Structured Logging for Python
structlog: Structured Logging for Python
structlog makes logging in Python less painful and more powerful by adding structure to your log entries.
It’s up to you whether you want structlog to take care about the output of your log entries or whether you prefer to forward them to an existing logging system like the standard library’s logging module.
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) 2016-04-20 16:20.13 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) 2016-04-20 16:20.13 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:
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 structlog works like a wrapper that formats a string and passes them off into existing systems that won’t ever know that structlog even exists. Or the other way round: structlog comes with a logging formatter that allows for processing third party log records.
- Don’t format it to a string at all! structlog passes 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.
Please use the structlog tag on StackOverflow to get help.
Answering questions of your fellow developers is also great way to help the project!
structlog targets Python 2.7, 3.4 and newer, and PyPy.
- As announced in 18.1.0, pip install -e .[dev] now installs all development dependencies. Sorry for the inconveniences this undoubtedly will cause!
- structlog.ReturnLogger and structlog.PrintLogger now have a fatal() log method. #181
- Under certain (rather unclear) circumstances, the frame extraction could throw an SystemError: error return without exception set. A workaround has been added. #174
- structlog now tolerates passing through dicts to stdlib logging. #187 #188 #189
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