fixup! Use set instead of list for block level elements

Author: pawamoy

markdown/core.py

@@ -113,6 +113,12 @@ def __init__(self, **kwargs):
         ]
         """ List of characters which get the backslash escape treatment. """
 
+
+        # `BLOCK_LEVEL_ELEMENTS` is actually a hybrid list/set container.
+        # It supports list methods for backwards compatibility.
+        # We expliclitly lie here, so that users running type checkers will get
+        # warnings when they use the container as a list. This is a very effective
+        # way of communicating the change, and deprecating list-like usage.
         self.block_level_elements: set[str] = BLOCK_LEVEL_ELEMENTS.copy()
 
         self.registeredExtensions: list[Extension] = []

markdown/util.py

@@ -312,6 +312,9 @@ def update(self, *others: set[str]) -> None:
 
 
 # Type it as `set[str]` to express our intent for it to be used as such.
+# We expliclitly lie here, so that users running type checkers will get
+# warnings when they use the container as a list. This is a very effective
+# way of communicating the change, and deprecating list-like usage.
 BLOCK_LEVEL_ELEMENTS: set[str] = _BlockLevelElements([
     # Elements which are invalid to wrap in a `<p>` tag.
     # See https://w3c.github.io/html/grouping-content.html#the-p-element