Enable color logging on Windows via colorama

[mivade]

This is a slight rework of #1487 which was closed when I reorganized my clone. I'm not sure what the proper way to test this would be, but I used the following script to visually check that the output does as expected:

import logging
from tornado.log import enable_pretty_logging

logger = logging.getLogger()
enable_pretty_logging()
logger.setLevel(logging.DEBUG)

logger.info("INFO")
logger.warning("WARNING")
logger.debug("DEBUG")
logger.error("ERROR")
print("normal")

[bdarnell]

This also needs to be conditioned on the color variable and _stderr_supports_color(). Make this

    if color and _stderr_supports_color():
        if curses is not None:
           ...
        elif colorama is not None:
           ..
        else:
            raise RuntimeError("no supported color terminal library")

[mivade]

Any other comments, @bdarnell?

Just pinging re: your recent post on the mailing list.

[bdarnell]

I'm not really comfortable calling colorama.init at import time. It looks like it may not be idempotent, and replacing sys.std{out,err} is intrusive enough that there should be a more definite opt-in than "the colorama package is installed" (colorama's issue tracker shows that its approach has quite a few quirks).

What if we leave the call to colorama.init() to the application and replace the if colorama is not None calls below with if sys.stderr is getattr(colorama, 'wrapped_stderr', object())? That way we'd be able to detect the use of colorama as an alternative indicator that ANSI color is supported (instead of curses's inspection of $TERM and the terminfo database)

It looks like windows 10 also has a native ANSI-aware terminal (see tartley/colorama#104), so another approach we could take would be to ignore colorama and figure out how to detect that terminal in the absence of curses and terminfo.

[mivade]

I agree with your objection to calling colorama.init explicitly. I've removed that and added language to the LogFormatter docstring pointing out that colorama must be initialized by the user.

I'm reluctant to rely on Windows 10 ANSI-aware terminal support for now since there is still a lot of use of Windows 7.

[bdarnell]

Looking good, just one more thing (I think) to fix the build. Sphinx is complaining that it can't find colorama.init (single backticks are links in RST format, not just monospace like in markdown). Those need to be double backticks instead.

[mivade]

Thanks for the input. I was using the :func: directive there, but with colorama not installed, that resulted in the error. I've replaced with double backticks instead.

[bdarnell]

Thanks!