python logging string formatting

I am using python's log formatter to format log records and i have a fmt value of

fmt = "[%(filename)s:%(lineno)s] %(message)s"

What i would like is that "[file.py:20]" to be stretched to 10 characters wide (for example). If it was one valu开发者_JAVA技巧e that would have been easy but is there any way to stretch this entire structure to a specified length? I want something like:

tmp = "[%(filename)s:%(lineno)s]"
fmt = "%(tmp)10s %(message)s"

I would like to know if this is possible using string formatting or if I can trick python's formatter somehow to get what i want..

As an example, this Formatter ensures a fixed width "[%(filename)s:%(lineno)s]" by either truncating the filename, or right-padding (after the line number) with spaces.

class MyFormatter(logging.Formatter):
    width = 10

    def format(self, record):
        max_filename_width = self.width - 3 - len(str(record.lineno))
        filename = record.filename
        if len(record.filename) > max_filename_width:
            filename = record.filename[:max_filename_width]
        a = "%s:%s" % (filename, record.lineno)
        return "[%s] %s" % (a.ljust(self.width), record.msg)

if __name__ == '__main__':
    logger = logging.getLogger('simple_example')
    logger.setLevel(logging.DEBUG)
    ch = logging.StreamHandler()
    ch.setLevel(logging.DEBUG)
    formatter = MyFormatter()
    ch.setFormatter(formatter)
    logger.addHandler(ch)

    logger.debug('No one expects the spammish repetition')

EDIT:

If you want to ensure a minimum width of 10 characters, ditch the filename stuff.

def format(self, record):
    a = "%s:%s" % (record.filename, record.lineno)
    return "[%s] %s" % (a.ljust(self.width), record.msg)

Option 1

Start here: http://docs.python.org/library/logging.html#formatter-objects

You'll create your own customized subclass of Formatter that provides it's own unique format method.

Then you must be sure to call setFormatter() in each of your Handlers so that they use your new formatter.

Option 2

Create your own subclass of LogRecord with the additional property.

Subclass Logger and override makeRecord to create your new subclass of LogRecord.

Provide a customized format that uses this new property value.

Using @rob-cowie's answer as a basis, I've found the following useful:

class MyFormatter(logging.Formatter):
    width = 24
    datefmt='%Y-%m-%d %H:%M:%S'

    def format(self, record):
        cpath = '%s:%s:%s' % (record.module, record.funcName, record.lineno)
        cpath = cpath[-self.width:].ljust(self.width)
        record.message = record.getMessage()
        s = "%-7s %s %s : %s" % (record.levelname, self.formatTime(record, self.datefmt), cpath, record.getMessage())
        if record.exc_info:
            # Cache the traceback text to avoid converting it multiple times
            # (it's constant anyway)
            if not record.exc_text:
                record.exc_text = self.formatException(record.exc_info)
        if record.exc_text:
            if s[-1:] != "\n":
                s = s + "\n"
            s = s + record.exc_text
        #if record.stack_info:
        #    if s[-1:] != "\n":
        #        s = s + "\n"
        #    s = s + self.formatStack(record.stack_info)
        return s

logFormatter = MyFormatter()
logger = logging.getLogger("example")
logger.setFormatter(logFormatter)

Which gives output like:

WARNING 2014-03-28 16:05:09 module:function:31       : Message
WARNING 2014-03-28 16:05:09 dule:longerfunctions:140 : Message