New option --get-filename to print output filename

When using youtube-dl within scripts, it is very useful to know
what will be the final output filename with all the title settings and
filename templates applied. Add option to the quiet mode operations
to print that info.

For this I had to move the filename-generation into its own function.
As much as I can tell it should work almost always well, ie. not to
break things if one not actually interested in the title, like in
case of other forced printing. That is, unless there's an invalid
system charset or the user specified a wrong output template. In that
case probably could be assumed that the user does have a problem
(the former) or did want to mess with the filename (the latter).

Signed-off-by: Gergely Imreh <imrehg@gmail.com>

youtube-dl

@@ -258,6 +258,7 @@ class FileDownloader(object):
 	forcetitle:       Force printing title.
 	forcethumbnail:   Force printing thumbnail URL.
 	forcedescription: Force printing description.
+	forcefilename:    Force printing final filename.
 	simulate:         Do not download the video files.
 	format:           Video format code.
 	format_limit:     Highest quality format to try.
@@ -494,8 +495,21 @@ class FileDownloader(object):
 		"""Increment the ordinal that assigns a number to each file."""
 		self._num_downloads += 1
 
+	def prepare_filename(self, info_dict):
+		"""Generate the output filename."""
+		try:
+			template_dict = dict(info_dict)
+			template_dict['epoch'] = unicode(long(time.time()))
+			template_dict['autonumber'] = unicode('%05d' % self._num_downloads)
+			filename = self.params['outtmpl'] % template_dict
+			return filename
+		except (ValueError, KeyError), err:
+			self.trouble(u'ERROR: invalid system charset or erroneous output template')
+			return None
+
 	def process_info(self, info_dict):
 		"""Process a single dictionary returned by an InfoExtractor."""
+		filename = self.prepare_filename(info_dict)
 		# Do nothing else if in simulate mode
 		if self.params.get('simulate', False):
 			# Forced printings
@@ -507,16 +521,12 @@ class FileDownloader(object):
 				print info_dict['thumbnail'].encode(preferredencoding(), 'xmlcharrefreplace')
 			if self.params.get('forcedescription', False) and 'description' in info_dict:
 				print info_dict['description'].encode(preferredencoding(), 'xmlcharrefreplace')
+			if self.params.get('forcefilename', False) and filename is not None:
+				print filename.encode(preferredencoding(), 'xmlcharrefreplace')
 
 			return
 
-		try:
-			template_dict = dict(info_dict)
-			template_dict['epoch'] = unicode(long(time.time()))
-			template_dict['autonumber'] = unicode('%05d' % self._num_downloads)
-			filename = self.params['outtmpl'] % template_dict
-		except (ValueError, KeyError), err:
-			self.trouble(u'ERROR: invalid system charset or erroneous output template')
+		if filename is None:
 			return
 		if self.params.get('nooverwrites', False) and os.path.exists(filename):
 			self.to_stderr(u'WARNING: file exists and will be skipped')
@@ -2384,6 +2394,8 @@ if __name__ == '__main__':
 				action='store_true', dest='getthumbnail', help='simulate, quiet but print thumbnail URL', default=False)
 		verbosity.add_option('--get-description',
 				action='store_true', dest='getdescription', help='simulate, quiet but print video description', default=False)
+		verbosity.add_option('--get-filename',
+				action='store_true', dest='getfilename', help='simulate, quiet but print output filename', default=False)
 		verbosity.add_option('--no-progress',
 				action='store_true', dest='noprogress', help='do not print progress bar', default=False)
 		verbosity.add_option('--console-title',
@@ -2503,12 +2515,13 @@ if __name__ == '__main__':
 			'usenetrc': opts.usenetrc,
 			'username': opts.username,
 			'password': opts.password,
-			'quiet': (opts.quiet or opts.geturl or opts.gettitle or opts.getthumbnail or opts.getdescription),
+			'quiet': (opts.quiet or opts.geturl or opts.gettitle or opts.getthumbnail or opts.getdescription or opts.getfilename),
 			'forceurl': opts.geturl,
 			'forcetitle': opts.gettitle,
 			'forcethumbnail': opts.getthumbnail,
 			'forcedescription': opts.getdescription,
-			'simulate': (opts.simulate or opts.geturl or opts.gettitle or opts.getthumbnail or opts.getdescription),
+			'forcefilename': opts.getfilename,
+			'simulate': (opts.simulate or opts.geturl or opts.gettitle or opts.getthumbnail or opts.getdescription or opts.getfilename),
 			'format': opts.format,
 			'format_limit': opts.format_limit,
 			'outtmpl': ((opts.outtmpl is not None and opts.outtmpl.decode(preferredencoding()))