I would like to simply print a "hello world" to the python console after /button is called by the user.

This is my naive approach:

@app.route('/button/')
def button_clicked():
    print 'Hello world!'
    return redirect('/')

Background: I would like to execute other python commands from flask (not shell). "print" should be the easiest case. I believe I have not understood a basic twist here. Thanks in advance!

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