import logging

class QueueHandler(logging.Handler):
    """
    This handler sends events to a queue. Typically, it would be used together
    with a multiprocessing Queue to centralise logging to file in one process
    (in a multi-process application), so as to avoid file write contention
    between processes.

    This code is new in Python 3.2, but this class can be copy pasted into
    user code for use with earlier Python versions.
    """

    def __init__(self, queue):
        """
        Initialise an instance, using the passed queue.
        """
        logging.Handler.__init__(self)
        self.queue = queue

    def enqueue(self, record):
        """
        Enqueue a record.

        The base implementation uses put_nowait. You may want to override
        this method if you want to use blocking, timeouts or custom queue
        implementations.
        """
        self.queue.put_nowait(record)

    def prepare(self, record):
        """
        Prepares a record for queueing. The object returned by this
        method is enqueued.
        
        The base implementation formats the record to merge the message
        and arguments, and removes unpickleable items from the record
        in-place.
        
        You might want to override this method if you want to convert
        the record to a dict or JSON string, or send a modified copy
        of the record while leaving the original intact.
        """
        # The format operation gets traceback text into record.exc_text
        # (if there's exception data), and also puts the message into
        # record.message. We can then use this to replace the original
        # msg + args, as these might be unpickleable. We also zap the
        # exc_info attribute, as it's no longer needed and, if not None,
        # will typically not be pickleable.
        self.format(record)
        record.msg = record.message
        record.args = None
        record.exc_info = None
        return record

    def emit(self, record):
        """
        Emit a record.

        Writes the LogRecord to the queue, preparing it first.
        """
        try:
            self.enqueue(self.prepare(record))
        except (KeyboardInterrupt, SystemExit):
            raise
        except:
            self.handleError(record)
