[utils] traverse_obj: More fixes (#6959)

- Fix result when branching with `traverse_string`
- Fix `slice` path on `dict`s
- Fix tests and docstrings from 21b5ec86c2
- Add `is_iterable_like` helper function

Authored by: Grub4K

yt_dlp/utils.py

@@ -3273,8 +3273,14 @@ def multipart_encode(data, boundary=None):
     return out, content_type
 
 
-def variadic(x, allowed_types=(str, bytes, dict)):
-    return x if isinstance(x, collections.abc.Iterable) and not isinstance(x, allowed_types) else (x,)
+def is_iterable_like(x, allowed_types=collections.abc.Iterable, blocked_types=NO_DEFAULT):
+    if blocked_types is NO_DEFAULT:
+        blocked_types = (str, bytes, collections.abc.Mapping)
+    return isinstance(x, allowed_types) and not isinstance(x, blocked_types)
+
+
+def variadic(x, allowed_types=NO_DEFAULT):
+    return x if is_iterable_like(x, blocked_types=allowed_types) else (x,)
 
 
 def dict_get(d, key_or_keys, default=None, skip_false_values=True):
@@ -5467,7 +5473,7 @@ def traverse_obj(
         obj, *paths, default=NO_DEFAULT, expected_type=None, get_all=True,
         casesense=True, is_user_input=False, traverse_string=False):
     """
-    Safely traverse nested `dict`s and `Sequence`s
+    Safely traverse nested `dict`s and `Iterable`s
 
     >>> obj = [{}, {"key": "value"}]
     >>> traverse_obj(obj, (1, "key"))
@@ -5475,7 +5481,7 @@ def traverse_obj(
 
     Each of the provided `paths` is tested and the first producing a valid result will be returned.
     The next path will also be tested if the path branched but no results could be found.
-    Supported values for traversal are `Mapping`, `Sequence` and `re.Match`.
+    Supported values for traversal are `Mapping`, `Iterable` and `re.Match`.
     Unhelpful values (`{}`, `None`) are treated as the absence of a value and discarded.
 
     The paths will be wrapped in `variadic`, so that `'key'` is conveniently the same as `('key', )`.
@@ -5492,7 +5498,7 @@ def traverse_obj(
                             Read as: `[traverse_obj(obj, branch) for branch in branches]`.
         - `function`:       Branch out and return values filtered by the function.
                             Read as: `[value for key, value in obj if function(key, value)]`.
-                            For `Sequence`s, `key` is the index of the value.
+                            For `Iterable`s, `key` is the index of the value.
                             For `re.Match`es, `key` is the group number (0 = full match)
                             as well as additionally any group names, if given.
         - `dict`            Transform the current object and return a matching dict.
@@ -5540,7 +5546,9 @@ def traverse_obj(
         result = None
 
         if obj is None and traverse_string:
-            pass
+            if key is ... or callable(key) or isinstance(key, slice):
+                branching = True
+                result = ()
 
         elif key is None:
             result = obj
@@ -5563,7 +5571,7 @@ def traverse_obj(
             branching = True
             if isinstance(obj, collections.abc.Mapping):
                 result = obj.values()
-            elif isinstance(obj, collections.abc.Iterable) and not isinstance(obj, (str, bytes)):
+            elif is_iterable_like(obj):
                 result = obj
             elif isinstance(obj, re.Match):
                 result = obj.groups()
@@ -5577,7 +5585,7 @@ def traverse_obj(
             branching = True
             if isinstance(obj, collections.abc.Mapping):
                 iter_obj = obj.items()
-            elif isinstance(obj, collections.abc.Iterable) and not isinstance(obj, (str, bytes)):
+            elif is_iterable_like(obj):
                 iter_obj = enumerate(obj)
             elif isinstance(obj, re.Match):
                 iter_obj = itertools.chain(
@@ -5601,7 +5609,7 @@ def traverse_obj(
             } or None
 
         elif isinstance(obj, collections.abc.Mapping):
-            result = (obj.get(key) if casesense or (key in obj) else
+            result = (try_call(obj.get, args=(key,)) if casesense or try_call(obj.__contains__, args=(key,)) else
                       next((v for k, v in obj.items() if casefold(k) == key), None))
 
         elif isinstance(obj, re.Match):
@@ -5613,7 +5621,7 @@ def traverse_obj(
                 result = next((v for k, v in obj.groupdict().items() if casefold(k) == key), None)
 
         elif isinstance(key, (int, slice)):
-            if isinstance(obj, collections.abc.Sequence) and not isinstance(obj, (str, bytes)):
+            if is_iterable_like(obj, collections.abc.Sequence):
                 branching = isinstance(key, slice)
                 with contextlib.suppress(IndexError):
                     result = obj[key]