Format json logs as plain text

jsonlog.py

#!/usr/bin/env python3
"""Read a stream of one-per-line json events and print them in a more human-readable format"""

import json
import sys
import string
import argparse

class SkipFormatter(string.Formatter):
    """A string.Formatter that returns '-' for missing fields"""
    def get_field(self, field_name, args, kwargs):
        try:
            val = super().get_field(field_name, args, kwargs)
        except (KeyError, AttributeError):
            val = None, field_name
        return val

    def format_field(self, value, format_spec):
        if value is None:
            return '-'
        return super().format_field(value, format_spec)

DEFAULT_FORMAT = '{timestamp} {level} [{logger}] {message}'

def printlogs(logformat=DEFAULT_FORMAT, infile=sys.stdin, outfile=sys.stdout, flush=False):
    """Read logs and print them"""
    if isinstance(infile, str):
        infile = open(infile, 'r')
    if isinstance(outfile, str):
        outfile = open(outfile, 'w')

    fmt = SkipFormatter()

    for line in infile:
        line = line.strip()
        try:
            data = json.loads(line)
            log = fmt.format(logformat, **data)
            print(log, file=outfile)
        except json.decoder.JSONDecodeError:
            print('[plain]', line, file=outfile)
        if flush:
            outfile.flush()

def main():
    """Maining the mains since 1976"""
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument('--format', '-f', dest='logformat', default=DEFAULT_FORMAT,
                        help='specify an output format (default: "' + DEFAULT_FORMAT + '")')
    parser.add_argument('--flush', '-F', action='store_true',
                        help='flush output after every log line')
    parser.add_argument('--input', '-i', dest='infile', default=sys.stdin,
                        help='file to write to (default: standard input)')
    parser.add_argument('--output', '-o', dest='outfile', default=sys.stdout,
                        help='file to write to (default: standard output)')
    args = parser.parse_args()
    printlogs(**vars(args))

if __name__ == '__main__':
    main()