allow init of playlist from id

pytube/contrib/playlist.py

@@ -24,7 +24,11 @@ class Playlist:
         if proxies:
             install_proxy(proxies)
 
-        self.playlist_id: str = parse_qs(url.split("?")[1])["list"][0]
+        try:
+            self.playlist_id: str = parse_qs(url.split("?")[1])["list"][0]
+        except IndexError:  # assume that url is just the id
+            self.playlist_id = url
+
         self.playlist_url: str = (
             "https://www.youtube.com/playlist?list=" + self.playlist_id
         )

tests/contrib/test_playlist.py

@@ -39,6 +39,16 @@ def test_init_with_watch_url(request_get):
     )
 
 
+@mock.patch("pytube.contrib.playlist.request.get")
+def test_init_with_watch_id(request_get):
+    request_get.return_value = ""
+    playlist = Playlist("PLS1QulWo1RIaJECMeUT4LFwJ-ghgoSH6n")
+    assert (
+        playlist.playlist_url
+        == "https://www.youtube.com/playlist?list=PLS1QulWo1RIaJECMeUT4LFwJ-ghgoSH6n"
+    )
+
+
 @mock.patch("pytube.contrib.playlist.request.get")
 def test_parse_links(request_get, playlist_html):
     url = "https://www.fakeurl.com/playlist?list=whatever"