Account for a paragraph that starts with a colon when processing def_lists

Fixes #171. While that report provided an example of an unordered list item that started with a colon, any block that starts with a colon and has no siblings before it (paragraph as begining if document, list item, etc) all exhibit this same behavior. Following PHP Markdown Extra's lead, these are not definition items as they have no term before them.

3 files changed, 50 insertions, 4 deletions

diff --git a/markdown/extensions/def_list.py b/markdown/extensions/def_list.py
index da1726a..382445c 100644
--- a/markdown/extensions/def_list.py
+++ b/markdown/extensions/def_list.py
@@ -34,10 +34,11 @@ class DefListProcessor(markdown.blockprocessors.BlockProcessor):
         return bool(self.RE.search(block))
 
     def run(self, parent, blocks):
-        block = blocks.pop(0)
-        m = self.RE.search(block)
-        terms = [l.strip() for l in block[:m.start()].split('\n') if l.strip()]
-        block = block[m.end():]
+
+        raw_block = blocks.pop(0)
+        m = self.RE.search(raw_block)
+        terms = [l.strip() for l in raw_block[:m.start()].split('\n') if l.strip()]
+        block = raw_block[m.end():]
         no_indent = self.NO_INDENT_RE.match(block)
         if no_indent:
             d, theRest = (block, None)
@@ -48,6 +49,11 @@ class DefListProcessor(markdown.blockprocessors.BlockProcessor):
         else:
             d = m.group(2)
         sibling = self.lastChild(parent)
+        if not terms and sibling is None:
+            # This is not a definition item. Most likely a paragraph that 
+            # starts with a colon at the begining of a document or list.
+            blocks.insert(0, raw_block)
+            return False
         if not terms and sibling.tag == 'p':
             # The previous paragraph contains the terms
             state = 'looselist'
diff --git a/tests/extensions/extra/def-in-list.html b/tests/extensions/extra/def-in-list.html
new file mode 100644
index 0000000..21cddaa
--- /dev/null
+++ b/tests/extensions/extra/def-in-list.html
@@ -0,0 +1,25 @@
+<p>: a paragraph that starts with a colon</p>
+<ul>
+<li>A List item</li>
+<li>
+<dl>
+<dt>A def term</dt>
+<dd>A def item</dd>
+<dd>a second</dd>
+</dl>
+</li>
+<li>
+<dl>
+<dt>Another def term</dt>
+<dd>
+<p>a loose item</p>
+</dd>
+<dd>
+<p>a second</p>
+</dd>
+</dl>
+</li>
+<li>
+<p>: a list item that starts with a colon</p>
+</li>
+</ul>
\ No newline at end of file
diff --git a/tests/extensions/extra/def-in-list.txt b/tests/extensions/extra/def-in-list.txt
new file mode 100644
index 0000000..7a292ab
--- /dev/null
+++ b/tests/extensions/extra/def-in-list.txt
@@ -0,0 +1,15 @@
+: a paragraph that starts with a colon
+
+* A List item
+* 
+    A def term
+    : A def item
+    : a second
+
+* Another def term
+
+    : a loose item
+    
+    : a second
+
+* : a list item that starts with a colon