Logging stdout to gunicorn access log?

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

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