structlog 20.2.0

Structured Logging for Python

structlog makes logging in Python faster, 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.

Once you feel inspired to try it out, check out our friendly Getting Started tutorial that also contains detailed installation instructions!

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.”.

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)
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!

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)
2020-11-18 09:18.28 [info     ] 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 a 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/.

We collect useful third party extension in our wiki.

structlog targets Python 3.6 and newer, and PyPy3.

If you need support for older Python versions, the last release with support for Python 2.7 and 3.5 was 20.1.0. The package meta data should ensure that you get the correct version.

structlog for Enterprise

Available as part of the Tidelift Subscription.

The maintainers of structlog and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use. Learn more.

Release Information

20.2.0 (2020-12-31)

Backward-incompatible changes:

• Python 2.7 and 3.5 aren’t supported anymore. The package meta data should ensure that you keep getting 20.1.0 on those versions. #244

• structlog is now fully type-annotated. This won’t break your applications, but if you use Mypy, it will most likely break your CI.

  Check out the new chapter on typing for details.

Deprecations:

• Accessing the _context attribute of a bound logger is now deprecated. Please use the new structlog.get_context().

Changes:

• structlog has now type hints for all of its APIs! Since structlog is highly dynamic and configurable, this led to a few concessions like a specialized structlog.stdlib.get_logger() whose only difference to structlog.get_logger() is that it has the correct type hints.

  We consider them provisional for the time being – i.e. the backward compatibility does not apply to them in its full strength until we feel we got it right. Please feel free to provide feedback! #223, #282

• Added structlog.make_filtering_logger that can be used like configure(wrapper_class=make_filtering_bound_logger(logging.INFO)). It creates a highly optimized bound logger whose inactive methods only consist of a return None. This is now also the default logger.

• As a complement, structlog.stdlib.add_log_level() can now additionally be imported as structlog.processors.add_log_level since it just adds the method name to the event dict.

• structlog.processors.add_log_level() is now part of the default configuration.

• structlog.stdlib.ProcessorFormatter no longer uses exceptions for control flow, allowing foreign_pre_chain processors to use sys.exc_info() to access the real exception.

• Added structlog.BytesLogger to avoid unnecessary encoding round trips. Concretely this is useful with orjson which returns bytes. #271

• The final processor now also may return bytes that are passed untouched to the wrapped logger.

• structlog.get_context() allows you to retrieve the original context of a bound logger. #266,

• structlog.PrintLogger now supports copy.deepcopy(). #268

• Added structlog.testing.CapturingLogger for more unit testing goodness.

• Added structlog.stdlib.AsyncBoundLogger that executes logging calls in a thread executor and therefore doesn’t block. #245

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.