diff -r 681979c6e6b2 Lib/idlelib/Percolator.py
--- a/Lib/idlelib/Percolator.py	Mon Jul 14 23:07:32 2014 -0400
+++ b/Lib/idlelib/Percolator.py	Thu Jul 17 12:20:46 2014 +0530
@@ -1,6 +1,7 @@
 from idlelib.WidgetRedirector import WidgetRedirector
 from idlelib.Delegator import Delegator
 
+
 class Percolator:
 
     def __init__(self, text):
@@ -16,8 +17,10 @@
         while self.top is not self.bottom:
             self.removefilter(self.top)
         self.top = None
-        self.bottom.setdelegate(None); self.bottom = None
-        self.redir.close(); self.redir = None
+        self.bottom.setdelegate(None)
+        self.bottom = None
+        self.redir.close()
+        self.redir = None
         self.text = None
 
     def insert(self, index, chars, tags=None):
@@ -51,23 +54,27 @@
             f.setdelegate(filter.delegate)
             filter.setdelegate(None)
 
+
 def _percolator(parent):
     import tkinter as tk
     import re
+
     class Tracer(Delegator):
         def __init__(self, name):
             self.name = name
             Delegator.__init__(self, None)
+
         def insert(self, *args):
             print(self.name, ": insert", args)
             self.delegate.insert(*args)
+
         def delete(self, *args):
             print(self.name, ": delete", args)
             self.delegate.delete(*args)
     root = tk.Tk()
     root.title("Test Percolator")
     width, height, x, y = list(map(int, re.split('[x+]', parent.geometry())))
-    root.geometry("+%d+%d"%(x, y + 150))
+    root.geometry("+%d+%d" % (x, y + 150))
     text = tk.Text(root)
     p = Percolator(text)
     t1 = Tracer("t1")
@@ -100,5 +107,9 @@
     root.mainloop()
 
 if __name__ == "__main__":
+    import unittest
+    unittest.main('idlelib.idle_test.test_percolator', verbosity=2,
+                  exit=False)
+
     from idlelib.idle_test.htest import run
     run(_percolator)
diff -r 681979c6e6b2 Lib/idlelib/idle_test/test_percolator.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/idle_test/test_percolator.py	Thu Jul 17 12:20:46 2014 +0530
@@ -0,0 +1,116 @@
+import unittest
+from test.support import requires
+from tkinter import Text, Tk, END
+from idlelib.Percolator import Percolator, Delegator
+
+
+class MyFilter(Delegator):
+    def __init__(self):
+        Delegator.__init__(self, None)
+
+    def insert(self, *args):
+        self.insert_called_with = args
+        self.delegate.insert(*args)
+
+    def delete(self, *args):
+        self.delete_called_with = args
+        self.delegate.delete(*args)
+
+    def uppercase_insert(self, index, chars, tags=None):
+        chars = chars.upper()
+        self.delegate.insert(index, chars)
+
+    def lowercase_insert(self, index, chars, tags=None):
+        chars = chars.lower()
+        self.delegate.insert(index, chars)
+
+    def dont_insert(self, index, chars, tags=None):
+        pass
+
+
+class PercolatorTest(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        requires('gui')
+        cls.root = Tk()
+        cls.text = Text(cls.root)
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.text.destroy()
+        cls.root.destroy()
+        del cls.text, cls.root
+
+    def setUp(self):
+        self.percolator = Percolator(self.text)
+        self.filter_one = MyFilter()
+        self.filter_two = MyFilter()
+        self.percolator.insertfilter(self.filter_one)
+        self.percolator.insertfilter(self.filter_two)
+
+    def tearDown(self):
+        self.percolator.close()
+        self.text.delete('1.0', END)
+
+    def test_insertfilter(self):
+        self.assertIsNotNone(self.filter_one.delegate)
+        self.assertEqual(self.percolator.top, self.filter_two)
+        self.assertEqual(self.filter_two.delegate, self.filter_one)
+        self.assertEqual(self.filter_one.delegate, self.percolator.bottom)
+
+    def test_removefilter(self):
+        filter_three = MyFilter()
+        self.percolator.removefilter(self.filter_two)
+        self.assertEqual(self.percolator.top, self.filter_one)
+        self.assertIsNone(self.filter_two.delegate)
+
+        filter_three = MyFilter()
+        self.percolator.insertfilter(self.filter_two)
+        self.percolator.insertfilter(filter_three)
+        self.percolator.removefilter(self.filter_one)
+        self.assertEqual(self.percolator.top, filter_three)
+        self.assertEqual(filter_three.delegate, self.filter_two)
+        self.assertEqual(self.filter_two.delegate, self.percolator.bottom)
+        self.assertIsNone(self.filter_one.delegate)
+
+    def test_insert(self):
+        self.text.insert('insert', 'foo')
+        self.assertEqual(self.text.get('1.0', END), 'foo\n')
+        self.assertTupleEqual(self.filter_one.insert_called_with,
+                              ('insert', 'foo', None))
+
+    def test_modify_insert(self):
+        self.filter_one.insert = self.filter_one.uppercase_insert
+        self.text.insert('insert', 'bAr')
+        self.assertEqual(self.text.get('1.0', END), 'BAR\n')
+
+    def test_modify_chain_insert(self):
+        filter_three = MyFilter()
+        self.percolator.insertfilter(filter_three)
+        self.filter_two.insert = self.filter_two.uppercase_insert
+        self.filter_one.insert = self.filter_one.lowercase_insert
+        self.text.insert('insert', 'BaR')
+        self.assertEqual(self.text.get('1.0', END), 'bar\n')
+
+    def test_dont_insert(self):
+        self.filter_one.insert = self.filter_one.dont_insert
+        self.text.insert('insert', 'foo bar')
+        self.assertEqual(self.text.get('1.0', END), '\n')
+        self.filter_one.insert = self.filter_one.dont_insert
+        self.text.insert('insert', 'foo bar')
+        self.assertEqual(self.text.get('1.0', END), '\n')
+
+    def test_without_filter(self):
+        self.text.insert('insert', 'hello')
+        self.assertEqual(self.text.get('1.0', 'end'), 'hello\n')
+
+    def test_delete(self):
+        self.text.insert('insert', 'foo')
+        self.text.delete('1.0', '1.2')
+        self.assertEqual(self.text.get('1.0', END), 'o\n')
+        self.assertTupleEqual(self.filter_one.delete_called_with,
+                              ('1.0', '1.2'))
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)