Default encoding for python for stderr?

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)