Issue #4116: Make infile and outfile parameters of flrender script from python-abp optional

abp/filters/render_script.py

 # This file is part of Adblock Plus <https://adblockplus.org/>,
 # Copyright (C) 2006-present eyeo GmbH
 #
 # Adblock Plus is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 3 as
 # published by the Free Software Foundation.
 #
 # Adblock Plus is distributed in the hope that it will be useful,
 # but WITHOUT ANY WARRANTY; without even the implied warranty of
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
@@ skipping 12 matching lines @@
 from .sources import FSSource, TopSource, WebSource, NotFound
 from .renderer import render_filterlist, IncludeError, MissingHeader
 
 __all__ = ['main']
 
 
 def parse_args():
     parser = argparse.ArgumentParser(description='Render a filter list.')
     parser.add_argument(
         'infile', help='top level filter list fragment from which the '
-        'rendering starts')
-    parser.add_argument('outfile', help='output filter list file')
+        'rendering starts', default='-', nargs='?')
+    parser.add_argument('outfile', help='output filter list file',
+                        default='-', nargs='?')
     parser.add_argument(
         '-i', '--include', action='append', default=[], metavar='NAME=PATH',
         help='define include path (could be given multiple times)')
     parser.add_argument(
         '-v', '--verbose', action='store_true', default=False,
         help='log included files and URLs')
     return parser.parse_args()
 
 
 def main():
     """Entry point for the rendering script (flrender)."""
     sources = {
         'http': WebSource('http'),
         'https': WebSource('https'),
     }
     args = parse_args()
-
     if args.verbose:
         logging.basicConfig(level=logging.INFO, stream=sys.stderr,
                             format='%(message)s')
 
     for include_path in args.include:
         name, path = include_path.split('=', 1)
         sources[name] = FSSource(path)
 
     try:
         lines = render_filterlist(args.infile, sources, TopSource())
-        with io.open(args.outfile, 'w', encoding='utf-8') as out_fp:
+        if args.outfile == '-':
             for line in lines:
-                out_fp.write(line.to_string() + '\n')
+                sys.stdout.write(line.to_string() + '\n')
+        else:
+            with io.open(args.outfile, 'w', encoding='utf-8') as out_fp:
+                for line in lines:
+                    out_fp.write(line.to_string() + '\n')
     except (MissingHeader, NotFound, IncludeError) as exc:
         sys.exit(exc)