Change `set.append` -> `set.add` in `headerid.unique`

Fixes #195. This was getting missed because the HeadrerId extension's
reset method was resetting the IDs to a list. However, some third party
extensions may want to call the unique function and it should work as
documented. Interestingly, the TOC extension was using it and passing in
a list as well. All fixed now. Also added a test of the `unique` function
directly so we shouldn't repeat this in the future.

3 files changed, 10 insertions, 3 deletions

diff --git a/markdown/extensions/headerid.py b/markdown/extensions/headerid.py
index b6a12e8..cf3df17 100644
--- a/markdown/extensions/headerid.py
+++ b/markdown/extensions/headerid.py
@@ -101,7 +101,7 @@ def unique(id, ids):
             id = '%s_%d'% (m.group(1), int(m.group(2))+1)
         else:
             id = '%s_%d'% (id, 1)
-    ids.append(id)
+    ids.add(id)
     return id
 
 
@@ -191,7 +191,7 @@ class HeaderIdExtension (markdown.Extension):
             md.treeprocessors.add('headerid', self.processor, '>prettify')
 
     def reset(self):
-        self.processor.IDs = []
+        self.processor.IDs = set()
 
 
 def makeExtension(configs=None):
diff --git a/markdown/extensions/toc.py b/markdown/extensions/toc.py
index 7edc0cc..1d6639e 100644
--- a/markdown/extensions/toc.py
+++ b/markdown/extensions/toc.py
@@ -127,7 +127,7 @@ class TocTreeprocessor(markdown.treeprocessors.Treeprocessor):
         self.use_anchors = self.config["anchorlink"] in [1, '1', True, 'True', 'true']
         
         # Get a list of id attributes
-        used_ids = []
+        used_ids = set()
         for c in doc.getiterator():
             if "id" in c.attrib:
                 used_ids.append(c.attrib["id"])
diff --git a/tests/test_extensions.py b/tests/test_extensions.py
index fa9a801..d9d77b8 100644
--- a/tests/test_extensions.py
+++ b/tests/test_extensions.py
@@ -199,6 +199,13 @@ class TestHeaderId(unittest.TestCase):
         self.assertEqual(self.md.convert(text),
             '<h1 id="some-header">Some Header</h1>')
 
+    def testUniqueFunc(self):
+        """ Test 'unique' function. """
+        from markdown.extensions.headerid import unique
+        ids = set(['foo'])
+        self.assertEqual(unique('foo', ids), 'foo_1')
+        self.assertEqual(ids, set(['foo', 'foo_1']))
+
     def testUniqueIds(self):
         """ Test Unique IDs. """