[downloader/external] Simplify

youtube_dl/downloader/external.py

@@ -47,8 +47,9 @@ class ExternalFD(FileDownloader):
 
     def _source_address(self, command_option):
         command_part = []
-        if self.ydl.params['source_address'] is not None:
-            command_part = [command_option, self.ydl.params['source_address']]
+        source_address = self.ydl.params.get('source_address')
+        if source_address:
+            command_part = [command_option, source_address]
         return command_part
 
     def _call_downloader(self, tmpfilename, info_dict):