[utils] Improve `LazyList`

* Add `repr` and `str` that mimics `list`
* Add `reversed`. Unlike `[::-1]`, reversed does not exhaust the iterable and modifies the `LazyList` in-place
* Add tests

test/test_utils.py

@@ -12,6 +12,7 @@ sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 # Various small unit tests
 import io
+import itertools
 import json
 import xml.etree.ElementTree
 
@@ -108,6 +109,7 @@ from yt_dlp.utils import (
     cli_bool_option,
     parse_codecs,
     iri_to_uri,
+    LazyList,
 )
 from yt_dlp.compat import (
     compat_chr,
@@ -1525,6 +1527,47 @@ Line 1
         self.assertEqual(clean_podcast_url('https://www.podtrac.com/pts/redirect.mp3/chtbl.com/track/5899E/traffic.megaphone.fm/HSW7835899191.mp3'), 'https://traffic.megaphone.fm/HSW7835899191.mp3')
         self.assertEqual(clean_podcast_url('https://play.podtrac.com/npr-344098539/edge1.pod.npr.org/anon.npr-podcasts/podcast/npr/waitwait/2020/10/20201003_waitwait_wwdtmpodcast201003-015621a5-f035-4eca-a9a1-7c118d90bc3c.mp3'), 'https://edge1.pod.npr.org/anon.npr-podcasts/podcast/npr/waitwait/2020/10/20201003_waitwait_wwdtmpodcast201003-015621a5-f035-4eca-a9a1-7c118d90bc3c.mp3')
 
+    def test_LazyList(self):
+        it = list(range(10))
+
+        self.assertEqual(list(LazyList(it)), it)
+        self.assertEqual(LazyList(it).exhaust(), it)
+        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)[6:2:-2], it[6:2:-2])
+        self.assertEqual(LazyList(it)[::-1], it[::-1])
+
+        self.assertTrue(LazyList(it))
+        self.assertFalse(LazyList(range(0)))
+        self.assertEqual(len(LazyList(it)), len(it))
+        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])
+
+    def test_LazyList_laziness(self):
+
+        def test(ll, idx, val, cache):
+            self.assertEqual(ll[idx], val)
+            self.assertEqual(getattr(ll, '_LazyList__cache'), list(cache))
+
+        ll = LazyList(range(10))
+        test(ll, 0, 0, range(1))
+        test(ll, 5, 5, range(6))
+        test(ll, -3, 7, range(10))
+
+        ll = reversed(LazyList(range(10)))
+        test(ll, -1, 0, range(1))
+        test(ll, 3, 6, range(10))
+
+        ll = LazyList(itertools.count())
+        test(ll, 10, 10, range(11))
+        reversed(ll)
+        test(ll, -15, 14, range(15))
+
 
 if __name__ == '__main__':
     unittest.main()

yt_dlp/utils.py

@@ -3954,10 +3954,14 @@ class LazyList(collections.Sequence):
     def __init__(self, iterable):
         self.__iterable = iter(iterable)
         self.__cache = []
+        self.__reversed = False
 
     def __iter__(self):
-        for item in self.__cache:
-            yield item
+        if self.__reversed:
+            # We need to consume the entire iterable to iterate in reverse
+            yield from self.exhaust()[::-1]
+            return
+        yield from self.__cache
         for item in self.__iterable:
             self.__cache.append(item)
             yield item
@@ -3965,29 +3969,39 @@ class LazyList(collections.Sequence):
     def exhaust(self):
         ''' Evaluate the entire iterable '''
         self.__cache.extend(self.__iterable)
+        return self.__cache
+
+    @staticmethod
+    def _reverse_index(x):
+        return -(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 1 if step > 0 else -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)
         elif isinstance(idx, int):
+            if self.__reversed:
+                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
-            self.exhaust()
-        else:
-            n = max(start, stop) - len(self.__cache) + 1
-            if n > 0:
-                self.__cache.extend(itertools.islice(self.__iterable, n))
+            return self.exhaust()[idx]
+
+        n = max(start, stop) - len(self.__cache) + 1
+        if n > 0:
+            self.__cache.extend(itertools.islice(self.__iterable, n))
         return self.__cache[idx]
 
     def __bool__(self):
         try:
-            self[0]
+            self[-1] if self.__reversed else self[0]
         except IndexError:
             return False
         return True
@@ -3996,6 +4010,17 @@ class LazyList(collections.Sequence):
         self.exhaust()
         return len(self.__cache)
 
+    def __reversed__(self):
+        self.__reversed = not self.__reversed
+        return self
+
+    def __repr__(self):
+        # repr and str should mimic a list. So we exhaust the iterable
+        return repr(self.exhaust())
+
+    def __str__(self):
+        return repr(self.exhaust())
+
 
 class PagedList(object):
     def __len__(self):