Add default extra fields to messages

[Fuco1]

I have a json appender where I would like to automatically add some meta-data without having to specify them in the log function every time.

For example, I have a loop which operates on some entities, I'd like to automatically add the entity ID to each log message emitted in the loop.

The motivation is removal of extra clutter in the logging code, where often I'd have 4-5 extra arguments which don't change inside the entire function/loop body.

In code (non-working):

appender$set_default_meta(a = "b", id = 33)

...

lg$info("Hello")
lg$warn("Warning")

Both logs would include the a and id extra keys without me having to specify them repeatedly.

[s-fleck]

This is already possible with (abusing) Filters! And since this is actually something I also needed, lgr already includes a constructor for Filters for your exact usecase!

lg$add_filter(FilterInject$new(a = "b", id = 33), "inject")

see ?EventFilter for more details

you can afterwards remove the filter with lg$remove_filter("inject"). Note that Filters can be attached to Loggers as well as Appenders.

[Fuco1]

Genius! Is this described somewhere in the docs? Maybe a short vignette or a blog post would be nice, I think this is fairly common use-case in data processing.

[s-fleck]

Ok great that it works :)

No it's not super well documented yet, only in the help of ?EventFilter and with_log_value() (which is just a wrapper for temporary setting a Filter that injects extra values)

I'm leaving this issue open because I agree it should be better documented (how it works, and also WHY it works). Sadly, refining the documentation is a lot of work :(. In the meanwhile, note that lgr is basically a very close clone of python logging and most of the documentation of python logging applies - with small modifications - also to lgr (in this case Filter Objects)

[s-fleck]

I just saw, this was actually documented in the vignette already, I just forgot about it ;) I gave the section a slightly better heading now.