Thread-Local Context


You should call some functions with some arguments.

—David Reid

structlog does its best to have as little global state as possible to achieve its goals. In an ideal world, you would just stick to its immutable bound loggers and reap all the rewards of having purely immutable state.

However, we realize that passing loggers around is rather clunky and intrusive in practice. And since practicality beats purity, structlog ships with the structlog.threadlocal module to help you to safely have global context storage.

The merge_threadlocal Processor

structlog provides a simple set of functions that allow explicitly binding certain fields to a global (thread-local) context and merge them later using a processor into the event dict.

The general flow of using these functions is:

>>> from structlog.threadlocal import (
...     bind_threadlocal,
...     clear_threadlocal,
...     get_merged_threadlocal,
...     get_threadlocal,
...     merge_threadlocal,
... )
>>> from structlog import configure
>>> configure(
...     processors=[
...         merge_threadlocal,
...         structlog.processors.KeyValueRenderer(),
...     ]
... )
>>> log = structlog.get_logger()
>>> # At the top of your request handler (or, ideally, some general
>>> # middleware), clear the thread-local context and bind some common
>>> # values:
>>> clear_threadlocal()
>>> bind_threadlocal(a=1)
>>> # Then use loggers as per normal
>>> # (perhaps by using structlog.get_logger() to create them).
>>> log.msg("hi")
a=1 event='hi'
>>> # You can access the current thread-local state.
>>> get_threadlocal()
{'a': 1}
>>> # Or get it merged with a bound logger.
>>> get_merged_threadlocal(log.bind(example=True))
{'a': 1, 'example': True}
>>> # And when we clear the thread-local state again, it goes away.
>>> clear_threadlocal()
>>> log.msg("hi there")
event='hi there'

Thread-local Contexts

structlog also provides thread-local context storage in a form that you may already know from Flask and that makes the entire context global to your thread or greenlet.

This makes its behavior more difficult to reason about which is why we generally recommend to use the merge_threadlocal route.

Wrapped Dicts

In order to make your context thread-local, structlog ships with a function that can wrap any dict-like class to make it usable for thread-local storage: structlog.threadlocal.wrap_dict.

Within one thread, every instance of the returned class will have a common instance of the wrapped dict-like class:

>>> from structlog.threadlocal import wrap_dict
>>> WrappedDictClass = wrap_dict(dict)
>>> d1 = WrappedDictClass({"a": 1})
>>> d2 = WrappedDictClass({"b": 2})
>>> d3 = WrappedDictClass()
>>> d3["c"] = 3
>>> d1 is d3
>>> d1 == d2 == d3 == WrappedDictClass()
>>> d3  
<WrappedDict-...({'a': 1, 'b': 2, 'c': 3})>

To enable thread-local context use the generated class as the context class:



Creation of a new BoundLogger initializes the logger’s context as context_class(initial_values), and then adds any values passed via .bind(). As all instances of a wrapped dict-like class share the same data, in the case above, the new logger’s context will contain all previously bound values in addition to the new ones.

structlog.threadlocal.wrap_dict returns always a completely new wrapped class:

>>> from structlog.threadlocal import wrap_dict
>>> WrappedDictClass = wrap_dict(dict)
>>> AnotherWrappedDictClass = wrap_dict(dict)
>>> WrappedDictClass() != AnotherWrappedDictClass()
>>> WrappedDictClass.__name__  
>>> AnotherWrappedDictClass.__name__   

In order to be able to bind values temporarily to a logger, structlog.threadlocal comes with a context manager: structlog.threadlocal.tmp_bind:

>>> log.bind(x=42)  
<BoundLoggerFilteringAtNotset(context=<WrappedDict-...({'x': 42})>, ...)>
>>> log.msg("event!")
x=42 event='event!'
>>> with tmp_bind(log, x=23, y="foo") as tmp_log:
...     tmp_log.msg("another event!")
x=23 y='foo' event='another event!'
>>> log.msg("one last event!")
x=42 event='one last event!'

The state before the with statement is saved and restored once it’s left.

If you want to detach a logger from thread-local data, there’s structlog.threadlocal.as_immutable.

Downsides & Caveats

The convenience of having a thread-local context comes at a price though:


  • If you can’t rule out that your application re-uses threads, you must remember to initialize your thread-local context at the start of each request using new() (instead of bind()). Otherwise you may start a new request with the context still filled with data from the request before.

  • Don’t stop assigning the results of your bind()s and new()s!


    log =
    log = log.bind(x=42)


    Although the state is saved in a global data structure, you still need the global wrapped logger produce a real bound logger. Otherwise each log call will result in an instantiation of a temporary BoundLogger.

    See Configuration for more details.

  • It doesn’t play well with os.fork and thus multiprocessing (unless configured to use the spawn start method).

The general sentiment against thread-locals is that they’re hard to test. In this case we feel like this is an acceptable trade-off. You can easily write deterministic tests using a call-capturing processor if you use the API properly (cf. warning above).

This big red box is also what separates immutable local from mutable global data.