diff --git a/Lib/idlelib/AutoExpand.py b/Lib/idlelib/AutoExpand.py
index 9e93d57..7059054 100644
--- a/Lib/idlelib/AutoExpand.py
+++ b/Lib/idlelib/AutoExpand.py
@@ -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 +34,7 @@
         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 +61,7 @@
         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 +92,13 @@
         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)
diff --git a/Lib/idlelib/idle_test/test_autoexpand.py b/Lib/idlelib/idle_test/test_autoexpand.py
new file mode 100644
index 0000000..7f112a0
--- /dev/null
+++ b/Lib/idlelib/idle_test/test_autoexpand.py
@@ -0,0 +1,141 @@
+"""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.auto_expand = AutoExpand(Dummy_Editwin(cls.text))
+
+    @classmethod
+    def tearDownClass(cls):
+        if hasattr(cls, 'tk'):
+            cls.tk.destroy()
+            del cls.tk
+        del cls.text, cls.auto_expand
+
+    def tearDown(self):
+        self.text.delete('1.0', 'end')
+
+    def test_get_prevword(self):
+        text = self.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_before_only(self):
+        previous = self.auto_expand.getprevword
+        expand = self.auto_expand.expand_word_event
+        equal = self.assertEqual
+
+        self.text.insert('insert', 'ab ac bx ad ab a')
+        equal(self.auto_expand.getwords(), ['ab', 'ad', 'ac', 'a'])
+        expand('event')
+        equal(previous(), 'ab')
+        expand('event')
+        equal(previous(), 'ad')
+        expand('event')
+        equal(previous(), 'ac')
+        expand('event')
+        equal(previous(), 'a')
+
+    def test_after_only(self):
+        # Also add punctuation 'noise' that shoud be ignored.
+        text = self.text
+        previous = self.auto_expand.getprevword
+        expand = self.auto_expand.expand_word_event
+        equal = self.assertEqual
+
+        text.insert('insert', 'a, [ab] ac: () bx"" cd ac= ad ya')
+        text.mark_set('insert', '1.1')
+        equal(self.auto_expand.getwords(), ['ab', 'ac', 'ad', 'a'])
+        expand('event')
+        equal(previous(), 'ab')
+        expand('event')
+        equal(previous(), 'ac')
+        expand('event')
+        equal(previous(), 'ad')
+        expand('event')
+        equal(previous(), 'a')
+
+    def test_both_before_after(self):
+        text = self.text
+        previous = self.auto_expand.getprevword
+        expand = self.auto_expand.expand_word_event
+        equal = self.assertEqual
+
+        text.insert('insert', 'ab xy yz\n')
+        text.insert('insert', 'a ac by ac')
+
+        text.mark_set('insert', '2.1')
+        equal(self.auto_expand.getwords(), ['ab', 'ac', 'a'])
+        expand('event')
+        equal(previous(), 'ab')
+        expand('event')
+        equal(previous(), 'ac')
+        expand('event')
+        equal(previous(), 'a')
+
+    def test_other_expand_cases(self):
+        text = self.text
+        expand = self.auto_expand.expand_word_event
+        equal = self.assertEqual
+
+        # no expansion candidate found
+        equal(self.auto_expand.getwords(), [])
+        equal(expand('event'), 'break')
+
+        text.insert('insert', 'bx cy dz a')
+        equal(self.auto_expand.getwords(), [])
+
+        # reset state by successfully expanding once
+        # move cursor to another position and expand again
+        text.insert('insert', 'ac xy a ac ad a')
+        text.mark_set('insert', '1.7')
+        expand('event')
+        initial_state = self.auto_expand.state
+        text.mark_set('insert', '1.end')
+        expand('event')
+        new_state = self.auto_expand.state
+        self.assertNotEqual(initial_state, new_state)
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)