new_contents: #!/usr/bin/env python
  from __future__ import absolute_import
  
  import argparse
  import sys
  from subprocess import check_call
  from typing import List, Dict, Union
  
  from .lister import list_files
  
  def do_replace(listing, old_string, new_string):
      # type: (Union[Dict[str, List[str]], List[str]], str, str) -> None
      for filename in listing:
          regex = 's/{}/{}/g'.format(old_string, new_string)
          check_call(['sed', '-i', regex, filename])
  
  if __name__ == "__main__":
      parser = argparse.ArgumentParser(description='Replace string in files')
      parser.add_argument('old_string', help="String to replace")
      parser.add_argument('new_string', help="String to replace with")
      parser.add_argument('targets', nargs='*', default=[],
                          help='files and directories to include in the result. '
                               'If this is not specified, the current directory '
                               'is used')
      parser.add_argument('-m', '--modified', action='store_true',
                          default=False, help='list only modified files')
      parser.add_argument('-f', '--ftypes', nargs='+', default=[],
                          help='list of file types to filter on. All files are '
                               'included if this option is absent')
      parser.add_argument('--ext-only', dest='extonly', action='store_true',
                          default=False,
                          help='only use extension to determine file type')
      parser.add_argument('--exclude', nargs='+', default=[],
                          help='list of files and directories to exclude from '
                               'results, relative to repo root')
      parser.add_argument('--extless-only', dest='extless_only',
                          action='store_true', default=False,
                          help='only include extensionless files in output')
      args = parser.parse_args()
  
      listing = list_files(targets=args.targets, ftypes=args.ftypes,
                           use_shebang=not args.extonly,
                           modified_only=args.modified,
                           exclude=args.exclude,
                           extless_only=args.extless_only)
  
      do_replace(listing, args.old_string, args.new_string)
  
subject: Add inline string replacer for files.