Use set instead of list for block level elements

Using a set allows for better performances when checking for membership of a tag within block level elements.

Issue-1507: #1507

Author: pawamoy

markdown/core.py

@@ -114,6 +114,7 @@ def __init__(self, **kwargs):
         """ List of characters which get the backslash escape treatment. """
 
         self.block_level_elements: list[str] = BLOCK_LEVEL_ELEMENTS.copy()
+        self._block_level_elements: set[str] = set(BLOCK_LEVEL_ELEMENTS)
 
         self.registeredExtensions: list[Extension] = []
         self.docType = ""  # TODO: Maybe delete this. It does not appear to be used anymore.
@@ -308,7 +309,7 @@ def is_block_level(self, tag: Any) -> bool:
 
         """
         if isinstance(tag, str):
-            return tag.lower().rstrip('/') in self.block_level_elements
+            return tag.lower().rstrip('/') in self._block_level_elements
         # Some ElementTree tags are not strings, so return False.
         return False
 

markdown/extensions/md_in_html.py

@@ -42,7 +42,7 @@ class HTMLExtractorExtra(HTMLExtractor):
 
     def __init__(self, md: Markdown, *args, **kwargs):
         # All block-level tags.
-        self.block_level_tags = set(md.block_level_elements.copy())
+        self.block_level_tags = set(md.block_level_elements)
         # Block-level tags in which the content only gets span level parsing
         self.span_tags = set(
             ['address', 'dd', 'dt', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'legend', 'li', 'p', 'summary', 'td', 'th']