allow filtering '--list-extractors' results

with blacklist/whitelist syntax, e.g.

--list-extractors pixiv
--list-extractors pixiv:user pixiv:work
--list-extractors :search

docs/options.md

@@ -49,7 +49,8 @@
                                 values for the given URLs
     -e, --error-file FILE       Add input URLs which returned an error to FILE
     --list-modules              Print a list of available extractor modules
-    --list-extractors           Print a list of extractor classes with
+    --list-extractors CATEGORIES
+                                Print a list of extractor classes with
                                 description, (sub)category and example URL
     --write-log FILE            Write logging output to FILE
     --write-unsupported FILE    Write URLs, which get emitted by other

gallery_dl/__init__.py

@@ -202,12 +202,18 @@ def main():
             extractor.modules.append("")
             sys.stdout.write("\n".join(extractor.modules))
 
-        elif args.list_extractors:
+        elif args.list_extractors is not None:
             write = sys.stdout.write
             fmt = ("{}{}\nCategory: {} - Subcategory: {}"
                    "\nExample : {}\n\n").format
 
-            for extr in extractor.extractors():
+            extractors = extractor.extractors()
+            if args.list_extractors:
+                fltr = util.build_extractor_filter(
+                    args.list_extractors, negate=False)
+                extractors = filter(fltr, extractors)
+
+            for extr in extractors:
                 write(fmt(
                     extr.__name__,
                     "\n" + extr.__doc__ if extr.__doc__ else "",

gallery_dl/option.py

@@ -344,7 +344,7 @@ def build_parser():
     )
     output.add_argument(
         "--list-extractors",
-        dest="list_extractors", action="store_true",
+        dest="list_extractors", metavar="CATEGORIES", nargs="*",
         help=("Print a list of extractor classes "
               "with description, (sub)category and example URL"),
     )

gallery_dl/util.py

@@ -760,8 +760,9 @@ def build_extractor_filter(categories, negate=True, special=None):
     if catsub:
         def test(extr):
             for category, subcategory in catsub:
-                if category in (extr.category, extr.basecategory) and \
+                if subcategory == extr.subcategory and (
-                        subcategory == extr.subcategory:
+                        category == extr.category or
+                        category == extr.basecategory):
                     return not negate
             return negate
         tests.append(test)