How to save python Errors (stderr) to a log file ONLY in case errors actually happen?

Question

I have a python script that runs once a minute. For debugging, I want to create text files with errors/exceptions in case they occur. In case errors are absent, I don't want any log files.

EDIT: I can't wrap all my code in a try/except loop, so this doesn't help.

First, I am saving stderr to .txt doing:

sys.stderr = open('ERRORS_%s.txt' % (infile), 'a')
with open("%s_Error_Messages.txt" % (MSKSN), "a") as myfile:
    myfile.close()

Second, I then try to delete empty "%s_Error_Messages.txt" % (MSKSN) files before any raise SystemExit positions since no error was raised (otherwise, the raise SystemExit position would not have been reached).

if os.path.getsize('ERRORS_%s.txt' % (infile)) == 0:
    os.remove('ERRORS_%s.txt' % (infile))

What I get is WindowsError: [Error 32] The process cannot access the file because it is being used by another process . I assume that this is because stderr is always in use by python.exe as long as the script runs.

For the first step, I also tried:

if sys.stderr != 0: 
   sys.stderr = open('ERRORS_%s.txt' % (infile), 'a')

Which does not work since sys.stderr is never 0.

How to write sys.stderr to .txt only in case of actual errors?

Answer 1

Unless you have a huge volume of error data to worry about, you can just redirect stderr into a stringIO object and then write it out (or not) depending on contents

import StringIO
import sys

sys.stderr = StringIO.StringIO()
# do stuff
if sys.stderr.getvalue():
   with open ("path/to/log.txt") as output:
        output.write(sys.stderr.getvalue())

This is a great case for a context manager that will set the replacement stderr, run your code, then reset stderr back to sys.__stderr__ when done.

If you can't wrap the code in a try, you could also try setting this functionality using sys.excepthook