Omit metadata tag if it's empty

yuyichao · yuyichao · commit 79c26b4dd25e · 2020-07-20T17:57:52.000-04:00
diff --git a/lib/matplotlib/backends/backend_svg.py b/lib/matplotlib/backends/backend_svg.py
@@ -370,23 +370,30 @@ def _write_metadata(self, metadata):
             else:
                 metadata['Date'] = datetime.datetime.today().isoformat()
 
-        mid = writer.start('metadata')
-        writer.start('rdf:RDF', attrib={
+        mid = None
+        def ensure_metadata(mid):
+            if mid is not None:
+                return mid
+            mid = writer.start('metadata')
+            writer.start('rdf:RDF', attrib={
                 'xmlns:dc': "http://purl.org/dc/elements/1.1/",
                 'xmlns:cc': "http://creativecommons.org/ns#",
                 'xmlns:rdf': "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
             })
-        writer.start('cc:Work')
+            writer.start('cc:Work')
+            return mid
 
         uri = metadata.pop('Type', None)
         if uri is not None:
+            mid = ensure_metadata(mid)
             writer.element('dc:type', attrib={'rdf:resource': uri})
 
         # Single value only.
         for key in ['title', 'coverage', 'date', 'description', 'format',
                     'identifier', 'language', 'relation', 'source']:
             info = metadata.pop(key.title(), None)
             if info is not None:
+                mid = ensure_metadata(mid)
                 writer.element(f'dc:{key}', text=info)
 
         # Multiple Agent values.
@@ -398,6 +405,7 @@ def _write_metadata(self, metadata):
             if isinstance(agents, str):
                 agents = [agents]
 
+            mid = ensure_metadata(mid)
             writer.start(f'dc:{key}')
             for agent in agents:
                 writer.start('cc:Agent')
@@ -411,14 +419,16 @@ def _write_metadata(self, metadata):
             if isinstance(keywords, str):
                 keywords = [keywords]
 
+            mid = ensure_metadata(mid)
             writer.start('dc:subject')
             writer.start('rdf:Bag')
             for keyword in keywords:
                 writer.element('rdf:li', text=keyword)
             writer.end('rdf:Bag')
             writer.end('dc:subject')
 
-        writer.close(mid)
+        if mid is not None:
+            writer.close(mid)
 
         if metadata:
             raise ValueError('Unknown metadata key(s) passed to SVG writer: ' +
