Question

I’ve got a noisy python script that I want to silence by directing its stderr output to /dev/null (using bash BTW).

Like so:

python -u parse.py  1> /tmp/output3.txt 2> /dev/null 

but it quickly exits prematurely. Hmm. I can’t see the traceback because of course that goes out with stderr. It runs noisily and normally if I don’t direct stderr somewhere.

So let’s try redirecting it to a file somewhere rather than /dev/null, and take a look at what it’s outputting:

python -u parse.py  1> /tmp/output3.txt 2> /tmp/foo || tail /tmp/foo  Traceback (most recent call last):   File 'parse.py', line 79, in <module>     parseit('pages-articles.xml')   File 'parse.py', line 33, in parseit     print >>sys.stderr, 'bad page title', page_title UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128) 

So, the stderr that’s being generated contains utf8, and for some reason python refuses to print non-ascii when it’s being redirected, even though it’s being directed to /dev/null (though of course python doesn’t know that).

How can I silence the stderr of a python script even though it contains utf8? Is there any way to do it without re-writing every print to stderr in this script?

Answer 1

You can silence stderr by binding it to a custom writer:

#!/usr/bin/env python import codecs, sys  class NullWriter:     def write(self, *args, **kwargs):         pass  if len(sys.argv) == 2:    if sys.argv[1] == '1':       sys.stderr = NullWriter()    elif sys.argv[1] == '2':       #NOTE: sys.stderr.encoding is *read-only*        #      therefore the whole stderr should be replaced       # encode all output using 'utf8'       sys.stderr = codecs.getwriter('utf8')(sys.stderr)  print >>sys.stderr, u'\u20AC' # euro sign print 'ok' 

Example:

$ python silence_stderr.py Traceback (most recent call last):   File 'silence_stderr.py', line 11, in <module>     print >>sys.stderr, u'\u20AC' UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 0: ordinal not in range(128) 

Silenced stderr:

$ python silence_stderr.py 1 ok 

Encoded stderr:

$ python silence_stderr.py 2 € ok 

NOTE: I’ve got the above outputs inside emacs therefore to emulate it in a terminal you could do:

$ python ... 2>out.txt $ cat out.txt 

NOTE: Inside Windows console (after chcp 65001 that switch to ‘utf-8’ and with truetype font (Lucida Console)) I’ve got strange results:

C:\> python silence_stderr.py 2 Traceback (most recent call last):   File 'silence_stderr.py', line 14, in <module>     print >>sys.stderr, u'\u20AC' # euro sign   File 'C:\pythonxy\python\lib\codecs.py', line 304, in write     self.stream.write(data) IOError: [Errno 13] Permission denied 

If the font is not truetype then the exception doesn’t raise but the output is wrong.

Perl works for the truetype font:

C:\> perl  -E'say qq(\x{20ac})' Wide character in print at -e line 1. € 

Redirection works though:

C:\>python silence_stderr.py 2 2>tmp.log ok C:\>cat tmp.log € cat: write error: Permission denied 

re comment

From codecs.getwriter documentation:

Look up the codec for the given encoding and return its StreamWriter class or factory function. Raises a LookupError in case the encoding cannot be found.

An oversimplified view:

class UTF8StreamWriter:     def __init__(self, writer):         self.writer = writer     def write(self, s):         self.writer.write(s.encode('utf-8'))  sys.stderr = UTF8StreamWriter(sys.stderr)