How to redirect stderr to variable in Python 2.7?

Question

I am trying to redirect the stderr of a script to a variable to use it in if/else statements. What I need is the program to behave in one way or another according to the stderr. I have found this post using StringIO which may suit my needs, but I cannot figure out how to make it to redirect stderr. I'm sure it is quite easy, but I'm quite new to python and any detailed help will be really appreciated. Thanks!

Dani

Use sys.stderr instead of sys.stdout and do what the post says. But why do you need it. Who is writing to stderr? — Anand S Kumar
– Anand S Kumar, Commented Aug 4, 2015 at 18:04

Piotr Dabkowski · Accepted Answer · 2015-08-04 18:08:09Z

In order to intercept writes to stderr use following 'hack':

import sys STDERR = '' def new_stderr(old): def new(*args): # put your code here, you will intercept writes to stderr print('Intercepted: ' + repr(args)) global STDERR # add new write to STDERR STDERR += args[0] old(*args) return new sys.stderr.write = new_stderr(sys.stderr.write)

All writes to stderr will be stored in STDERR variable

In my case the interpreter protests pointing at the last line, with a message: "AttributeError: 'file' object attribute 'write' is read-only"

Robert Jacobs · Accepted Answer · 2015-08-04 18:07:16Z

Make a copy of sys.stderr.
Open some StringIO variable (may already be done)
Assign variable to sys.stderr
All writes to stderr will now go to the new file.
Copy your saved original version of sys.stderr back to return to old behavior

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