Add LogfmtRenderer

[airwoodix]

Summary

This PR adds a LogfmtRenderer processor to format log lines in the logfmt format.

The core of the work is done by the logfmt library, which is added as global dependency. Or should this be changed to feature-based optional?

Refactored the code in KeyValueRenderer to share the sort_keys/key_order/drop_missing logic. Tests are directly adapted from KeyValueRenderer.

Closes #322

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[hynek]

Wow, this is great thank you! logfmt doesn't seem to be too well-maintained these days, but we can give it a try before we implement it ourselves.

As you've suspected, we need to make logfmt optional, I've added instructions to the pyproject.toml lines

Tasks that I couldn't add to the diff:

• add test env with logfmt to tox.ini (analogous to the rich env)
• add a usage example to typing_examples.py

[hynek]

Um, so I've had a look at logfmt (of course one second after sending the review 🤦‍♂️) and I don't think we need a dependency for 17 LoC?

def format_line(extra):
    outarr = []
    for k, v in extra.items():
        if v is None:
            outarr.append("%s=" % k)
            continue


        if isinstance(v, bool):
            v = "true" if v else "false"
        elif isinstance(v, numbers.Number):
            pass
        else:
            if isinstance(v, (dict, object)):
                v = str(v)
            v = '"%s"' % v.replace('"', '\\"')
        outarr.append("%s=%s" % (k, v))
    return " ".join(outarr)

[airwoodix]

Thanks for the fast and detailed review!

I don't think we need a dependency for 17 LoC?

Agreed. I added the logfmt rendering logic to LogfmtRenderer.__call__ in 44a19e6.

Since there's no formal specification, I oriented myself to the behavior of the go and node implementations. The go docs have an example and a pseudo-spec.

I also quickly checked that Grafana's LogQL logfmt filter/parser is happy with quoted/unquoted strings, empty values (key=) and boolean flags (key).

add test env with logfmt to tox.ini (analogous to the rich env)

I guess that this is obsolete with the dependency to logfmt removed?

add a usage example to typing_examples.py

I added a block similar to the ones for JSONRenderer in f3b5ffe, though I'm not sure how the typing_examples.py mechanism works.

[hynek]

Thanks for the fast and detailed review!

Thanks for the quick turnaround!

I don't think we need a dependency for 17 LoC?

Agreed. I added the logfmt rendering logic to LogfmtRenderer.__call__ in 44a19e6.

Since there's no formal specification, I oriented myself to the behavior of the go and node implementations. The go docs have an example and a pseudo-spec.

Wonderful, looks great! We might want to add a default mechanism like json has, but good enough start!

add test env with logfmt to tox.ini (analogous to the rich env)
I guess that this is obsolete with the dependency to logfmt removed?

yep

add a usage example to typing_examples.py
I added a block similar to the ones for JSONRenderer in f3b5ffe, though I'm not sure how the typing_examples.py mechanism works.

it just uses the APIs and looks whether mypy is happy or not. the code is never executed.