[Audio] Playlist download addition (#2482)

Bot owners can use `[p]playlist download playlistname` to download a playlist file that is compatible with the playlist upload command. Songs can also be exported for v2 use by using `[p]playlist download playlistname True`, which strips out tracks besides YouTube or Soundcloud URLs.

redbot/cogs/audio/audio.py

@@ -4,6 +4,8 @@ import datetime
 import discord
 from fuzzywuzzy import process
 import heapq
+from io import StringIO
+import json
 import lavalink
 import logging
 import math
@@ -1248,6 +1250,44 @@ class Audio(commands.Cog):
                 return await self._embed_msg(ctx, _("No playlist with that name."))
         await self._embed_msg(ctx, _("{name} playlist deleted.").format(name=playlist_name))
 
+    @checks.is_owner()
+    @playlist.command(name="download")
+    async def _playlist_download(self, ctx, playlist_name, v2=False):
+        """Download a copy of a playlist.
+
+        These files can be used with the [p]playlist upload command.
+        Red v2-compatible playlists can be generated by passing True
+        for the v2 variable."""
+        if not await self._playlist_check(ctx):
+            return
+        playlists = await self.config.guild(ctx.guild).playlists.get_raw()
+        v2_valid_urls = ["https://www.youtube.com/watch?v=", "https://soundcloud.com/"]
+        song_list = []
+        playlist_url = None
+
+        try:
+            if playlists[playlist_name]["playlist_url"]:
+                playlist_url = playlists[playlist_name]["playlist_url"]
+            for track in playlists[playlist_name]["tracks"]:
+                if v2:
+                    if track["info"]["uri"].startswith(tuple(v2_valid_urls)):
+                        song_list.append(track["info"]["uri"])
+                else:
+                    song_list.append(track["info"]["uri"])
+        except TypeError:
+            return await self._embed_msg(ctx, _("That playlist has no tracks."))
+        except KeyError:
+            return await self._embed_msg(ctx, _("That playlist doesn't exist."))
+
+        playlist_data = json.dumps(
+            {"author": ctx.author.id, "link": playlist_url, "playlist": song_list}
+        )
+        to_write = StringIO()
+        to_write.write(playlist_data)
+        to_write.seek(0)
+        await ctx.send(file=discord.File(to_write, filename=f"{playlist_name}.txt"))
+        to_write.close()
+
     @playlist.command(name="info")
     async def _playlist_info(self, ctx, playlist_name):
         """Retrieve information from a saved playlist."""