    return render_template('datum.js')

@app.after_request
def after_request(response):
    response.headers.add('Access-Control-Allow-Origin', '*')
    response.headers.add('Access-Control-Allow-Headers', 'Content-Type,Authorization')
    response.headers.add('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE')
    return response

if __name__ == "__main__":
    app.debug = True
    app.run()

How can i debug for error ?

Answer 1

I tried this locally and found out you have you can see what is wrong by setting a logger in your application and making it print to stdout

from flask import Flask, render_template
import sys
import logging
app = Flask(__name__)

app.logger.addHandler(logging.StreamHandler(sys.stdout))
app.logger.setLevel(logging.ERROR)

That way when you run on heroku, any errors will be printed on heroku logs

Funny though it that when trying to setup this for testing, I ran into a problem similar to yours. The templates (html) must be inside a templates folder on the root of your project, my test template wasn't and that caused the 500 for me, it might be the cause of your 500 too.