Add support for per-loglevel formatting

[mdraw]

This makes it possible to specify a formatter dictionary that enables colorlog to format messages differently based on their log level.

Use case:

• If you want to log informational output using logger.info(), you are rarely interested in the module and line number where the message comes from.
• On the other hand, it can be useful to prefix warnings, errors and criticals with 'WARNING: ' etc. and add line number and module at the end, for example.

This is why I added the level_fmts argument to ColoredFormatter.

I tested the new code with Python 2.7 and Python 3.5.

Possible issues:

• default_level_formats has nothing to do with colorlog's normal default_formats. Could this be confusing?
• We could reuse the fmt argument instead of introducing the new level_fmts. We would need to check if fmt is a dict or a string in ColoredFormatter.__init__() and could use it differently depending on the type. I don't know if that would be better. What is your opinion?

[borntyping]

This is pretty cool, though is quite far from the modules original scope. I think this behaviour might be better placed in a subclass of the ColoredFormatter class, which is already quite complex.

default_level_formats has nothing to do with colorlog's normal default_formats. Could this be confusing?

Not worried about the naming of default_level_formats - it's an internal constant that users shouldn't see anyway. It would be good to have the formats defined it resemble those in default_formats though, as they currently have a very different layout of the log messages. It might be easier to go a step further and not include a set of defaults, since it's a very user-specific function.

We could reuse the fmt argument instead of introducing the new level_fmts. We would need to check if fmt is a dict or a string in ColoredFormatter.init() and could use it differently depending on the type. I don't know if that would be better. What is your opinion?

I like this idea of reusing fmt, that would keep the API much simpler.

[mdraw]

Thanks for the feedback! I hope you don't mean it is too far from your original idea of colorlog to get merged.

I think this behaviour might be better placed in a subclass of the ColoredFormatter class, which is already quite complex.

Actually, this is how I had originally implemented it, but I decided to merge it into ColoredFormatter itself to avoid duplicating code from the base class (I would have to copy-paste most of the ColoredFormatter.format() and ColoredFormatter.__init__() functions to my new class, because my new code has to be in the middle of the format() and __init__() functions).

It might be easier to go a step further and not include a set of defaults, since it's a very user-specific function.

So should I just remove the whole default_level_formats dict from the code? Should we re-add it as an example in the documentation then?

I like this idea of reusing fmt, that would keep the API much simpler.

OK, I'll rewrite it to reuse fmt and remove the new separate level_fmts argument.

[borntyping]

Yep - happy to use this in colorlog, I'd just greatly prefer to put it in a subclass.

I would have to copy-paste most of the ColoredFormatter.format() and ColoredFormatter.__init__() functions to my new class, because my new code has to be in the middle of the format() and __init__() functions

I think __init__ doesn't need so many changes if we drop default_level_formats and use a single fmt argument. An approach to format() might be to add some optional arguments that override the format/style (so that the subclass can call it with a specific format).

[mdraw]

I was a bit confused about when I have to call the superclass functions at first, but I've just managed to write my new code exclusively into a subclass without duplicating any code from ColoredFormatter.

Do you have a suggestion how to name this new class? I chose "ColoredLevelFormatter" for now, but that might sound a bit awkward.

[borntyping]

Maybe just colorlog.LevelFormatter? Everything else looks great.

[mdraw]

colorlog.LevelFormatter sounds good, I just renamed it. Would you like to merge the PR in this state?

[mdraw]

Thanks a lot for merging this!
I'd like to remind you to update https://github.com/borntyping/python-colorlog/blob/master/colorlog/colorlog.py#L158 before tagging a new release.

[borntyping]

Thanks for all the work you've put in :)