Python Standard Library

Ideally, structlog should be able to be used as a drop-in replacement for standard library’s logging by wrapping it. In other words, you should be able to replace your call to logging.getLogger() by a call to structlog.get_logger() and things should keep working as before (if structlog is configured right, see Suggested Configuration below).

If you run into incompatibilities, it is a bug so please take the time to report it! If you’re a heavy logging user, your help to ensure a better compatibility would be highly appreciated!

Concrete Bound Logger

To make structlog’s behavior less magicy, it ships with a standard library-specific wrapper class that has an explicit API instead of improvising: structlog.stdlib.BoundLogger. It behaves exactly like the generic structlog.BoundLogger except:

• it’s slightly faster due to less overhead,
• has an explicit API that mirrors the log methods of standard library’s logging.Logger,
• hence causing less cryptic error messages if you get method names wrong.

Processors

structlog comes with a few standard library-specific processors:

filter_by_level():

Checks the log entry’s log level against the configuration of standard library’s logging. Log entries below the threshold get silently dropped. Put it at the beginning of your processing chain to avoid expensive operations happen in the first place.

add_logger_name():

Adds the name of the logger to the event dictionary under the key logger.

add_log_level():

Adds the log level to the event dictionary under the key level.

PositionalArgumentsFormatter:

This processes and formats positional arguments (if any) passed to log methods in the same way the logging module would do, e.g. logger.info("Hello, %s", name).

Suggested Configuration

A basic configuration to output structured logs in JSON format looks like this:

import structlog

structlog.configure(
    processors=[
        structlog.stdlib.filter_by_level,
        structlog.stdlib.add_logger_name,
        structlog.stdlib.add_log_level,
        structlog.stdlib.PositionalArgumentsFormatter(),
        structlog.processors.TimeStamper(fmt='iso'),
        structlog.processors.StackInfoRenderer(),
        structlog.processors.format_exc_info,
        structlog.processors.JSONRenderer()
    ],
    context_class=dict,
    logger_factory=structlog.stdlib.LoggerFactory(),
    wrapper_class=structlog.stdlib.BoundLogger,
    cache_logger_on_first_use=True,
)

To make your program behave like a proper 12 factor app that outputs only JSON to stdout, configure the logging module like this:

import logging
import sys

handler = logging.StreamHandler(sys.stdout)
root_logger = logging.getLogger()
root_logger.addHandler(handler)

If you plan to hook up the logging output to logstash, as suggested in Logging Best Practices, the simplest approach is to configure logstash-forwarder to pick up the output from your application. To achieve this, configure your process supervisor (such as runit or supervisord) to store the output in a file, and have logstash-forwarder monitor that file to ship it to the central log collection server. This approach also applies to other centralized logging solutions.