Add YAML support to Meta extension

By default, this only supports YAML deliminators (`---`) and adds no
additional behavior. In other words, parsing is unchanged. However, with
the `yaml` option set, PyYAML will parse the metadata.

Thanks to @kernc for suggesting the idea and doing the work on this.

1 files changed, 41 insertions, 7 deletions

diff --git a/markdown/extensions/meta.py b/markdown/extensions/meta.py
index a733a8a..cf8074f 100644
--- a/markdown/extensions/meta.py
+++ b/markdown/extensions/meta.py
@@ -20,36 +20,68 @@ from __future__ import unicode_literals
 from . import Extension
 from ..preprocessors import Preprocessor
 import re
+import logging
+
+try:  # pragma: no cover
+    import yaml
+    try:
+        from yaml import CSafeLoader as SafeLoader
+    except ImportError:
+        from yaml import SafeLoader
+except ImportError:
+    yaml = None
+
+log = logging.getLogger('MARKDOWN')
 
 # Global Vars
 META_RE = re.compile(r'^[ ]{0,3}(?P<key>[A-Za-z0-9_-]+):\s*(?P<value>.*)')
 META_MORE_RE = re.compile(r'^[ ]{4,}(?P<value>.*)')
+YAML_BEGIN_RE = re.compile(r'^-{3}(\s.*)?')
+YAML_END_RE = re.compile(r'^(-{3}|\.{3})(\s.*)?')
 
 
 class MetaExtension (Extension):
     """ Meta-Data extension for Python-Markdown. """
+    def __init__(self, *args, **kwargs):
+        self.config = {
+            'yaml': [False, "Parse meta data specified as a "
+                            "'---' delimited YAML front matter"],
+            }
+        super(MetaExtension, self).__init__(*args, **kwargs)
 
     def extendMarkdown(self, md, md_globals):
         """ Add MetaPreprocessor to Markdown instance. """
-
-        md.preprocessors.add(
-            "meta", MetaPreprocessor(md), ">normalize_whitespace"
-        )
+        md.preprocessors.add("meta",
+                             MetaPreprocessor(md, self.getConfigs()),
+                             ">normalize_whitespace")
 
 
 class MetaPreprocessor(Preprocessor):
     """ Get Meta-Data. """
 
+    def __init__(self, md, config):
+        self.config = config
+        super(MetaPreprocessor, self).__init__(md)
+
     def run(self, lines):
         """ Parse Meta-Data and store in Markdown.Meta. """
         meta = {}
         key = None
+        yaml_block = []
+        have_yaml = False
+        if lines and YAML_BEGIN_RE.match(lines[0]):
+            have_yaml = True
+            lines.pop(0)
+            if self.config['yaml'] and not yaml:  # pragma: no cover
+                log.warning('Document with YAML header, but PyYAML unavailable')
         while lines:
             line = lines.pop(0)
-            if line.strip() == '':
-                break  # blank line - done
             m1 = META_RE.match(line)
-            if m1:
+            if line.strip() == '' or have_yaml and YAML_END_RE.match(line):
+                break  # blank line or end of YAML header - done
+            elif have_yaml and self.config['yaml'] and yaml:
+                yaml_block.append(line)
+            elif m1:
                 key = m1.group('key').lower().strip()
                 value = m1.group('value').strip()
                 try:
@@ -64,6 +96,8 @@ class MetaPreprocessor(Preprocessor):
                 else:
                     lines.insert(0, line)
                     break  # no meta data - done
+        if yaml_block:
+            meta = yaml.load('\n'.join(yaml_block), SafeLoader)
         self.markdown.Meta = meta
         return lines