Context Variables#
The contextvars
module in the Python standard library allows having a global structlog
context that is local to the current execution context.
The execution context can be thread-local, concurrent code such as code using asyncio
, or greenlet.
For example, you may want to bind certain values like a request ID or the peer’s IP address at the beginning of a web request and have them logged out along with the local contexts you build within our views.
For that structlog
provides the structlog.contextvars
module with a set of functions to bind variables to a context-local context.
This context is safe to be used both in threaded as well as asynchronous code.
The general flow is:
Use
structlog.configure
withstructlog.contextvars.merge_contextvars
as your first processor.Call
structlog.contextvars.clear_contextvars
at the beginning of your request handler (or whenever you want to reset the context-local context).Call
structlog.contextvars.bind_contextvars
andstructlog.contextvars.unbind_contextvars
instead of your bound logger’sbind()
andunbind()
when you want to bind and unbind key-value pairs to the context-local context. You can also use thestructlog.contextvars.bound_contextvars
context manager/decorator.Use
structlog
as normal. Loggers act as they always do, but thestructlog.contextvars.merge_contextvars
processor ensures that any context-local binds get included in all of your log messages.If you want to access the context-local storage, you use
structlog.contextvars.get_contextvars
andstructlog.contextvars.get_merged_contextvars
.
We’re sorry the word context means three different things in this itemization depending on…context.
>>> from structlog.contextvars import (
... bind_contextvars,
... bound_contextvars,
... clear_contextvars,
... merge_contextvars,
... unbind_contextvars,
... )
>>> from structlog import configure
>>> configure(
... processors=[
... merge_contextvars,
... structlog.processors.KeyValueRenderer(key_order=["event", "a"]),
... ]
... )
>>> log = structlog.get_logger()
>>> # At the top of your request handler (or, ideally, some general
>>> # middleware), clear the contextvars-local context and bind some common
>>> # values:
>>> clear_contextvars()
>>> bind_contextvars(a=1, b=2)
{'a': <Token var=<ContextVar name='structlog_a' default=Ellipsis at ...> at ...>, 'b': <Token var=<ContextVar name='structlog_b' default=Ellipsis at ...> at ...>}
>>> # Then use loggers as per normal
>>> # (perhaps by using structlog.get_logger() to create them).
>>> log.msg("hello")
event='hello' a=1 b=2
>>> # Use unbind_contextvars to remove a variable from the context.
>>> unbind_contextvars("b")
>>> log.msg("world")
event='world' a=1
>>> # You can also bind key/value pairs temporarily.
>>> with bound_contextvars(b=2):
... log.msg("hi")
event='hi' a=1 b=2
>>> # Now it's gone again.
>>> log.msg("hi")
event='hi' a=1
>>> # And when we clear the contextvars state again, it goes away.
>>> # a=None is printed due to the key_order argument passed to
>>> # KeyValueRenderer, but it is NOT present anymore.
>>> clear_contextvars()
>>> log.msg("hi there")
event='hi there' a=None
Support for contextvars.Token#
If e.g. your request handler calls a helper function that needs to temporarily override some contextvars before restoring them back to their original values, you can use the Token
s returned by bind_contextvars()
along with reset_contextvars()
to accomplish this (much like how contextvars.ContextVar.reset()
works):
def foo():
bind_contextvars(a=1)
_helper()
log.msg("a is restored!") # a=1
def _helper():
tokens = bind_contextvars(a=2)
log.msg("a is overridden") # a=2
reset_contextvars(**tokens)