Add slicing notation to --playlist-items

* Adds support for negative indices and step
* Add `-I` as alias for `--playlist-index`
* Deprecates `--playlist-start`, `--playlist-end`, `--playlist-reverse`, `--no-playlist-reverse`

Closes #2951, Closes #2853

yt_dlp/utils.py

@@ -2609,6 +2609,16 @@ def get_exe_version(exe, args=['--version'],
     return detect_exe_version(out, version_re, unrecognized) if out else False
 
 
+def frange(start=0, stop=None, step=1):
+    """Float range"""
+    if stop is None:
+        start, stop = 0, start
+    sign = [-1, 1][step > 0] if step else 0
+    while sign * start < sign * stop:
+        yield start
+        start += step
+
+
 class LazyList(collections.abc.Sequence):
     """Lazy immutable list from an iterable
     Note that slices of a LazyList are lists and not LazyList"""
@@ -2805,6 +2815,148 @@ class InAdvancePagedList(PagedList):
             yield from page_results
 
 
+class PlaylistEntries:
+    MissingEntry = object()
+    is_exhausted = False
+
+    def __init__(self, ydl, info_dict):
+        self.ydl, self.info_dict = ydl, info_dict
+
+    PLAYLIST_ITEMS_RE = re.compile(r'''(?x)
+        (?P<start>[+-]?\d+)?
+        (?P<range>[:-]
+            (?P<end>[+-]?\d+|inf(?:inite)?)?
+            (?::(?P<step>[+-]?\d+))?
+        )?''')
+
+    @classmethod
+    def parse_playlist_items(cls, string):
+        for segment in string.split(','):
+            if not segment:
+                raise ValueError('There is two or more consecutive commas')
+            mobj = cls.PLAYLIST_ITEMS_RE.fullmatch(segment)
+            if not mobj:
+                raise ValueError(f'{segment!r} is not a valid specification')
+            start, end, step, has_range = mobj.group('start', 'end', 'step', 'range')
+            if int_or_none(step) == 0:
+                raise ValueError(f'Step in {segment!r} cannot be zero')
+            yield slice(int_or_none(start), float_or_none(end), int_or_none(step)) if has_range else int(start)
+
+    def get_requested_items(self):
+        playlist_items = self.ydl.params.get('playlist_items')
+        playlist_start = self.ydl.params.get('playliststart', 1)
+        playlist_end = self.ydl.params.get('playlistend')
+        # For backwards compatibility, interpret -1 as whole list
+        if playlist_end in (-1, None):
+            playlist_end = ''
+        if not playlist_items:
+            playlist_items = f'{playlist_start}:{playlist_end}'
+        elif playlist_start != 1 or playlist_end:
+            self.ydl.report_warning('Ignoring playliststart and playlistend because playlistitems was given', only_once=True)
+
+        for index in self.parse_playlist_items(playlist_items):
+            for i, entry in self[index]:
+                yield i, entry
+                try:
+                    # TODO: Add auto-generated fields
+                    self.ydl._match_entry(entry, incomplete=True, silent=True)
+                except (ExistingVideoReached, RejectedVideoReached):
+                    return
+
+    @property
+    def full_count(self):
+        if self.info_dict.get('playlist_count'):
+            return self.info_dict['playlist_count']
+        elif self.is_exhausted and not self.is_incomplete:
+            return len(self)
+        elif isinstance(self._entries, InAdvancePagedList):
+            if self._entries._pagesize == 1:
+                return self._entries._pagecount
+
+    @functools.cached_property
+    def _entries(self):
+        entries = self.info_dict.get('entries')
+        if entries is None:
+            raise EntryNotInPlaylist('There are no entries')
+        elif isinstance(entries, list):
+            self.is_exhausted = True
+
+        indices = self.info_dict.get('requested_entries')
+        self.is_incomplete = bool(indices)
+        if self.is_incomplete:
+            assert self.is_exhausted
+            ret = [self.MissingEntry] * max(indices)
+            for i, entry in zip(indices, entries):
+                ret[i - 1] = entry
+            return ret
+
+        if isinstance(entries, (list, PagedList, LazyList)):
+            return entries
+        return LazyList(entries)
+
+    @functools.cached_property
+    def _getter(self):
+        if isinstance(self._entries, list):
+            def get_entry(i):
+                try:
+                    entry = self._entries[i]
+                except IndexError:
+                    entry = self.MissingEntry
+                    if not self.is_incomplete:
+                        raise self.IndexError()
+                if entry is self.MissingEntry:
+                    raise EntryNotInPlaylist(f'Entry {i} cannot be found')
+                return entry
+        else:
+            def get_entry(i):
+                try:
+                    return type(self.ydl)._handle_extraction_exceptions(lambda _, i: self._entries[i])(self.ydl, i)
+                except (LazyList.IndexError, PagedList.IndexError):
+                    raise self.IndexError()
+        return get_entry
+
+    def __getitem__(self, idx):
+        if isinstance(idx, int):
+            idx = slice(idx, idx)
+
+        # NB: PlaylistEntries[1:10] => (0, 1, ... 9)
+        step = 1 if idx.step is None else idx.step
+        if idx.start is None:
+            start = 0 if step > 0 else len(self) - 1
+        else:
+            start = idx.start - 1 if idx.start >= 0 else len(self) + idx.start
+
+        # NB: Do not call len(self) when idx == [:]
+        if idx.stop is None:
+            stop = 0 if step < 0 else float('inf')
+        else:
+            stop = idx.stop - 1 if idx.stop >= 0 else len(self) + idx.stop
+        stop += [-1, 1][step > 0]
+
+        for i in frange(start, stop, step):
+            if i < 0:
+                continue
+            try:
+                try:
+                    entry = self._getter(i)
+                except self.IndexError:
+                    self.is_exhausted = True
+                    if step > 0:
+                        break
+                    continue
+            except IndexError:
+                if self.is_exhausted:
+                    break
+                raise
+            yield i + 1, entry
+
+    def __len__(self):
+        return len(tuple(self[:]))
+
+    class IndexError(IndexError):
+        pass
+
+
 def uppercase_escape(s):
     unicode_escape = codecs.getdecoder('unicode_escape')
     return re.sub(