How to print from Flask @app.route to python console

An easy way to do this is by printing to stderr. You can do that like this:

from __future__ import print_function # In python 2.7
import sys

@app.route('/button/')
def button_clicked():
    print('Hello world!', file=sys.stderr)
    return redirect('/')

Flask will display things printed to stderr in the console. For other ways of printing to stderr, see this stackoverflow post

I think the core issue with Flask is that stdout gets buffered. I was able to print with print('Hi', flush=True). You can also disable buffering by setting the PYTHONUNBUFFERED environment variable (to any non-empty string).

We can also use logging to print data on the console.

Example:

import logging
from flask import Flask

app = Flask(__name__)

@app.route('/print')
def printMsg():
    app.logger.warning('testing warning log')
    app.logger.error('testing error log')
    app.logger.info('testing info log')
    return "Check your console"

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