I believe the widespread use of format strings in logging is based on two presumptions:
The first level consumer of a log message is a human.
The programmer knows what information is needed to debug an issue.
I believe these presumptions are no longer correct in server side software.
Structured logging means that you don’t write hard-to-parse and hard-to-keep-consistent prose in your logs but that you log events that happen in a context instead.
If you prefer videos over reading, check out this DjangoCon Europe 2019 talk by Markus Holtermann: “Logging Rethought 2: The Actions of Frank Taylor Jr.”.
You can stop writing prose and start thinking in terms of an event that happens in the context of key/value pairs:
>>> from structlog import get_logger >>> log = get_logger() >>> log.info("key_value_logging", out_of_the_box=True, effort=0) 2020-11-18 09:17.09 [info ] key_value_logging effort=0 out_of_the_box=True
Each log entry is a meaningful dictionary instead of an opaque string now!
Since log entries are dictionaries, you can start binding and re-binding key/value pairs to your loggers to ensure they are present in every following logging call:
>>> log = log.bind(user="anonymous", some_key=23) >>> log = log.bind(user="hynek", another_key=42) >>> log.info("user.logged_in", happy=True) 2020-11-18 09:18.28 [info ] user.logged_in another_key=42 happy=True some_key=23 user=hynek
Each log entry goes through a processor pipeline that is just a chain of functions that receive a dictionary and return a new dictionary that gets fed into the next function. That allows for simple but powerful data manipulation:
def timestamper(logger, log_method, event_dict): """Add a timestamp to each log entry.""" event_dict["timestamp"] = time.time() return event_dict
There are plenty of processors for most common tasks coming with
structlog is completely flexible about how the resulting log entry is emitted.
Since each log entry is a dictionary, it can be formatted to any format:
A colorful key/value format for local development,
JSON for easy parsing,
or some standard format you have parsers for like nginx or Apache httpd.
Internally, formatters are processors whose return value (usually a string) is passed into loggers that are responsible for the output of your message.
structlog comes with multiple useful formatters out-of-the-box.
structlog is also very flexible with the final output of your log entries:
A built-in lightweight printer like in the examples above. Easy to use and fast.
Use the standard library’s or Twisted’s logging modules for compatibility. In this case
structlogworks like a wrapper that formats a string and passes them off into existing systems that won’t ever know that
structlogeven exists. Or the other way round:
structlogcomes with a
loggingformatter that allows for processing third party log records.
Don’t format it to a string at all!
structlogpasses you a dictionary and you can do with it whatever you want. Reported uses cases are sending them out via network or saving them in a database.