[generic] Warn before fallback to automatic search

youtube_dl/__init__.py

@@ -256,7 +256,8 @@ def parseOpts(overrideArguments=None):
     general.add_option(
         '--bidi-workaround', dest='bidi_workaround', action='store_true',
         help=u'Work around terminals that lack bidirectional text support. Requires bidiv or fribidi executable in PATH')
-    general.add_option('--default-search',
+    general.add_option(
+        '--default-search',
         dest='default_search', metavar='PREFIX',
         help='Use this prefix for unqualified URLs. For example "gvsearch2:" downloads two videos from google videos for  youtube-dl "large apple". By default (with value "auto") youtube-dl guesses.')
     general.add_option(
@@ -676,7 +677,7 @@ def _real_main(argv=None):
         date = DateRange.day(opts.date)
     else:
         date = DateRange(opts.dateafter, opts.datebefore)
-    if opts.default_search not in ('auto', None) and ':' not in opts.default_search:
+    if opts.default_search not in ('auto', 'auto_warning', None) and ':' not in opts.default_search:
         parser.error(u'--default-search invalid; did you forget a colon (:) at the end?')
 
     # Do not download videos when there are audio-only formats

youtube_dl/extractor/generic.py

@@ -316,13 +316,16 @@ class GenericIE(InfoExtractor):
         if not parsed_url.scheme:
             default_search = self._downloader.params.get('default_search')
             if default_search is None:
-                default_search = 'auto'
+                default_search = 'auto_warning'
 
-            if default_search == 'auto':
+            if default_search in ('auto', 'auto_warning'):
                 if '/' in url:
                     self._downloader.report_warning('The url doesn\'t specify the protocol, trying with http')
                     return self.url_result('http://' + url)
                 else:
+                    if default_search == 'auto_warning':
+                        self._downloader.report_warning(
+                            'Falling back to youtube search for  %s . Set --default-search to "auto" to suppress this warning.' % url)
                     return self.url_result('ytsearch:' + url)
             else:
                 assert ':' in default_search