Python logging using custom timestamps (not current timestamp)

@m1keil's answer will work for simple cases, but gets cumbersome if you want, for example, to have multiple handlers which format the time in different ways, or if you don't need/want to specify the time every time you make a logging call, etc. Here is a solution which is slightly more complicated, but is the "more correct way", and integrates better with the logging system's normal timestamp handling:

First, you need to create a log filter (filters have the ability to modify log records that pass through loggers they're attached to):

class TimestampFilter (logging.Filter):
    """
    This is a logging filter which will check for a `timestamp` attribute on a
    given LogRecord, and if present it will override the LogRecord creation time
    (and msecs) to be that of the timestamp (specified as a time.time()-style
    value).
    This allows one to override the date/time output for log entries by specifying
    `timestamp` in the `extra` option to the logging call.
    """

    def filter(self, record):
        if hasattr(record, 'timestamp'):
            record.created = record.timestamp
            record.msecs = (record.timestamp % 1) * 1000
        return True

Then, simply create an instance of the filter and add it to an appropriate logger instance:

logger = logging.getLogger(__name__)
filter = TimestampFilter()
logger.addFilter(filter)

Then when you want to override the date/time of a log entry, when making the logging call, supply a timestamp in extra:

my_timestamp = time.time() + 86400  # Let's pretend it's tomorrow already
logger.warn("I am a warning from the future!", extra={'timestamp': my_timestamp})

You can do that simply by:

logging.basicConfig(level=logging.DEBUG,
                    format='%(date)s %(name)-12s %(levelname)-8s %(message)s',
                    filename=my_filename,
                    filemode='w')

and logging with:

logging.info(msg="Set up logger", extra={'date':'2010-01-01 19:10:22,762'})

Do note that since you hardcoded the date argument in the format, a log message without one will fail. If you want to make it optional, you'll need to use something like filters to modify your log message on the fly.