allow release notes sections to be single strings

Release notes entries may now be made up of single strings. This
simplifies formatting for smaller notes, and eliminates a class of
errors associated with escaping reStructuredText inside YAML lists.

Change-Id: I7f2fb2d2fd16f49e7ee061582df7bcdd4116f215
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

examples/notes/add-complex-example-6b5927c246456896.yaml

@@ -26,3 +26,6 @@ other:
       This example is also rendered
       correctly on multiple lines
       as a pre-formatted block.
+features:
+  This note is a simple string, and does not retain its
+  formatting when it is rendered in HTML.

releasenotes/notes/flexible-formatting-31c8de2599d3637d.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  Release notes entries may now be made up of single strings. This
+  simplifies formatting for smaller notes, and eliminates a class of
+  errors associated with escaping reStructuredText inside YAML lists.

reno/loader.py

@@ -101,6 +101,8 @@ class Loader(object):
             body = self._scanner.get_file_at_commit(filename, sha)
             content = yaml.safe_load(body)
 
+        cleaned_content = {}
+
         for section_name, section_content in content.items():
             if section_name == 'prelude':
                 if not isinstance(section_content, six.string_types):
@@ -111,10 +113,15 @@ class Loader(object):
                         filename,
                     )
             else:
-                if not isinstance(section_content, list):
+                if isinstance(section_content, six.string_types):
+                    # A single string is OK, but wrap it with a list
+                    # so the rest of the code can treat the data model
+                    # consistently.
+                    section_content = [section_content]
+                elif not isinstance(section_content, list):
                     LOG.warning(
                         ('The %s section of %s '
-                         'does not parse as a list of strings. '
+                         'does not parse as a string or list of strings. '
                          'Is the YAML input escaped properly?') % (
                              section_name, filename),
                     )
@@ -128,5 +135,6 @@ class Loader(object):
                                  ) % (item, section_name,
                                       filename, type(item)),
                             )
+            cleaned_content[section_name] = section_content
 
-        return content
+        return cleaned_content

reno/tests/test_loader.py

@@ -67,7 +67,7 @@ class TestValidate(base.TestCase):
         ldr.parse_note_file('note1', None)
         self.assertIn('prelude', self.logger.output)
 
-    def test_non_prelude_single_string(self):
+    def test_non_prelude_single_string_converted_to_list(self):
         note_bodies = yaml.safe_load(textwrap.dedent('''
         issues: |
           This is a single string.
@@ -75,8 +75,8 @@ class TestValidate(base.TestCase):
         print(type(note_bodies['issues']))
         self.assertIsInstance(note_bodies['issues'], six.string_types)
         ldr = self._make_loader(note_bodies)
-        ldr.parse_note_file('note1', None)
-        self.assertIn('list of strings', self.logger.output)
+        parse_results = ldr.parse_note_file('note1', None)
+        self.assertIsInstance(parse_results['issues'], list)
 
     def test_note_with_colon_as_dict(self):
         note_bodies = yaml.safe_load(textwrap.dedent('''