[utils] Fix slicing of reversed LazyList

Closes yt-dlp#589

test/test_utils.py

@@ -1537,8 +1537,11 @@ def test_LazyList(self):
         self.assertEqual(LazyList(it).exhaust(), it)
         self.assertEqual(LazyList(it)[5], it[5])
 
+        self.assertEqual(LazyList(it)[5:], it[5:])
+        self.assertEqual(LazyList(it)[:5], it[:5])
         self.assertEqual(LazyList(it)[::2], it[::2])
         self.assertEqual(LazyList(it)[1::2], it[1::2])
+        self.assertEqual(LazyList(it)[5::-1], it[5::-1])
         self.assertEqual(LazyList(it)[6:2:-2], it[6:2:-2])
         self.assertEqual(LazyList(it)[::-1], it[::-1])
 
@@ -1550,6 +1553,7 @@ def test_LazyList(self):
 
         self.assertEqual(list(LazyList(it).reverse()), it[::-1])
         self.assertEqual(list(LazyList(it).reverse()[1:3:7]), it[::-1][1:3:7])
+        self.assertEqual(list(LazyList(it).reverse()[::-1]), it)
 
     def test_LazyList_laziness(self):
 

yt_dlp/utils.py

@@ -3993,28 +3993,27 @@ def exhaust(self):
 
     @staticmethod
     def __reverse_index(x):
-        return -(x + 1)
+        return None if x is None else -(x + 1)
 
     def __getitem__(self, idx):
         if isinstance(idx, slice):
-            step = idx.step or 1
-            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
-                idx = slice(start, stop, step)
+                idx = slice(self.__reverse_index(idx.start), self.__reverse_index(idx.stop), -(idx.step or 1))
+            start, stop, step = idx.start, idx.stop, idx.step or 1
         elif isinstance(idx, int):
             if self.__reversed:
                 idx = self.__reverse_index(idx)
-            start = stop = idx
+            start, stop, step = idx, idx, 0
         else:
             raise TypeError('indices must be integers or slices')
-        if start < 0 or stop < 0:
+        if ((start or 0) < 0 or (stop or 0) < 0
+                or (start is None and step < 0)
+                or (stop is None and step > 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]
 
-        n = max(start, stop) - len(self.__cache) + 1
+        n = max(start or 0, stop or 0) - len(self.__cache) + 1
         if n > 0:
             self.__cache.extend(itertools.islice(self.__iterable, n))
         return self.__cache[idx]