Flask doesn't print to console


I’m new to flask, and I’m trying to add print info to debug server side code.
When launch my flask app with debug=True, i can’t get any info print to console

I tried to use logging instead, but no success.
So how to debug flask program with console.

@app.route('/getJSONResult', methods=['GET', 'POST'])
def getJSONResult():

    if request.method == 'POST':
        uut = request.form['uut']
        notes = request.form['notes']
        temperature = request.form['temperature']

        logging.info("enter getJSONReuslt")
        print('enter getJSONReuslt')
        filter_by_query = {k: v for k, v in {
            'uut': uut, 'notes': notes, 'temperature': temperature}.items() if v != ""}
        s = session.query(UUT_TEST_INFO).filter_by(**filter_by_query).first()
        return jsonify(s.serialize)

if __name__ == '__main__':
    app.secret_key = ''.join(random.choice(
        string.ascii_uppercase + string.digits) for x in range(32))
    app.debug = True
    app.run(host='', port=5000)

> - - [07/Jun/2017 15:20:48] "GET /qyer HTTP/1.1" 200 -
> - - [07/Jun/2017 15:20:48] "GET /static/css/bootstrap.min.css HTTP/1.1" 200 -
> - - [07/Jun/2017 15:20:48] "GET /static/js/bootstrap.min.js HTTP/1.1" 200 -
> - - [07/Jun/2017 15:20:51] "GET /static/css/bootstrap.min.css.map HTTP/1.1" 200 -
> - - [07/Jun/2017 15:21:58] "POST /getJSONResult HTTP/1.1" 500 -

I fixed server side 500 error issue, now request get 200 code, and console displays following info

$ python project.py
INFO:werkzeug: * Running on (Press CTRL+C to quit)
INFO:werkzeug: * Restarting with stat
WARNING:werkzeug: * Debugger is active!
INFO:werkzeug: * Debugger pin code: 158-624-607
INFO:werkzeug: - - [08/Jun/2017 11:33:33] "GET /qyer HTTP/1.1" 200 -
INFO:root:Enter getJSONResult
INFO:werkzeug: - - [08/Jun/2017 11:33:43] "POST /getJSONResult HTTP/1.1" 200 -

Still no info from print command

Asked By: X.Z



By default the level for logging is warning. So you won’t see a logging message of level DEBUG. To fix this just enable debug logging with the basicConfig() function of the logging module:

import logging
Answered By: MrLeeh

Try this and see if it helps:

For python2:

from __future__ import print_function
import sys

print('This is error output', file=sys.stderr)
print('This is standard output', file=sys.stdout)

For python3 you don’t need to import from future print_function:

import sys

print('This is error output', file=sys.stderr)
print('This is standard output', file=sys.stdout)

See if it helps to print to console.

Answered By: Nurjan

Had the same printing problem. Using sys.stdout.flush() after the print solved the issue.

Answered By: GiedriusL

You can force to flush stdout directly from print:

print('enter getJSONReuslt', flush=True)

This way you don’t have to print to sys.stderr (which flushes by default).

The reason for your problem is line buffering. Line buffering makes I/O more efficient with the drawback of not immediately showing prints under some conditions.

Answered By: benjy

You can use the app instance in development mode, because the logging level is set to DEBUG, ie.:

app.logger.info('This is info output')

In production mode you need to use a more severe level or you can set the logging level to DEBUG, ie.:

from flask import Flask
import logging

app = Flask(__name__)


def hello_world():
    app.logger.info('Processing default request')
    return 'Hello World!'

if __name__ == '__main__':

This article talks about logging into flask: https://www.scalyr.com/blog/getting-started-quickly-with-flask-logging/

Answered By: Bassirou Diaby
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