diff -r 28665dc3a696 Lib/idlelib/AutoExpand.py
--- a/Lib/idlelib/AutoExpand.py	Mon Jun 02 20:42:56 2014 -0400
+++ b/Lib/idlelib/AutoExpand.py	Tue Jun 03 17:02:15 2014 +0530
@@ -1,3 +1,17 @@
+'''Complete the current word before the cursor with words in the editor.
+
+Each menu selection or shortcut key selection replaces the word with a
+different word with the same prefix. The search for matches begins
+before the target and moves toward the top of the editor. It then starts
+after the cursor and moves down. It then returns to the original word and
+the cycle starts again.
+
+Changing the current text line or leaving the cursor in a different
+place before requesting the next selection causes AutoExpand to reset
+its state.
+
+This is an extension file and there is only one instance of AutoExpand.
+'''
 import string
 import re
 
@@ -20,6 +34,8 @@
         self.state = None
 
     def expand_word_event(self, event):
+        '''Replace the current word with the next expansion.
+        '''   
         curinsert = self.text.index("insert")
         curline = self.text.get("insert linestart", "insert lineend")
         if not self.state:
@@ -46,6 +62,7 @@
         return "break"
 
     def getwords(self):
+        "Return a list of words that match the prefix before the cursor."
         word = self.getprevword()
         if not word:
             return []
@@ -76,8 +93,14 @@
         return words
 
     def getprevword(self):
+        "Return the word prefix before the cursor."
         line = self.text.get("insert linestart", "insert")
         i = len(line)
         while i > 0 and line[i-1] in self.wordchars:
             i = i-1
         return line[i:]
+
+if __name__ == '__main__':
+    import unittest
+    unittest.main('idlelib.idle_test.test_autoexpand',
+                   verbosity=2, exit=False)
diff -r 28665dc3a696 Lib/idlelib/idle_test/test_autoexpand.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/idle_test/test_autoexpand.py	Tue Jun 03 17:02:15 2014 +0530
@@ -0,0 +1,149 @@
+"""Unit tests for idlelib.AutoExpand"""
+import unittest
+from test.support import requires
+from tkinter import Text, Tk
+#from idlelib.idle_test.mock_tk import Text
+from idlelib.AutoExpand import AutoExpand
+
+
+class Dummy_Editwin:
+    # AutoExpand.__init__ only needs .text
+    def __init__(self, text):
+        self.text = text
+
+class AutoExpandTest(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        if 'tkinter' in str(Text):
+            requires('gui')
+            cls.tk = Tk()
+            cls.text = Text(cls.tk)
+        else:
+            cls.text = Text()
+        cls.editwin = Dummy_Editwin(cls.text)
+        cls.auto_expand = AutoExpand(cls.editwin)
+
+    @classmethod
+    def tearDownClass(cls):
+        if hasattr(cls, 'tk'):
+            cls.tk.destroy()
+            del cls.tk
+
+    def tearDown(self):
+        self.text.delete('1.0', 'end')
+
+    def test_get_prevword(self):
+        text = self.editwin.text
+        previous = self.auto_expand.getprevword
+        equal = self.assertEqual
+
+        equal(previous(), '')
+
+        text.insert('insert', 't')
+        equal(previous(), 't')
+
+        text.insert('insert', 'his')
+        equal(previous(), 'this')
+
+        text.insert('insert', ' ')
+        equal(previous(), '')
+
+        text.insert('insert', 'is')
+        equal(previous(), 'is')
+
+        text.insert('insert', '\nsample\nstring')
+        equal(previous(), 'string')
+
+        text.delete('3.0', 'insert')
+        equal(previous(), '')
+
+        text.delete('1.0', 'end')
+        equal(previous(), '')
+
+    def test_get_words(self):
+        text = self.editwin.text
+        words = self.auto_expand.getwords
+        equal = self.assertEqual
+
+        equal(words(), [])
+
+        text.insert('insert', 'a ab')
+        equal(words(), [])
+        text.delete('1.0', 'end')
+
+        text.insert('insert', 'ab a')
+        equal(words(), ['ab', 'a'])
+        text.delete('1.0', 'end')
+
+        text.insert('insert', 'aab aba\n acb\n')
+        text.insert('insert', 'a')
+        equal(words(), ['acb', 'aba', 'aab', 'a'])
+        text.insert('insert', 'a')
+        equal(words(), ['aab', 'aa'])
+        text.insert('insert', 'b')
+        equal(words(), [])
+        text.delete('1.0', 'end')
+
+        text.insert('insert', 'a\nb\nc\nd')
+        text.insert('insert', 'abc')
+        equal(words(), [])
+        text.delete('1.0', 'end')
+
+    def test_expand_event(self):
+        text = self.editwin.text
+        expand = self.auto_expand.expand_word_event
+        previous = self.auto_expand.getprevword
+        equal = self.assertEqual
+
+        expand('event')
+        equal(previous(), '')
+        text.delete('1.0', 'end')
+
+        # candidates only before 'insert'
+        text.insert('insert', 'ab ac xy ab ad yz\na')
+        expand('event')
+        equal(previous(), 'ad')
+        expand('event')
+        equal(previous(), 'ab')
+        expand('event')
+        equal(previous(), 'ac')
+        expand('event')
+        equal(previous(), 'a')
+        text.delete('1.0', 'end')
+
+        # candidates both before and after 'insert'
+        text.insert('insert', 'ab xy ac yz ad\n')
+        text.insert('insert', 'a \n')
+        text.insert('insert', 'ax az bx ay az')
+        text.mark_set('insert', '2.1')
+        expand('event')
+        expand('event')
+        expand('event')
+        expand('event')
+        equal(previous(), 'ax')
+        expand('event')
+        equal(previous(), 'az')
+        expand('event')
+        equal(previous(), 'ay')
+        expand('event')
+        equal(previous(), 'a')
+        expand('event')
+        equal(previous(), 'ad')
+        text.delete('1.0', 'end')
+
+        # candidates only after 'insert'
+        text.insert('insert', 'a \n')
+        text.insert('insert', 'ax by ay ax az')
+        text.mark_set('insert', '2.1')
+        expand('event')
+        equal(previous(), 'ay')
+        expand('event')
+        equal(previous(), 'ax')
+        expand('event')
+        equal(previous(), 'az')
+        expand('event')
+        equal(previous(), 'a')
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2, exit=False)