Python logging handler to append to list
Here is a naive, non thread-safe implementation:
import logging
class ListHandler(logging.Handler): # Inherit from logging.Handler
def __init__(self, log_list):
# run the regular Handler __init__
logging.Handler.__init__(self)
# Our custom argument
self.log_list = log_list
def emit(self, record):
# record.message is the log message
self.log_list.append(record.msg)
@imriqwe's answer is correct for a non thread-safe implementation, but if you need to be thread-safe, one solution is to use a queue.Queue()
instead of a list. Here is some code I am using in an in-process project to generate a tkinter log window.
import logging
import queue
class QueuingHandler(logging.Handler):
"""A thread safe logging.Handler that writes messages into a queue object.
Designed to work with LoggingWidget so log messages from multiple
threads can be shown together in a single ttk.Frame.
The standard logging.QueueHandler/logging.QueueListener can not be used
for this because the QueueListener runs in a private thread, not the
main thread.
Warning: If multiple threads are writing into this Handler, all threads
must be joined before calling logging.shutdown() or any other log
destinations will be corrupted.
"""
def __init__(self, *args, message_queue, **kwargs):
"""Initialize by copying the queue and sending everything else to superclass."""
logging.Handler.__init__(self, *args, **kwargs)
self.message_queue = message_queue
def emit(self, record):
"""Add the formatted log message (sans newlines) to the queue."""
self.message_queue.put(self.format(record).rstrip('\n'))
To use, create a queue, create the handler using the queue, then add it to the logger (this example also creates a log file in the current directory):
LOG_FORMAT = '%(asctime)s: %(name)8s: %(levelname)8s: %(message)s'
# Setup root logger to write to a log file.
logging.basicConfig(filename='gui-test.log',
filemode='w',
format=LOG_FORMAT,
level=logging.DEBUG
)
# Get a child logger
logger = logging.getLogger(name='gui')
# Build our QueuingHandler
message_queue = queue.Queue()
handler = QueuingHandler(message_queue=message_queue, level=logging.DEBUG)
# Change the date/time format for the GUI to drop the date
formatter = logging.Formatter(LOG_FORMAT)
formatter.default_time_format = '%H:%M:%S'
handler.setFormatter(formatter)
# Add our QueuingHandler into the logging heirarchy at the lower level
logger.addHandler(handler)
Now all you have to do is read your messages from the queue.