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

Question

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!

Solution 1

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

Solution 2

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)

Solution 3

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).

Solution 4

I tried running @Viraj Wadate's code, but couldn't get the output from app.logger.info on the console.

To get INFO, WARNING, and ERROR messages in the console, the dictConfig object can be used to create logging configuration for all logs (source):

from logging.config import dictConfig
from flask import Flask
dictConfig({
    'version': 1,
    'formatters': {'default': {
        'format': '[%(asctime)s] %(levelname)s in %(module)s: %(message)s',
    }},
    'handlers': {'wsgi': {
        'class': 'logging.StreamHandler',
        'stream': 'ext://flask.logging.wsgi_errors_stream',
        'formatter': 'default'
    }},
    'root': {
        'level': 'INFO',
        'handlers': ['wsgi']
    }
})
app = Flask(__name__)
@app.route('/')
def index():
    return "Hello from Flask's test environment"
@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)

View more solutions

Comments

[Daniel Roseman]

You're confusing two things here. You can call any functions you like from a handler; but the issue with print is what Flask is doing to stdout.

[Robert Filter]

Hi @DanielRoseman and thanks for the comment! So flask is somewhat routing the print to http? What should I do to prevent this? Sorry if the question is silly :)

[Ciaran Liedeman]

There are no silly questions :)

[dirn]

Flask does not route print to the response. If you are running the development server from a terminal session, you will see the output there. If you are running it through a WSGI server such as uWSGI, the output will appear in the logs instead.

[Ciaran Liedeman]

How are you starting flask?

[Robert Filter]

Hi @ciaran, i am starting the script from pyscripter. This should be equivalent to python myscript.py

[Robert Filter]

Just to add that all have been right. Print is simply not meant here to provide output in the console.

[Austin Pray]

Might be worth checking this out: gitlab.com/meltano/meltano/issues/106#note_105372174

[Robert Filter]

Thx @Gabe, this seems to be a way to go.

[e271p314]

Do I really need to go over all files and add from __future__ import print_function also file=sys.stderr for every print? is there a short way to it?

[Gabe]

I would recommend taking a look at the post I linked to in the original answer. There's one person recommends defining a function which always prints to stderr (you could put this in a util file that you already import). Another person recommends sys.stderr.write.

[phoenix]

You could also save a little bit of repetition with: from sys import stderr, file=stderr. In Python 3+, you don't need from __future__ import print_function, that is the default functionality.

[jcroll]

If dumping an object this seems to work pprint(vars(myobject), sys.stderr)

[gunslingor]

See this if you cannot get the info logger to work: flask.palletsprojects.com/en/1.0.x/logging

[learner0000]

This is the best way for print-debugging, especially in small projects

[thanos.a]

What is the value that should be set in the PYTHONUNBUFFERED env variable ?

[Chris]

@thanos.a You can set it to any non-empty string. I updated the answer.

[thanos.a]

You can add an example like PYTHONUNBUFFERED="anything_here"

[Anonymous12332313]

I'm using flask run to run the app from command prompt. I'm using pycharm as my IDE. In run configurations I have it set to emulate terminal in output console. When I try this, nothing prints in either the terminal or the python console. Any idea of what's not working?