add unbind_threadlocal and try_unbind_threadlocal methods

[c-wilson]

I really like the new processor-style interface for thread local context, but I was hoping we could extend it with unbind methods that mirror the BoundLoggerBase class. I frequently have the need to enter and exit a logging context, but I'm a bit worried about unintended side-effects using the old interface and threadlocal.tmp_bind.

adds:

• structlog.threadlocal.unbind_threadlocal
• tests

import structlog

log= structlog.get_logger()

def myfunction():
  structlog.threadlocal.bind_threadlocal(x=12, y=2)
  log.msg('my message')
  # enter another function, module, etc.
  structlog.threadlocal.unbind_threadlocal('x', 'y')

If I'm violating the spirit of threadlocal context, just let me know. I would also be happy to add an actual logging context manager here or in another PR if it would be helpful!

with LogContext(a=12, b=32):
  log.msg('mymsg')

[MarkusH]

I'm not sure we need the strict and "lenient" implementations. Logging interactions should IMO not cause an application to fail. I'd thus go with a "fail-silent" approach as I've done in the contextvars implementation:

structlog/src/structlog/contextvars.py

Lines 70 to 72 in 062aa21

	ctx = _get_context()
	for key in args:
	ctx.pop(key, None)

[hynek]

hmmm the problem is that it's not consistent with structlog's bind/unbind. I guess best way forward would be to add an option and deprecate try_unbind?

[c-wilson]

Ha, now that I know about the contextvars implementation, I'm second-guessing whether to use threadlocal at all... still happy to see this through though!

While I do tend to agree that "logging should not make your app fail", there are definitely contexts where production of accurate logs are an important side-effect for guiding decisions (ie how much infra do I need, what is my ETL task failure rate, etc.).

Bringing to light double-removals of context could be valuable ie:

def main(var)
  bind_threadlocal(var=var)
  fun1(var)
  fun2()
  log.msg("I finished something with 'a' in my context")
  unbind_threadlocal("var")

def fun1(var):
  bind_threadlocal(var=var)
  # do something
  unbind_threadlocal("var")

def fun2():
  pass

but maybe this highlights that we should use a context manager to roll-back context instead of this interface.

Anyways, let me know what you think, and I can remove try_unbind_threadlocal if that makes sense to you.

[hynek]

Yeah let's just mirror the approach in contextvars please -- thanks!

[hynek]

Thanks!

Could you add

• a change log entry
• and a .. versionadded:: tag?

Thanks!

[c-wilson]

Sure thing! Maybe this is a dumb question, but the versionadded will be 20.1.0?

[hynek]

Yeah but I'll do it myself real quick – thanks!