Pyoslog

Pyoslog allows you to send messages to the macOS unified logging system using Python.

from pyoslog import os_log, OS_LOG_DEFAULT
os_log(OS_LOG_DEFAULT, 'Hello from Python!')

Installation

Pyoslog requires macOS 10.12 or later and Python 3.6 or later. Install from PyPI using pip:

python -m pip install pyoslog

The module will install and import without error on earlier macOS versions, or on unsupported Operating Systems or incompatible Python versions. Use pyoslog.is_supported() if you need to support incompatible environments and want to know at runtime whether to use pyoslog. Please note that if is_supported() returns False then none of the module's other methods or constants will exist.

Usage

import pyoslog
if pyoslog.is_supported():
    pyoslog.log('This is an OS_LOG_TYPE_DEFAULT message via pyoslog')

Available methods

Pyoslog provides the following methods from Apple's unified logging header:

• os_log_create
• os_log_type_enabled (and info/debug variants)
• os_log_with_type
• os_log (and info/debug/error/fault variants).

All the pyoslog methods have the same signatures as their native versions, except for where a method requires a format parameter. The os_log system requires a constant (static) format specifier, and it is not possible to achieve this via Python. As a result, all instances of format strings use "%{public}s", and all messages are converted to a string before passing to the native methods.

Pyoslog also offers a helper method – log – that by default posts a message of type OS_LOG_TYPE_DEFAULT to OS_LOG_DEFAULT. For example, the shortcut log('message') is equivalent to os_log_with_type(OS_LOG_DEFAULT, OS_LOG_TYPE_DEFAULT, 'message').

The Handler class is designed for use with Python's inbuilt logging module. It works as a drop-in replacement for other Handler varieties.

See pyoslog's method documentation for a full reference.

Labelling subsystem and category

Create a log object using os_log_create and pass it to any of the log methods to add your own subsystem and category labels:

import pyoslog
log = pyoslog.os_log_create('ac.robinson.pyoslog', 'custom-category')
pyoslog.os_log_with_type(log, pyoslog.OS_LOG_TYPE_DEBUG, 'Message to log object', log, 'of type', pyoslog.OS_LOG_TYPE_DEBUG)

Enabling and disabling log output

Log output can be enabled or disabled globally by switching between the desired log object and pyoslog.OS_LOG_DISABLED:

import pyoslog
log = pyoslog.OS_LOG_DEFAULT
pyoslog.os_log(log, 'Log output enabled')  # will appear in the unified log
log = pyoslog.OS_LOG_DISABLED
pyoslog.os_log(log, 'Log output disabled')  # will not appear in the unified log

It is also possible to check whether individual log types are enabled for a particular log object:

import pyoslog
pyoslog.os_log_type_enabled(pyoslog.OS_LOG_DEFAULT, pyoslog.OS_LOG_TYPE_DEBUG)

It is not possible to directly set a log object's mode from Python, but see the config section of man log for documentation about doing this in sudo mode.

Integration with the logging module

Use the pyoslog Handler to direct messages to pyoslog:

import logging, pyoslog
handler = pyoslog.Handler()
handler.setSubsystem('org.example.your-app', 'filter-category')
logger = logging.getLogger()
logger.addHandler(handler)
logger.error('message')

Logger levels are mapped internally to the OS_LOG_TYPE_* values – for example, logger.debug('message') will generate a message of type OS_LOG_TYPE_DEBUG.

Receiving log messages

Logs can be viewed using Console.app or the log command. For example, messages sent using the default configuration can be streamed using:

log stream --predicate 'processImagePath CONTAINS [c] "python"'

Messages sent using custom log objects can be filtered more precisely. For example, to receive messages from the labelled subsystem used in the example above:

log stream --predicate 'subsystem == "ac.robinson.pyoslog"' --level debug

See the log tool's manpages (man log or online) for further details about the available options and filters.

Handling cleanup

When labelling subsystem and category using the native C methods there is a requirement to free the log object after use (using os_release). The pyoslog module handles this for you – there is no need to del or release these objects.

Limitations

As noted above, while the macOS os_log API allows use of a format string with many methods, this parameter is required to be a C string literal. As a result, pyoslog hardcodes all format strings to "%{public}s".

Testing

The pyoslog module's tests require the pyobjc OSLog framework wrappers and the storeWithScope initialiser in order to verify output so, as a result, can only be run on macOS 12 or later.

After installing the OSLog wrappers (via python -m pip install pyobjc-framework-OSLog), navigate to the tests directory and run:

python -m unittest

All of pyoslog's code is covered by tests, but please note that if Console.app is live-streaming messages, some tests may fail. See test_logging.py for discussion about why this is the case.

Alternatives

At the time this module was created there were no alternatives available on PyPI. Since then, the macos-oslog module has been released, with broadly equivalent functionality to pyoslog, except for the need to manually release the log object. There is also os-signpost, which uses cython to provide the OSSignposter API, and could easily be extended to provide os_log functionality.

In addition, there are other options available if PyPI access is not seen as a constraint:

• apple_os_log_py
• pymacoslog
• loggy

Note that the pyobjc module OSLog is for reading from the unified logging system rather than writing to it (and as a result is used for testing pyoslog). A log.h binding is on that project's roadmap, but not yet implemented.

License

Apache 2.0