Overhaul for efficiency, Py3k compatibility, less convoluted control flow.

 * New feature: `instance.has_summary` set to True if an explicitly-defined
   summary exists, False otherwise.  Templates can use this to inject a
   "read more" link for instance (the `read_more_link` plugin seems to be
   broken under Py3k).

 * Tested under Python 3.4; import unicode_literals for consistent behavior
   on both Py2 and Py3.

 * `instance._content` is modified directly instead of using the undocumented
   `instance._get_content` hook.
 * Control flow within `content_object_init` hook is streamlined.
 * Settings are looked up only once per article, which helps both
   efficiency and readability.

summary/Readme.rst

@@ -26,6 +26,11 @@ beginning marker was found, it starts at the top of the body. It is possible
 to leave out the end marker instead, in which case the summary will start at the
 beginning marker and continue to the end of the body.
 
+The plugin also sets a ``has_summary`` attribute on every article. It is True
+for articles with an explicitly-defined summary, and False otherwise.  (It is
+also False for an article truncated by ``SUMMARY_MAX_LENGTH``.)  Your templates
+can use this e.g. to add a link to the full text at the end of the summary.
+
 reST example
 ~~~~~~~~~~~~
 

summary/summary.py

@@ -2,12 +2,11 @@
 Summary
 -------
 
-This plugin allows easy, variable length summaries directly embedded into the 
+This plugin allows easy, variable length summaries directly embedded into the
 body of your articles.
 """
 
-import types
-
+from __future__ import unicode_literals
 from pelican import signals
 
 def initialized(pelican):
@@ -24,37 +23,51 @@ def initialized(pelican):
 
 def content_object_init(instance):
     # if summary is already specified, use it
-    if 'summary' in instance.metadata:
+    # if there is no content, there's nothing to do
+    if hasattr(instance, '_summary'):
+        instance.has_summary = True
+        return
+
+    if not instance._content:
+        instance.has_summary = False
         return
 
-    def _get_content(self):
-        content = self._content
-        if self.settings['SUMMARY_BEGIN_MARKER']:
-            content = content.replace(
-                self.settings['SUMMARY_BEGIN_MARKER'], '', 1)
-        if self.settings['SUMMARY_END_MARKER']:
-            content = content.replace(
-                self.settings['SUMMARY_END_MARKER'], '', 1)
-        return content
-    instance._get_content = types.MethodType(_get_content, instance)
+    begin_marker = instance.settings['SUMMARY_BEGIN_MARKER']
+    end_marker   = instance.settings['SUMMARY_END_MARKER']
 
     # extract out our summary
-    if not hasattr(instance, '_summary') and instance._content is not None:
-        content = instance._content
-        begin_summary = -1
-        end_summary = -1
-        if instance.settings['SUMMARY_BEGIN_MARKER']:
-            begin_summary = content.find(instance.settings['SUMMARY_BEGIN_MARKER'])
-        if instance.settings['SUMMARY_END_MARKER']:
-            end_summary = content.find(instance.settings['SUMMARY_END_MARKER'])
-        if begin_summary != -1 or end_summary != -1:
-            # the beginning position has to take into account the length
-            # of the marker
-            begin_summary = (begin_summary +
-                            len(instance.settings['SUMMARY_BEGIN_MARKER'])
-                            if begin_summary != -1 else 0)
-            end_summary = end_summary if end_summary != -1 else None
-            instance._summary = instance._update_content(content[begin_summary:end_summary], instance._context.get('localsiteurl', ''))
+    content = instance._content
+    begin_summary = -1
+    end_summary = -1
+    if begin_marker:
+        begin_summary = content.find(begin_marker)
+    if end_marker:
+        end_summary = content.find(end_marker)
+
+    if begin_summary == -1 and end_summary == -1:
+        instance.has_summary = False
+        return
+
+    # skip over the begin marker, if present
+    if begin_summary == -1:
+        begin_summary = 0
+    else:
+        begin_summary = begin_summary + len(begin_marker)
+
+    if end_summary == -1:
+        end_summary = None
+
+    summary = content[begin_summary:end_summary]
+
+    # remove the markers from the content
+    if begin_summary:
+        content = content.replace(begin_marker, '', 1)
+    if end_summary:
+        content = content.replace(end_marker, '', 1)
+
+    instance._content = content
+    instance._summary = summary
+    instance.has_summary = True
 
 def register():
     signals.initialized.connect(initialized)