feat: extend markdown to support macros

With this feature, the exercises can be added to the lesson as:

    {{ Exercise("two-circles") }}

This also added fenced_code extension that allows adding id and classes
to code blocks.

This uses Python-Markdown library instead of Markdown2 that is used
everywhere in Frappe. The Python-Markdown is more easily extensible than
Markdown2.

Issue #115

community/lms/md.py

@@ -0,0 +1,78 @@
+"""
+The md module extends markdown to add macros.
+
+Macros can be added to the markdown text in the following format.
+
+    {{ MacroName("macro-argument") }}
+
+These macros will be rendered using a pluggable mechanism.
+
+Apps can provide a hook community_markdown_macro_renderers, a
+dictionary mapping the macro name to the function that to render
+that macro. The function will get the argument passed to the macro
+as argument.
+"""
+
+import frappe
+import re
+from bs4 import BeautifulSoup
+import markdown
+from markdown import Extension
+from markdown.inlinepatterns import InlineProcessor
+import xml.etree.ElementTree as etree
+
+def markdown_to_html(text):
+    """Renders markdown text into html.
+    """
+    return markdown.markdown(text, extensions=['fenced_code', MacroExtension()])
+
+def get_macro_registry():
+    d = frappe.get_hooks("community_markdown_macro_renderers") or {}
+    return {name: frappe.get_attr(klass[0]) for name, klass in d.items()}
+
+def render_macro(macro_name, macro_argument):
+    # stripping the quotes on either side of the argument
+    macro_argument = macro_argument.strip(" '\"")
+
+    registry = get_macro_registry()
+    if macro_name in registry:
+        return registry[macro_name](macro_argument)
+    else:
+        return f"<p>Unknown macro: {macro_name}</p>"
+
+class MacroExtension(Extension):
+    """MacroExtension is a markdown extension to support macro syntax.
+    """
+    def extendMarkdown(self, md):
+        self.md = md
+
+        MACRO_RE = r'{{ *(\w+)\(([^{}]*)\) *}}'
+        pattern = MacroInlineProcessor(MACRO_RE)
+        pattern.md = md
+        md.inlinePatterns.register(pattern, 'macro', 75)
+
+class MacroInlineProcessor(InlineProcessor):
+    """MacroInlineProcessor is class that is handles the logic
+    of how to render each macro occurence in the markdown text.
+    """
+    def handleMatch(self, m, data):
+        """Handles each macro match and return rendered contents
+        for that macro as an etree node.
+        """
+        macro = m.group(1)
+        arg = m.group(2)
+        html = render_macro(macro, arg)
+        html = sanitize_html(str(html))
+        e = etree.fromstring(html)
+        return e, m.start(0), m.end(0)
+
+def sanitize_html(html):
+    """Sanotize the html using BeautifulSoup.
+
+    The markdown processor request the correct markup and crashes on
+    any broken tags. This makes sures that all those things are fixed
+    before passing to the etree parser.
+    """
+    soup = BeautifulSoup(html, features="lxml")
+    nodes = soup.body.children
+    return "<div>" + "\n".join(str(node) for node in nodes) + "</div>"

requirements.txt

@@ -1,2 +1,5 @@
 frappe
 websocket_client
+markdown
+beautifulsoup4
+lxml