structlog 19.1.0

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.

Easier Logging

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!

Data Binding

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'

Powerful Pipelines

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:

• Collectors of call stack information (“How did this log entry happen?”),
• …and exceptions (“What happened‽”).
• Unicode encoders/decoders.
• Flexible timestamping.

Formatting

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.

Output

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.

Getting Help

Please use the structlog tag on StackOverflow to get help.

Answering questions of your fellow developers is also great way to help the project!

Project Information

structlog is dual-licensed under Apache License, version 2 and MIT, available from PyPI, the source code can be found on GitHub, the documentation at https://www.structlog.org/.

structlog targets Python 2.7, 3.4 and newer, and PyPy.

Release Information

19.1.0 (2019-02-02)

Backward-incompatible changes:

• As announced in 18.1.0, pip install -e .[dev] now installs all development dependencies. Sorry for the inconveniences this undoubtedly will cause!

Deprecations:

none

Changes:

• 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

Full changelog.

Authors

structlog is written and maintained by Hynek Schlawack. It’s inspired by previous work done by Jean-Paul Calderone and David Reid.

The development is kindly supported by Variomedia AG.

A full list of contributors can be found on GitHub’s overview. Some of them disapprove of the addition of thread local context data. :)

The structlog logo has been contributed by Russell Keith-Magee.