Flask: Showing print on website instead of console?

Question

The error TypeError: can't concat bytes to str implies that you use Python 3 where python is more strict about mixing bytes and Unicode strings. You should also avoid mixing bytes and Unicode in Python 2 but python itself is more relax about it.

#!/usr/bin/env python3
import html
import sys
from subprocess import Popen, PIPE, STDOUT, DEVNULL
from textwrap import dedent

from flask import Flask, Response # $ pip install flask

app = Flask(__name__)

@app.route('/')
def index():
    def g():
        yield "<!doctype html><title>Stream subprocess output</title>"

        with Popen([sys.executable or 'python', '-u', '-c', dedent("""\
            # dummy subprocess
            import time
            for i in range(1, 51):
                print(i)
                time.sleep(.1) # an artificial delay
            """)], stdin=DEVNULL, stdout=PIPE, stderr=STDOUT,
                   bufsize=1, universal_newlines=True) as p:
            for line in p.stdout:
                yield "<code>{}</code>".format(html.escape(line.rstrip("\n")))
                yield "<br>\n"
    return Response(g(), mimetype='text/html')

if __name__ == "__main__":
    import webbrowser
    webbrowser.open('http://localhost:23423') # show the page in browser
    app.run(host='localhost', port=23423, debug=True) # run the server

See also Streaming data with Python and Flask.