ASAB logging is built on top of a standard Python logging module. It means that it logs to stderr when running on a console and ASAB also provides file and syslog output (both RFC5424 and RFC3164) for background mode of operations.

Log timestamps are captured with sub-second precision (depending on the system capabilities) and displayed including microsecond part.

Logging Levels

ASAB uses Python logging levels with the addition of LOG_NOTICE level. LOG_NOTICE level is similar to logging.INFO level but it is visible in even in non-verbose mode.

L.log(asab.LOG_NOTICE, "This message will be visible regardless verbose configuration.")
Level Numeric value Syslog Severity level
CRITICAL 50 Critical / crit / 2
ERROR 40 Error / err / 3
WARNING 30 Warning / warning / 4
LOG_NOTICE 25 Notice / notice / 5
INFO 20 Informational / info / 6
DEBUG 10 Debug / debug / 7

Verbose mode

The command-line argument -v enables verbose logging. It means that log entries with levels DEBUG and INFO will be visible. It also enables asyncio debug logging.

The actual verbose mode is avaiable at asab.Config["logging"]["verbose"] boolean option.

L.debug("This message will be visible only in verbose mode.")

Structured data

ASAB supports a structured data to be added to a log entry. It follows the RFC 5424, section STRUCTURED-DATA. Structured data are a dictionary, that has to be seriazable to JSON.

L.warning("Hello world!", struct_data={'key1':'value1', 'key2':2})

Example of the output to the console:

25-Mar-2018 23:33:58.044595 WARNING myapp.mymodule [sd key1="value1" key2="2"] Hello world!

Logging to file

The command-line argument -l on command-line enables logging to file. Also non-empty path option in the section [logging:file] of configuration file enables logging to file as well.

Example of the configuration file section:

format="%%(asctime)s %%(levelname)s %%(name)s %%(struct_data)s%%(message)s",
datefmt="%%d-%%b-%%Y %%H:%%M:%%S.%%f"

When the deployment expects more instances of the same application to be logging into the same file, it is recommended, that the variable hostname is used in the file path:


In this way, the applications will log to seperate log files in different folders, which is an intended behavior, since race conditions may occur when different application instances log into the same file.

Logging to console

ASAB will log to the console only if it detects that it runs in the foreground respectively on the terminal using os.isatty or if the environment variable ASABFORCECONSOLE is set to 1. This is useful setup for eg. PyCharm.

Log rotation

ASAB supports a log rotation. The log rotation is triggered by a UNIX signal SIGHUP, which can be used e.g. to integrate with logrotate utility. It is implemented using logging.handlers.RotatingFileHandler from a Python standard library. Also, a time-based log rotation can be configured using rotate_every option.

backup_count specifies a number of old files to be kept prior their removal. The system will save old log files by appending the extensions ‘.1’, ‘.2’ etc., to the filename.

rotate_every specifies an time interval of a log rotation. Default value is empty string, which means that the time-based log rotation is disabled. The interval is specified by an integer value and an unit, e.g. 1d (for 1 day) or 30M (30 minutes). Known units are H for hours, M for minutes, d for days and s for seconds.

Logging to syslog

The command-line argument -s enables logging to syslog.

A configuration section [logging:syslog] can be used to specify details about desired syslog logging.

Example of the configuration file section:


enabled is equivalent to command-line switch -s and it enables syslog logging target.

format speficies which logging format will be used. Possible values are:

  • 5 for (new) syslog format (RFC 5424 ) ,
  • 3 for old BSD syslog format (RFC 3164 ), typically used by /dev/log and
  • m for Mac OSX syslog flavour that is based on BSD syslog format but it is not fully compatible.

The default value is 3 on Linux and m on Mac OSX.

address specifies the location of the Syslog server. It could be a UNIX path such as /dev/log or URL. Possible URL values:

  • tcp://syslog.server.lan:1554/ for Syslog over TCP
  • udp://syslog.server.lan:1554/ for Syslog over UDP
  • unix-connect:///path/to/syslog.socket for Syslog over UNIX socket (stream)
  • unix-sendto:///path/to/syslog.socket for Syslog over UNIX socket (datagram), equivalent to /path/to/syslog.socket, used by a /dev/log.

The default value is a /dev/log on Linux or /var/run/syslog on Mac OSX.


class asab.log.AsyncIOHandler(loop, family, sock_type, address, facility=17)[source]

Bases: logging.Handler

A logging handler similar to a standard logging.handlers.SocketHandler that utilizes asyncio. It implements a queue for decoupling logging from a networking. The networking is fully event-driven via asyncio mechanisms.


This is the entry point for log entries.

class asab.log.Logging(app)[source]

Bases: object

class asab.log.MacOSXSyslogFormatter(fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: asab.log.StructuredDataFormatter

It implements Syslog formatting for Mac OSX syslog (aka format m).

class asab.log.StructuredDataFormatter(facility=16, fmt=None, datefmt=None, style='%', sd_id='sd', use_color: bool = False)[source]

Bases: logging.Formatter

The logging formatter that renders log messages that includes structured data.

BLUE = 4
CYAN = 6
RED = 1
empty_sd = ''

Format the specified record as text.

formatTime(record, datefmt=None)[source]

Return the creation time of the specified LogRecord as formatted text.


Return the string with structured data.

class asab.log.SyslogRFC3164Formatter(fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: asab.log.StructuredDataFormatter

Implementation of a legacy or BSD Syslog (RFC 3164) formatting (aka format 3).

class asab.log.SyslogRFC5424Formatter(fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: asab.log.StructuredDataFormatter

It implements Syslog formatting for Mac OSX syslog (aka format 5).

empty_sd = ' '