Trying to set up my Pi as a web server with flask. It works only sometimes, and then only for about 30 seconds. When I stop the script, it returns this error:

Unhandled exception in thread started by sys.exceptbook is missing lost sys.stderr

from flask import Flask, render_template
app = Flask(name)
def hello():
    templateData = {
    return render_template('main.html', **templateData)
if name == "main":
    app.run(host='', port=80, debug=True)

After it stops working the first time, the Pi must be restarted before it will work again. What did I do wrong?

  • 2
    Could you provide full error message (with stacktrace) and tell US show exactly do you run the script? Dec 20, 2013 at 21:52
  • Have you tried using the ip already assigned to your PI rather than Dec 21, 2013 at 0:58
  • I checked the ip with ifconfig. I ran the script from the command line: sudo python script.py
    – user11410
    Dec 21, 2013 at 23:25
  • It returns: running on, restarting with reloader. But I cannot access from another computer on the network. The funny thing is it does work sometimes, like 1/10 times
    – user11410
    Dec 21, 2013 at 23:27
  • Here is a self-contained project that uses flask. Might be useful: hackaday.io/project/…
    – SDsolar
    Jun 7, 2018 at 15:58

1 Answer 1


The problem was something to do with my router. It worked when I tried it with a different router at a friend's house!

  • So what happened when you brought it home?
    – SDsolar
    Jun 7, 2018 at 15:59

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