structlog can be easily installed using:
$ pip install structlog
If you’re running Python 2.6 and want to use
OrderedDicts for your context (which is the default), you also have to install the respective compatibility package:
$ pip install ordereddict
If the order of the keys of your context doesn’t matter (e.g. if you’re logging JSON that gets parsed anyway), simply use a vanilla
dict to avoid this dependency.
See Configuration on how to achieve that.
Your First Log Entry¶
A lot of effort went into making structlog accessible without reading pages of documentation. And indeed, the simplest possible usage looks like this:
>>> import structlog >>> log = structlog.get_logger() >>> log.msg('greeted', whom='world', more_than_a_string=[1, 2, 3]) whom='world' more_than_a_string=[1, 2, 3] event='greeted'
Here, structlog takes full advantage of its hopefully useful default settings:
- Output is sent to standard out instead of exploding into the user’s face. Yes, that seems a rather controversial attitude towards logging.
- All keywords are formatted using
structlog.processors.KeyValueRenderer. That in turn uses repr() to serialize all values to strings. Thus, it’s easy to add support for logging of your own objects[*].
It should be noted that even in most complex logging setups the example would still look just like that thanks to Configuration.
There you go, structured logging! However, this alone wouldn’t warrant its own package. After all, there’s even a recipe on structured logging for the standard library. So let’s go a step further.
Building a Context¶
Imagine a hypothetical web application that wants to log out all relevant data with just the API from above:
from structlog import get_logger log = get_logger() def view(request): user_agent = request.get('HTTP_USER_AGENT', 'UNKNOWN') peer_ip = request.client_addr if something: log.msg('something', user_agent=user_agent, peer_ip=peer_ip) return 'something' elif something_else: log.msg('something_else', user_agent=user_agent, peer_ip=peer_ip) return 'something_else' else: log.msg('else', user_agent=user_agent, peer_ip=peer_ip) return 'else'
The calls themselves are nice and straight to the point, however you’re repeating yourself all over the place. At this point, you’ll be tempted to write a closure like
def log_closure(event): log.msg(event, user_agent=user_agent, peer_ip=peer_ip)
inside of the view. Problem solved? Not quite. What if the parameters are introduced step by step? Do you really want to have a logging closure in each of your views?
Let’s have a look at a better approach:
from structlog import get_logger logger = get_logger() def view(request): log = logger.bind( user_agent=request.get('HTTP_USER_AGENT', 'UNKNOWN'), peer_ip=request.client_addr, ) foo = request.get('foo') if foo: log = log.bind(foo=foo) if something: log.msg('something') return 'something' elif something_else: log.msg('something_else') return 'something_else' else: log.msg('else') return 'else'
Suddenly your logger becomes your closure!
For structlog, a log entry is just a dictionary called event dict[ionary]:
- You can pre-build a part of the dictionary step by step. These pre-saved values are called the context.
- As soon as an event happens – which is a dictionary too – it is merged together with the context to an event dict and logged out.
- To keep as much order of the keys as possible, an
collections.OrderedDictis used for the context by default.
- The recommended way of binding values is the one in these examples: creating new loggers with a new context. If you’re okay with giving up immutable local state for convenience, you can also use thread/greenlet local storage for the context.
structlog and Standard Library’s logging¶
structlog’s primary application isn’t printing though. Instead, it’s intended to wrap your existing loggers and add structure and incremental context building to them. For that, structlog is completely agnostic of your underlying logger – you can use it with any logger you like.
The most prominent example of such an ‘existing logger’ is without doubt the logging module in the standard library. To make this common case as simple as possible, structlog comes with some tools to help you:
>>> import logging >>> logging.basicConfig() >>> from structlog import get_logger, configure >>> from structlog.stdlib import LoggerFactory >>> configure(logger_factory=LoggerFactory()) >>> log = get_logger() >>> log.warn('it works!', difficulty='easy') WARNING:structlog...:difficulty='easy' event='it works!'
In other words, you tell structlog that you would like to use the standard library logger factory and keep calling
get_logger() like before.
Since structlog is mainly used together with standard library’s logging, there’s more goodness to make it as fast and convenient as possible.
Liked what you saw?¶
Now you’re all set for the rest of the user’s guide. If you want to see more code, make sure to check out the Examples!
|[*]||In production, you’re more likely to use |