Make it work

docs/manual.txt

@@ -527,7 +527,7 @@ Markdown metadata
 
 Markdown Metadata only works in Markdown files, and requires the ``markdown.extensions.meta`` extension
 (see `MARKDOWN_EXTENSIONS <#markdown>`__). The exact format is described in
-the `markdown metadata extension docs <https://pythonhosted.org/Markdown/extensions/meta_data.html>`__
+the `markdown metadata extension docs. <https://pythonhosted.org/Markdown/extensions/meta_data.html>`__
 
 .. code:: text
 

nikola/nikola.py

@@ -546,6 +546,7 @@ def __init__(self, **config):
             'MARKDOWN_EXTENSIONS': ['fenced_code', 'codehilite'],  # FIXME: Add 'extras' in v8
             'MAX_IMAGE_SIZE': 1280,
             'MATHJAX_CONFIG': '',
+            'METADATA_FORMAT': 'Nikola',
             'METADATA_MAPPING': {},
             'NEW_POST_DATE_PATH': False,
             'NEW_POST_DATE_PATH_FORMAT': '%Y/%m/%d',
@@ -960,6 +961,11 @@ def __init__(self, **config):
             self._GLOBAL_CONTEXT['subtheme'] = config.get('THEME_REVEAL_CONFIG_SUBTHEME', 'sky')
             self._GLOBAL_CONTEXT['transition'] = config.get('THEME_REVEAL_CONFIG_TRANSITION', 'cube')
 
+        # The pelican metadata format requires a markdown extension
+        if config.get('METADATA_FORMAT', 'nikola') == 'pelican':
+            if 'markdown.extensions.meta' not in config.get('MARKDOWN_EXTENSIONS', []):
+                utils.LOGGER.warn('To use the pelican metadata format you may need to add "markdown.extensions.meta" to your MARKDOWN_EXTENSIONS setting.')
+
         # We use one global tzinfo object all over Nikola.
         try:
             self.tzinfo = dateutil.tz.gettz(self.config['TIMEZONE'])

nikola/plugins/compile/markdown/__init__.py

@@ -139,10 +139,11 @@ def create_post(self, path, **kw):
             if onefile:
                 format = self.site.config.get('METADATA_FORMAT', 'nikola')
                 if format == 'pelican':
-                    format = 'pelican_md'                
-                fd.write('<!-- \n')
-                fd.write(write_metadata(metadata, format))
-                fd.write('-->\n\n')
+                    format = 'pelican_md'
+                data = write_metadata(metadata, format)
+                if format == 'nikola':
+                    data = '<!--\n' + data + '-->\n\n'
+                fd.write(data)
             fd.write(content)
 
     def read_metadata(self, post, file_metadata_regexp=None, unslugify_titles=False, lang=None):
@@ -156,6 +157,12 @@ def read_metadata(self, post, file_metadata_regexp=None, unslugify_titles=False,
         source = post.translated_source_path(lang)
         with io.open(source, 'r', encoding='utf-8') as inf:
             # Note: markdown meta returns lowercase keys
+            data = inf.read()
+            # If the metadata starts with "---" it's actually YAML and
+            # we should not let markdown parse it, because it will do
+            # bad things like setting empty tags to "''"
+            if data.splitlines()[0] == '---':
+                return {}
             _, meta = self.converter.convert(inf.read())
         # Map metadata from other platforms to names Nikola expects (Issue #2817)
         map_metadata(meta, 'markdown_metadata', self.site.config)

nikola/post.py

@@ -1126,8 +1126,8 @@ def get_meta(post, file_metadata_regexp=None, unslugify_titles=False, lang=None)
 
     If ``file_metadata_regexp`` is given it will be tried to read
     metadata from the filename.
-    If ``unslugify_titles`` is True, the extracted title (if any) will be unslugified, as is done in galleries.
-    If any metadata is then found inside the file the metadata from the
+    If ``unslugify_titles`` is True, the extracted title (if any) will be unslugified, as is 
+    done in galleries. If any metadata is then found inside the file the metadata from the
     file will override previous findings.
     """
     meta = defaultdict(lambda: '')
@@ -1201,7 +1201,8 @@ def hyphenate(dom, _lang):
                 skippable_nodes = ['kbd', 'code', 'samp', 'mark', 'math', 'data', 'ruby', 'svg']
                 if node.getchildren():
                     for child in node.getchildren():
-                        if child.tag in skippable_nodes or (child.tag == 'span' and 'math' in child.get('class', [])):
+                        if child.tag in skippable_nodes or (child.tag == 'span' and 'math'
+                                                            in child.get('class', [])):
                             skip_node = True
                 elif 'math' in node.get('class', []):
                     skip_node = True

nikola/utils.py

@@ -1467,23 +1467,23 @@ def write_metadata(data, format='nikola'):
     if format == 'yaml':
         if yaml is None:
             req_missing('pyyaml', 'use YAML metadata', optional=False)
-            return '\n'.join(('---', yaml.dump(data), '---'), '')
+        return '\n'.join(('---', yaml.safe_dump(data, default_flow_style=False), '---', ''))
     elif format == 'toml':
         if toml is None:
             req_missing('toml', 'use TOML metadata', optional=False)
-            return '\n'.join(('+++', toml.dumps(data), '+++'), '')
+        return '\n'.join(('+++', toml.dumps(data), '+++', ''))
     elif format == 'pelican_rest':
         title = data.pop('title')
         results = [
             '=' * len(title),
             title,
             '=' * len(title),
             ''
-        ] + [':{0}: {1}'.format(k, v) for k, v in data.items()] + ['']
+        ] + [':{0}: {1}'.format(k, v) for k, v in data.items() if v] + ['']
         return '\n'.join(results)
 
     elif format == 'pelican_md':
-        ['{0}: {1}'.format(k, v) for k, v in data.items()] + ['']
+        results = ['{0}: {1}'.format(k, v) for k, v in data.items() if v] + ['', '']
         return '\n'.join(results)
 
     else:  # Nikola, default