[Tutor] logging to cmd.exe

[Peter Otten]

Albert-Jan Roskam wrote:

> Hi,
> 
> 
> With Python 3.5 under Windows I am using the logging module to log
> messages to stdout (and to a file), but this occasionally causes logging
> errors because some characters cannot be represented in the codepage used
> by cmd.exe (cp850, aka OEM codepage, I think). What is the best way to
> prevent this from happening? The program runs fine, but the error is
> distracting. I know I can use s.encode(sys.stdout.encoding, 'replace') and
> log that, but this is ugly and tedious to do when there are many log
> messages. I also don't understand why %r (instead of %s) still causes an
> error. I thought that the character representation uses only ascii
> characters?!

Not in Python 3. You can enforce ascii with "%a":

>>> euro = '\u20ac'
>>> print("%r" % euro)
'€'
>>> print("%a" % euro)
'\u20ac'

Or you can set an error handler with PYTHONIOENCODING (I have to use 
something that is not utf-8-encodable for the demo):

$ python3 -c 'print("\udc85")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
UnicodeEncodeError: 'utf-8' codec can't encode character '\udc85' in 
position 0: surrogates not allowed

$ PYTHONIOENCODING=:backslashreplace python3 -c 'print("\udc85")'
\udc85

Or you follow the convention and log to stderr:

$ python3 -c 'import sys; print("\udc85", file=sys.stderr)'
\udc85
$ $ python3 -c 'import logging; logging.basicConfig(); 
logging.getLogger().warn("\udc85")' > to_prove_it_s_not_stdout
WARNING:root:\udc85

> import logging
> import sys
> 
> assert sys.version_info.major > 2
> logging.basicConfig(filename="d:/log.txt",
> level=logging.DEBUG,format='%(asctime)s %(message)s') handler =
> logging.StreamHandler(stream=sys.stdout) logger =
> logging.getLogger(__name__) logger.addHandler(handler)
> 
> s = '\u20ac'
> logger.info("euro sign: %r", s)
> 
> 
> 
> --- Logging error ---
> Traceback (most recent call last):
>   File "c:\python3.5\lib\logging\__init__.py", line 982, in emit
>     stream.write(msg)
>   File "c:\python3.5\lib\encodings\cp850.py", line 19, in encode
>     return codecs.charmap_encode(input,self.errors,encoding_map)[0]
> UnicodeEncodeError: 'charmap' codec can't encode character '\u20ac' in
> position 12: character maps to <undefined> Call stack:
>   File "q:\temp\logcheck.py", line 10, in <module>
>     logger.info("euro sign: %r", s)
> Message: 'euro sign: %r'
> Arguments: ('\u20ac',)
> 
> 
> Thanks in advance for your replies!
> 
> 
> Albert-Jan
> 
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