[YoutubeDL] Improve _default_format_spec (closes #14461)

test/test_YoutubeDL.py

@@ -466,12 +466,18 @@ class TestFormatSelection(unittest.TestCase):
         ydl = YDL({'simulate': True})
         self.assertEqual(ydl._default_format_spec({}), 'bestvideo+bestaudio/best')
 
+        ydl = YDL({'is_live': True})
+        self.assertEqual(ydl._default_format_spec({}), 'best/bestvideo+bestaudio')
+
+        ydl = YDL({'simulate': True, 'is_live': True})
+        self.assertEqual(ydl._default_format_spec({}), 'bestvideo+bestaudio/best')
+
         ydl = YDL({'outtmpl': '-'})
-        self.assertEqual(ydl._default_format_spec({}), 'best')
+        self.assertEqual(ydl._default_format_spec({}), 'best/bestvideo+bestaudio')
 
         ydl = YDL({})
         self.assertEqual(ydl._default_format_spec({}, download=False), 'bestvideo+bestaudio/best')
-        self.assertEqual(ydl._default_format_spec({'is_live': True}), 'best')
+        self.assertEqual(ydl._default_format_spec({'is_live': True}), 'best/bestvideo+bestaudio')
 
 
 class TestYoutubeDL(unittest.TestCase):

youtube_dl/YoutubeDL.py

@@ -1078,22 +1078,27 @@ class YoutubeDL(object):
         return _filter
 
     def _default_format_spec(self, info_dict, download=True):
-        req_format_list = []
 
-        def can_have_partial_formats():
+        def can_merge():
+            merger = FFmpegMergerPP(self)
+            return merger.available and merger.can_merge()
+
+        def prefer_best():
             if self.params.get('simulate', False):
-                return True
+                return False
             if not download:
-                return True
-            if self.params.get('outtmpl', DEFAULT_OUTTMPL) == '-':
                 return False
+            if self.params.get('outtmpl', DEFAULT_OUTTMPL) == '-':
+                return True
             if info_dict.get('is_live'):
-                return False
-            merger = FFmpegMergerPP(self)
-            return merger.available and merger.can_merge()
-        if can_have_partial_formats():
-            req_format_list.append('bestvideo+bestaudio')
-        req_format_list.append('best')
+                return True
+            if not can_merge():
+                return True
+            return False
+
+        req_format_list = ['bestvideo+bestaudio', 'best']
+        if prefer_best():
+            req_format_list.reverse()
         return '/'.join(req_format_list)
 
     def build_format_selector(self, format_spec):