Logging stdout to gunicorn access log? - flask

Question

When I wrap my Flask application in gunicorn writing to stdout no longer seems to go anywhere (simple print statements don't appear). Is there someway to either capture the stdout into the gunicorn access log, or get a handle to the access log and write to it directly?

Answer 1

Use the logging: set the stream to stdout
import logging
app.logger.addHandler(logging.StreamHandler(sys.stdout))
app.logger.setLevel(logging.DEBUG)
app.logger.debug("Hello World")

Answer 2

Two solutions to this problem. They are probably longer than others, but ultimately they tap into how logging is done under the hood in Python.
1. set logging configuration in the Flask app
The official Flask documentation on logging works for gunicorn. https://flask.palletsprojects.com/en/1.1.x/logging/#basic-configuration
some example code to try out:
from logging.config import dictConfig
from flask import Flask
dictConfig(
{
"version": 1,
"disable_existing_loggers": False,
"formatters": {
"default": {
"format": "[%(asctime)s] [%(process)d] [%(levelname)s] in %(module)s: %(message)s",
"datefmt": "%Y-%m-%d %H:%M:%S %z"
}
},
"handlers": {
"wsgi": {
"class": "logging.StreamHandler",
"stream": "ext://flask.logging.wsgi_errors_stream",
"formatter": "default",
}
},
"root": {"level": "DEBUG", "handlers": ["wsgi"]},
}
)
app = Flask(__name__)
#app.route("/")
def hello():
app.logger.debug("this is a DEBUG message")
app.logger.info("this is an INFO message")
app.logger.warning("this is a WARNING message")
app.logger.error("this is an ERROR message")
app.logger.critical("this is a CRITICAL message")
return "hello world"
run with gunicorn
gunicorn -w 2 -b 127.0.0.1:5000 --access-logfile - app:app
request it using curl
curl http://127.0.0.1:5000
this would generate the following logs
[2020-09-04 11:24:43 +0200] [2724300] [INFO] Starting gunicorn 20.0.4
[2020-09-04 11:24:43 +0200] [2724300] [INFO] Listening at: http://127.0.0.1:5000 (2724300)
[2020-09-04 11:24:43 +0200] [2724300] [INFO] Using worker: sync
[2020-09-04 11:24:43 +0200] [2724311] [INFO] Booting worker with pid: 2724311
[2020-09-04 11:24:43 +0200] [2724322] [INFO] Booting worker with pid: 2724322
[2020-09-04 11:24:45 +0200] [2724322] [DEBUG] in flog: this is a DEBUG message
[2020-09-04 11:24:45 +0200] [2724322] [INFO] in flog: this is an INFO message
[2020-09-04 11:24:45 +0200] [2724322] [WARNING] in flog: this is a WARNING message
[2020-09-04 11:24:45 +0200] [2724322] [ERROR] in flog: this is an ERROR message
[2020-09-04 11:24:45 +0200] [2724322] [CRITICAL] in flog: this is a CRITICAL message
127.0.0.1 - - [04/Sep/2020:11:24:45 +0200] "GET / HTTP/1.1" 200 11 "-" "curl/7.68.0"
2. set logging configuration in Gunicorn
same application code as above but without the dictConfig({...}) section
create a logging.ini file
[loggers]
keys=root
[handlers]
keys=consoleHandler
[formatters]
keys=simpleFormatter
[logger_root]
level=DEBUG
handlers=consoleHandler
[handler_consoleHandler]
class=StreamHandler
level=DEBUG
formatter=simpleFormatter
args=(sys.stdout,)
[formatter_simpleFormatter]
format=[%(asctime)s] [%(process)d] [%(levelname)s] - %(module)s - %(message)s
datefmt=%Y-%m-%d %H:%M:%S %z
run gunicorn with --log-config logging.ini option, i.e gunicorn -w 2 -b 127.0.0.1:5000 --access-logfile - --log-config logging.ini app:app

Answer 3

The solution from John mee works, but it duplicates log entries in the stdout from gunicorn.
I used this:
import logging
from flask import Flask
app = Flask(__name__)
if __name__ != '__main__':
gunicorn_logger = logging.getLogger('gunicorn.error')
app.logger.handlers = gunicorn_logger.handlers
app.logger.setLevel(gunicorn_logger.level)
and got I this from: https://medium.com/#trstringer/logging-flask-and-gunicorn-the-manageable-way-2e6f0b8beb2f

Answer 4

You can redirect standard output to a errorlog file, which is enough for me.
Note that:
capture_output
--capture-output
False
Redirect stdout/stderr to specified file in errorlog
My config file gunicorn.config.py setting
accesslog = 'gunicorn.log'
errorlog = 'gunicorn.error.log'
capture_output = True
Then run with gunicorn app_py:myapp -c gunicorn.config.py
The equivaluent command line would be
gunicorn app_py:myapp --error-logfile gunicorn.error.log --access-logfile gunicorn.log --capture-output