How to redirect stderr in Python?

Question

I would like to log all the output of a Python script. I tried:

import sys

log = []

class writer(object):
    def write(self, data):
        log.append(dat         


        

Answer 1

I have a piece of software I wrote for work that captures stderr to a file like so:

import sys
sys.stderr = open('C:\\err.txt', 'w')

so it's definitely possible.

I believe your problem is that you are creating two instances of writer.

Maybe something more like:

import sys

class writer(object):
    log = []

    def write(self, data):
        self.log.append(data)

logger = writer()
sys.stdout = logger
sys.stderr = logger

Answer 2

I found this approach to redirecting stderr particularly helpful. Essentially, it is necessary to understand if your output is stdout or stderr. The difference? Stdout is any output posted by a shell command (think an 'ls' list) while sterr is any error output.

It may be that you want to take a shell commands output and redirect it to a log file only if it is normal output. Using ls as an example here, with an all files flag:

# Imports
import sys
import subprocess

# Open file
log = open("output.txt", "w+")

# Declare command
cmd = 'ls -a'
# Run shell command piping to stdout
result = subprocess.run(cmd, stdout=subprocess.PIPE, shell=True)
# Assuming utf-8 encoding
txt = result.stdout.decode('utf-8')
# Write and close file
log.write(txt)
log.close()

If you wanted to make this an error log, you could do the same with stderr. It's exactly the same code as stdout with stderr in its place. This pipes an error messages that get sent to the console to the log. Doing so actually keeps it from flooding your terminal window as well!

Saw this was a post from a while ago, but figured this could save someone some time :)

Answer 3

To route the output and errors from Windows, you can use the following code outside of your Python file:

python a.py 1> a.out 2>&1

Source: https://support.microsoft.com/en-us/help/110930/redirecting-error-messages-from-command-prompt-stderr-stdout

Answer 4

Python will not execute your code if there is an error. But you can import your script in another script an catch exceptions. Example:

Script.py

print 'something#

FinalScript.py

from importlib.machinery import SourceFileLoader

try:
    SourceFileLoader("main", "<SCRIPT PATH>").load_module()
except Exception as e:
    # Handle the exception here