python logging - how do I truncate the pathname to just the last few characters or just the filename?

Question

I'm using python logging and I have a formatter that looks like the following:

formatter = logging.Formatter(
    '%(asctime)s - %(pathname)86s - %(lineno)4s - %(message)s', '%d %H:%M'
    )

As you can see, I like the information in my log files to line up neatly in columns. The reason I have 86 spaces reserved for the pathname is because the full paths to some of the files used in my program are that long. However, all I really need is the actual file name, not the full path. How can I get the logging module to give me just the file name? Better yet, since I have some long filenames, I'd like the first 3 characters of the filename, followed by '~', then the last 16 characters. So

/Users/Jon/important_dir/dev/my_project/latest/testing-tools/test_read_only_scenarios_happily.py

should become

tes~arios_happily.py

Answer 1

You'll have to implement your own Formatter subclass that truncates the path for you; the formatting string cannot do this:

import logging
import os

class PathTruncatingFormatter(logging.Formatter):
    def format(self, record):
        if isinstance(record.args, dict) and 'pathname' in record.args:
            # truncate the pathname
            filename = os.path.basename(record.args['pathname'])
            if len(filename) > 20:
                filename = '{}~{}'.format(filename[:3], filename[-16:])
            record.args['pathname'] = filename
        return super(PathTruncatingFormatter, self).format(record)

Use this class instead of the normal logging.Formatter instance:

formatter = logging.PathTruncatingFormatter(
    '%(asctime)s - %(pathname)86s - %(lineno)4s - %(message)s', '%d %H:%M'
    )

Answer 2

Like this:

import os

def shorten_filename(filename):
    f = os.path.split(filename)[1]
    return "%s~%s" % (f[:3], f[-16:]) if len(f) > 19 else f