[downloader/dash] Do not pollute ```self```

youtube_dl/downloader/dash.py

@@ -16,14 +16,14 @@ class DashSegmentsFD(FileDownloader):
         base_url = info_dict['url']
         segment_urls = info_dict['segment_urls']
 
-        self.byte_counter = 0
+        byte_counter = 0
 
         def append_url_to_file(outf, target_url, target_name):
             self.to_screen('[DashSegments] %s: Downloading %s' % (info_dict['id'], target_name))
             req = compat_urllib_request.Request(target_url)
             data = self.ydl.urlopen(req).read()
             outf.write(data)
-            self.byte_counter += len(data)
+            return len(data)
 
         def combine_url(base_url, target_url):
             if re.match(r'^https?://', target_url):
@@ -35,15 +35,16 @@ class DashSegmentsFD(FileDownloader):
                 outf, combine_url(base_url, info_dict['initialization_url']),
                 'initialization segment')
             for i, segment_url in enumerate(segment_urls):
-                append_url_to_file(
+                segment_len = append_url_to_file(
                     outf, combine_url(base_url, segment_url),
                     'segment %d / %d' % (i + 1, len(segment_urls)))
+                byte_counter += segment_len
 
         self.try_rename(tmpfilename, filename)
 
         self._hook_progress({
-            'downloaded_bytes': self.byte_counter,
+            'downloaded_bytes': byte_counter,
-            'total_bytes': self.byte_counter,
+            'total_bytes': byte_counter,
             'filename': filename,
             'status': 'finished',
         })