print() output not appearing in apache error.log with Python 3

[YKdvd]

I've got a Flask wsgi app, where there are print statements in the myapp.wsgi, which take advantage of the "send unhandled print stuff out through the apache error log".  This was working fine with Python 2.7/Ubuntu 16.04, and the output appeared in the apache error.log as expected.  But after conversion to Python 3.9/Ubuntu 22.04 (mod_wsgi 4.9.x, Flask 2.3.x, I think), the output of print() statements doesn't seem to get to the apache log anymore.  Later on in the request handling of the app, I have Python logging established (normal stdout, I think), and that output does get to the error log, but print() statements still don't.

I assume there is some sort of change in how Python 3 is handling stdout/stderr, or how Flask or mod_wsgi interact with that, and the print() text never gets out, or goes to a bitbucket, or something, but I can't figure out why or how I might fix it.  I've tried a couple things to monkey with Python's stdout/stderr before the first print statement, but didn't have any luck.  I could go through and replace some of the old print statements with Python logging calls, but I'd like to figure this out and fix it if possible, if anyone has any ideas or pointers as to what might be happening?

[Graham Dumpleton]

Have you tried just a WSGI hello world with print() statements in it to make sure nothing about any framework used is interfering.

import sys

def application(environ, start_response):
    status = '200 OK'
    output = b'Hello World!'

    print("Hi there!")

    print("Send to stdout explicitly", file=sys.stdout)

    print("Send to stderr explicitly", file=sys.stderr)

    print("stdout", type(sys.stdout))

    print("stderr", type(sys.stderr))

    response_headers = [('Content-type', 'text/plain'),
                        ('Content-Length', str(len(output)))]
    start_response(status, response_headers)

    return [output]

On Fri, 1 Mar 2024 at 07:50, YKdvd <david...@gmail.com> wrote:

I've got a Flask wsgi app, where there are print statements in the myapp.wsgi, which take advantage of the "send unhandled print stuff out through the apache error log".  This was working fine with Python 2.7/Ubuntu 16.04, and the output appeared in the apache error.log as expected.  But after conversion to Python 3.9/Ubuntu 22.04 (mod_wsgi 4.9.x, Flask 2.3.x, I think), the output of print() statements doesn't seem to get to the apache log anymore.  Later on in the request handling of the app, I have Python logging established (normal stdout, I think), and that output does get to the error log, but print() statements still don't.

I assume there is some sort of change in how Python 3 is handling stdout/stderr, or how Flask or mod_wsgi interact with that, and the print() text never gets out, or goes to a bitbucket, or something, but I can't figure out why or how I might fix it.  I've tried a couple things to monkey with Python's stdout/stderr before the first print statement, but didn't have any luck.  I could go through and replace some of the old print statements with Python logging calls, but I'd like to figure this out and fix it if possible, if anyone has any ideas or pointers as to what might be happening?

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