Some minor fixes and refactoring (see desc)

* [utils] Fix issues with reversal
* check_formats should catch `DownloadError`, not `ExtractorError`
* Simplify format selectors with `LazyList` and `yield from`

3 files changed, 38 insertions, 44 deletions

diff --git a/test/test_utils.py b/test/test_utils.py
index ade10a7b1..0067e1ec9 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -1545,8 +1545,8 @@ Line 1
         self.assertEqual(repr(LazyList(it)), repr(it))
         self.assertEqual(str(LazyList(it)), str(it))
 
-        self.assertEqual(list(reversed(LazyList(it))), it[::-1])
-        self.assertEqual(list(reversed(LazyList(it))[1:3:7]), it[::-1][1:3:7])
+        self.assertEqual(list(LazyList(it).reverse()), it[::-1])
+        self.assertEqual(list(LazyList(it).reverse()[1:3:7]), it[::-1][1:3:7])
 
     def test_LazyList_laziness(self):
 
@@ -1559,13 +1559,13 @@ Line 1
         test(ll, 5, 5, range(6))
         test(ll, -3, 7, range(10))
 
-        ll = reversed(LazyList(range(10)))
+        ll = LazyList(range(10)).reverse()
         test(ll, -1, 0, range(1))
         test(ll, 3, 6, range(10))
 
         ll = LazyList(itertools.count())
         test(ll, 10, 10, range(11))
-        reversed(ll)
+        ll.reverse()
         test(ll, -15, 14, range(15))
 
 
diff --git a/yt_dlp/YoutubeDL.py b/yt_dlp/YoutubeDL.py
index 27d94b63a..785a21e72 100644
--- a/yt_dlp/YoutubeDL.py
+++ b/yt_dlp/YoutubeDL.py
@@ -1758,6 +1758,8 @@ class YoutubeDL(object):
             return new_dict
 
         def _check_formats(formats):
+            if not check_formats:
+                yield from formats
             for f in formats:
                 self.to_screen('[info] Testing format %s' % f['format_id'])
                 temp_file = tempfile.NamedTemporaryFile(
@@ -1765,16 +1767,16 @@ class YoutubeDL(object):
                     dir=self.get_output_path('temp') or None)
                 temp_file.close()
                 try:
-                    dl, _ = self.dl(temp_file.name, f, test=True)
-                except (ExtractorError, IOError, OSError, ValueError) + network_exceptions:
-                    dl = False
+                    success, _ = self.dl(temp_file.name, f, test=True)
+                except (DownloadError, IOError, OSError, ValueError) + network_exceptions:
+                    success = False
                 finally:
                     if os.path.exists(temp_file.name):
                         try:
                             os.remove(temp_file.name)
                         except OSError:
                             self.report_warning('Unable to delete temporary file "%s"' % temp_file.name)
-                if dl:
+                if success:
                     yield f
                 else:
                     self.to_screen('[info] Unable to download format %s. Skipping...' % f['format_id'])
@@ -1785,8 +1787,7 @@ class YoutubeDL(object):
 
                 def selector_function(ctx):
                     for f in fs:
-                        for format in f(ctx):
-                            yield format
+                        yield from f(ctx)
                 return selector_function
 
             elif selector.type == GROUP:  # ()
@@ -1802,22 +1803,24 @@ class YoutubeDL(object):
                             return picked_formats
                     return []
 
+            elif selector.type == MERGE:  # +
+                selector_1, selector_2 = map(_build_selector_function, selector.selector)
+
+                def selector_function(ctx):
+                    for pair in itertools.product(
+                            selector_1(copy.deepcopy(ctx)), selector_2(copy.deepcopy(ctx))):
+                        yield _merge(pair)
+
             elif selector.type == SINGLE:  # atom
                 format_spec = selector.selector or 'best'
 
                 # TODO: Add allvideo, allaudio etc by generalizing the code with best/worst selector
                 if format_spec == 'all':
                     def selector_function(ctx):
-                        formats = list(ctx['formats'])
-                        if check_formats:
-                            formats = _check_formats(formats)
-                        for f in formats:
-                            yield f
+                        yield from _check_formats(ctx['formats'])
                 elif format_spec == 'mergeall':
                     def selector_function(ctx):
-                        formats = ctx['formats']
-                        if check_formats:
-                            formats = list(_check_formats(formats))
+                        formats = list(_check_formats(ctx['formats']))
                         if not formats:
                             return
                         merged_format = formats[-1]
@@ -1855,29 +1858,17 @@ class YoutubeDL(object):
 
                     def selector_function(ctx):
                         formats = list(ctx['formats'])
-                        if not formats:
-                            return
                         matches = list(filter(filter_f, formats)) if filter_f is not None else formats
                         if format_fallback and ctx['incomplete_formats'] and not matches:
                             # for extractors with incomplete formats (audio only (soundcloud)
                             # or video only (imgur)) best/worst will fallback to
                             # best/worst {video,audio}-only format
                             matches = formats
-                        if format_reverse:
-                            matches = matches[::-1]
-                        if check_formats:
-                            matches = list(itertools.islice(_check_formats(matches), format_idx))
-                        n = len(matches)
-                        if -n <= format_idx - 1 < n:
+                        matches = LazyList(_check_formats(matches[::-1 if format_reverse else 1]))
+                        try:
                             yield matches[format_idx - 1]
-
-            elif selector.type == MERGE:        # +
-                selector_1, selector_2 = map(_build_selector_function, selector.selector)
-
-                def selector_function(ctx):
-                    for pair in itertools.product(
-                            selector_1(copy.deepcopy(ctx)), selector_2(copy.deepcopy(ctx))):
-                        yield _merge(pair)
+                        except IndexError:
+                            return
 
             filters = [self._build_format_filter(f) for f in selector.filters]
 
@@ -1971,7 +1962,7 @@ class YoutubeDL(object):
                     t['resolution'] = '%dx%d' % (t['width'], t['height'])
                 t['url'] = sanitize_url(t['url'])
             if self.params.get('check_formats'):
-                info_dict['thumbnails'] = reversed(LazyList(filter(test_thumbnail, thumbnails[::-1])))
+                info_dict['thumbnails'] = LazyList(filter(test_thumbnail, thumbnails[::-1])).reverse()
 
     def process_video_result(self, info_dict, download=True):
         assert info_dict.get('_type', 'video') == 'video'
@@ -3267,7 +3258,7 @@ class YoutubeDL(object):
         multiple = write_all and len(thumbnails) > 1
 
         ret = []
-        for t in thumbnails[::1 if write_all else -1]:
+        for t in thumbnails[::-1]:
             thumb_ext = determine_ext(t['url'], 'jpg')
             suffix = '%s.' % t['id'] if multiple else ''
             thumb_display_id = '%s ' % t['id'] if multiple else ''
diff --git a/yt_dlp/utils.py b/yt_dlp/utils.py
index c9599af53..f0d0097bb 100644
--- a/yt_dlp/utils.py
+++ b/yt_dlp/utils.py
@@ -3976,20 +3976,23 @@ class LazyList(collections.Sequence):
     def __iter__(self):
         if self.__reversed:
             # We need to consume the entire iterable to iterate in reverse
-            yield from self.exhaust()[::-1]
+            yield from self.exhaust()
             return
         yield from self.__cache
         for item in self.__iterable:
             self.__cache.append(item)
             yield item
 
-    def exhaust(self):
-        ''' Evaluate the entire iterable '''
+    def __exhaust(self):
         self.__cache.extend(self.__iterable)
         return self.__cache
 
+    def exhaust(self):
+        ''' Evaluate the entire iterable '''
+        return self.__exhaust()[::-1 if self.__reversed else 1]
+
     @staticmethod
-    def _reverse_index(x):
+    def __reverse_index(x):
         return -(x + 1)
 
     def __getitem__(self, idx):
@@ -3998,18 +4001,18 @@ class LazyList(collections.Sequence):
             start = idx.start if idx.start is not None else 0 if step > 0 else -1
             stop = idx.stop if idx.stop is not None else -1 if step > 0 else 0
             if self.__reversed:
-                start, stop, step = map(self._reverse_index, (start, stop, step))
+                (start, stop), step = map(self.__reverse_index, (start, stop)), -step
                 idx = slice(start, stop, step)
         elif isinstance(idx, int):
             if self.__reversed:
-                idx = self._reverse_index(idx)
+                idx = self.__reverse_index(idx)
             start = stop = idx
         else:
             raise TypeError('indices must be integers or slices')
         if start < 0 or stop < 0:
             # We need to consume the entire iterable to be able to slice from the end
             # Obviously, never use this with infinite iterables
-            return self.exhaust()[idx]
+            return self.__exhaust()[idx]
 
         n = max(start, stop) - len(self.__cache) + 1
         if n > 0:
@@ -4027,7 +4030,7 @@ class LazyList(collections.Sequence):
         self.exhaust()
         return len(self.__cache)
 
-    def __reversed__(self):
+    def reverse(self):
         self.__reversed = not self.__reversed
         return self