diff -r 39c67beb483a Doc/library/idle.rst
--- a/Doc/library/idle.rst	Fri Aug 15 01:23:02 2014 -0400
+++ b/Doc/library/idle.rst	Sat Aug 16 11:50:25 2014 +0530
@@ -235,6 +235,10 @@
    Open a pane at the top of the edit window which shows the block context
    of the section of code which is scrolling off the top of the window.
 
+Line Numbers (Editor Window only)
+   Open a column to the left of the edit window which shows the linenumber
+   of each line of text.
+
 Windows menu (Shell and Editor)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -550,3 +554,4 @@
 
 * CodeContext
 
+* LineNumber
diff -r 39c67beb483a Lib/idlelib/EditorWindow.py
--- a/Lib/idlelib/EditorWindow.py	Fri Aug 15 01:23:02 2014 -0400
+++ b/Lib/idlelib/EditorWindow.py	Sat Aug 16 11:50:25 2014 +0530
@@ -805,6 +805,7 @@
                 idleConf.GetOption('main','EditorWindow','font-size',
                                    type='int'),
                 fontWeight))
+        self.text.event_generate('<<font-changed>>')
 
     def RemoveKeybindings(self):
         "Remove the keybindings before they are changed."
diff -r 39c67beb483a Lib/idlelib/LineNumber.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/LineNumber.py	Sat Aug 16 11:50:25 2014 +0530
@@ -0,0 +1,171 @@
+"""Linenumbering implementation for IDLE as an extension.
+Includes BaseSideBar which can be extended for other sidebar based extensions
+"""
+import tkinter as tk
+from idlelib.Delegator import Delegator
+from idlelib.configHandler import idleConf
+
+DISABLED = False
+ENABLED = True
+
+get_end = lambda text: int(float(text.index('end-1c')))
+
+
+class BaseSideBar:
+    """
+    The base class for extensions which require a sidebar.
+    """
+    def __init__(self, editwin):
+        self.editwin = editwin
+        self.text = editwin.text
+        self.text.bind('<<font-changed>>', self.update_sidebar_text_font)
+        self.parent = self.text.nametowidget(self.text.winfo_parent())
+        self.sidebar_text = tk.Text(self.parent, width=1, wrap=tk.NONE)
+        self.sidebar_text.config(state=tk.DISABLED)
+        self.text['yscrollcommand'] = self.vbar_set
+        self.sidebar_text['yscrollcommand'] = self.vbar_set
+
+    def update_sidebar_text_font(self, event=''):
+        """
+        Implement in subclass to update font config values of sidebar_text
+        when font config values of editwin.text changes
+        """
+
+    def show_sidebar(self, side=tk.LEFT):
+        """
+        side - Valid values are tk.LEFT, tk.RIGHT, tk.TOP, tk.BOTTOM
+        """
+        try:
+            self.sidebar_text.pack(side=tk.LEFT, fill=tk.Y, before=self.text)
+        except tk.TclError:
+            self.sidebar_text.pack(side=tk.LEFT, fill=tk.Y)
+        self.state = ENABLED
+
+    def hide_sidebar(self):
+        self.sidebar_text.pack_forget()
+        self.state = DISABLED
+
+    def vbar_set(self, *args, **kwargs):
+        """Redirect scrollbar's set command to editwin.text and sidebar_text
+        """
+        self.editwin.vbar.set(*args)
+        self.sidebar_text.yview_moveto(args[0])
+        self.text.yview_moveto(args[0])
+
+    def redirect_event(self, event, event_name):
+        """Set focus to editwin.text and redirect 'event' to editwin.text.
+        """
+        self.text.focus_set()
+        if event_name == '<MouseWheel>':
+            self.text.event_generate(event_name, x=event.x, y=event.y,
+                                     delta=event.delta)
+        else:
+            self.text.event_generate(event_name, x=event.x, y=event.y)
+
+
+class EndLineDelegator(Delegator):
+    """Generate callbacks with the current end line number after
+       insert or delete operations"""
+    def __init__(self, changed_callback, end=1):
+        """
+        changed_callback - Callable, will be called after insert
+                           or delete operations with the current
+                           end line number.
+        end - int, inital value of the end line number"""
+        Delegator.__init__(self)
+        self.changed_callback = changed_callback
+        self.changed_callback(end)
+
+    def insert(self, index, chars, tags=None):
+        self.delegate.insert(index, chars, tags)
+        self.changed_callback(get_end(self.delegate))
+
+    def delete(self, index1, index2=None):
+        self.delegate.delete(index1, index2)
+        self.changed_callback(get_end(self.delegate))
+
+
+class LineNumber(BaseSideBar):
+
+    menudefs = [
+        ('options', [
+            ("Line Numbers", "<<toggle-linenumbering>>"),
+        ])
+    ]
+
+    def __init__(self, editwin):
+        BaseSideBar.__init__(self, editwin)
+        self.prev_end = 1
+        end = get_end(self.text)
+        self.update_sidebar_text_font()
+        self.sidebar_text.config(state=tk.NORMAL)
+        self.sidebar_text.insert('insert', '1', 'linenumber')
+        self.sidebar_text.config(state=tk.DISABLED)
+        for event_name in ('<Button-2>', '<Button-3>', '<Button-4>',
+                           '<Button-5>', '<B2-Motion>', '<B3-Motion>',
+                           '<ButtonRelease-2>', '<ButtonRelease-3>',
+                           '<Double-Button-1>', '<Double-Button-2>',
+                           '<Double-Button-3>', '<Enter>', '<Leave>',
+                           '<2>', '<3>', '<MouseWheel>',
+                           '<FocusIn>'):
+            self.sidebar_text.bind(event_name,
+                                   lambda event, event_name=event_name:
+                                   self.redirect_event(event, event_name))
+        self.end_line_delegator = EndLineDelegator(self.update_sidebar_text,
+                                                   end)
+        self.editwin.per.insertfilter(self.end_line_delegator)
+        self.state = idleConf.GetOption('extensions', 'LineNumber', 'visible',
+                                        type='bool')
+        # Note : We invert state here, and call toggle_linenumbering_event
+        # to get our desired state
+        self.state = not self.state
+        self.toggle_linenumbering_event('')
+
+    def update_sidebar_text_font(self, event=''):
+        """Update the font of sidebar_text when font of editwin.font
+        changes
+        """
+        bg = idleConf.GetOption('extensions', 'LineNumber', 'bgcolor')
+        fg = idleConf.GetOption('extensions', 'LineNumber', 'fgcolor')
+        self.sidebar_text.tag_config('linenumber', justify=tk.RIGHT)
+        config = {'fg': fg, 'bg': bg, 'font': self.text['font'],
+                  'relief': tk.FLAT, 'selectforeground': fg,
+                  'selectbackground': bg}
+        if tk.TkVersion >= 8.5:
+            config['inactiveselectbackground'] = bg
+        self.sidebar_text.config(**config)
+        # The below lines below are required to allow tk to "catch up" with
+        # changes in font to the main text widget
+        #
+        sidebar_text = self.sidebar_text.get('1.0', 'end')
+        self.sidebar_text.delete('1.0', 'end')
+        self.sidebar_text.insert('1.0', sidebar_text)
+        self.text.update_idletasks()
+        self.sidebar_text.update_idletasks()
+
+    def toggle_linenumbering_event(self, event):
+        self.show_sidebar() if self.state == DISABLED else self.hide_sidebar()
+
+    def update_sidebar_text(self, end):
+        """
+        Perform the following action:
+        Each line sidebar_text contains the linenumber for that line
+        Synchronize with editwin.text so that both sidebar_text and
+        editwin.text contain the same number of lines"""
+        if end == self.prev_end:
+            return
+        width_difference = len(str(end)) - len(str(self.prev_end))
+        self.sidebar_text['width'] += width_difference
+        self.sidebar_text.config(state=tk.NORMAL)
+        if end > self.prev_end:
+            for i in range(self.prev_end + 1, end + 1):
+                self.sidebar_text.insert('{}.0'.format(i), '\n{}'.format(i),
+                                         'linenumber')
+        else:
+            self.sidebar_text.delete('{}.0'.format(end+1), 'end')
+        self.sidebar_text.config(state=tk.DISABLED)
+        self.prev_end = end
+
+if __name__ == '__main__':
+    from unittest import main
+    main('idlelib.idle_test.test_linenumber', verbosity=2)
diff -r 39c67beb483a Lib/idlelib/config-extensions.def
--- a/Lib/idlelib/config-extensions.def	Fri Aug 15 01:23:02 2014 -0400
+++ b/Lib/idlelib/config-extensions.def	Sat Aug 16 11:50:25 2014 +0530
@@ -94,3 +94,12 @@
 enable_shell=0
 enable_editor=1
 
+[LineNumber]
+enable=1
+enable_editor=1
+# Should LineNumbering be visible on EditorWindow startup?
+visible=0 
+bgcolor=Gray
+fgcolor=Black
+[LineNumber_bindings]
+toggle-linenumbering=
diff -r 39c67beb483a Lib/idlelib/help.txt
--- a/Lib/idlelib/help.txt	Fri Aug 15 01:23:02 2014 -0400
+++ b/Lib/idlelib/help.txt	Sat Aug 16 11:50:25 2014 +0530
@@ -366,3 +366,4 @@
                 ParenMatch
                 AutoComplete
                 CodeContext
+                LineNumber
diff -r 39c67beb483a Lib/idlelib/idle_test/test_linenumber.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/idle_test/test_linenumber.py	Sat Aug 16 11:50:25 2014 +0530
@@ -0,0 +1,138 @@
+"""Unittest for idlelib.LineNumber"""
+import unittest
+from test.support import requires
+import tkinter as tk
+from idlelib.Percolator import Percolator
+from idlelib.LineNumber import LineNumber
+
+
+class Dummy_editwin:
+    def __init__(self, text):
+        self.text = text
+        self.per = Percolator(text)
+
+
+class LineNumberTest(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        requires('gui')
+        cls.root = tk.Tk()
+        cls.text = tk.Text(cls.root)
+        cls.editwin = Dummy_editwin(cls.text)
+        cls.editwin.vbar = tk.Scrollbar(cls.root)
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.editwin.per.close()
+        cls.text.destroy()
+        cls.root.destroy()
+        del cls.text, cls.root
+
+    def setUp(self):
+        self.linenumber = LineNumber(self.editwin)
+
+    def tearDown(self):
+        self.text.delete('1.0', 'end')
+
+    def test_init_empty(self):
+        get = self.linenumber.sidebar_text.get
+        self.assertEqual(get('1.0', 'end'), '1\n')
+
+    def test_init_not_empty(self):
+        self.text.insert('insert', 'foo bar\n'*3)
+        linenumber = LineNumber(self.editwin)
+        get = linenumber.sidebar_text.get
+        self.assertEqual(get('1.0', 'end'), '1\n2\n3\n4\n')
+
+    def test_toggle_linenumbering(self):
+        ENABLED = True
+        DISABLED = False
+
+        self.linenumber.state = ENABLED
+        self.linenumber.toggle_linenumbering_event('')
+        self.assertEqual(self.linenumber.state, DISABLED)
+
+        self.linenumber.state = DISABLED
+        self.linenumber.toggle_linenumbering_event('')
+        self.assertEqual(self.linenumber.state, ENABLED)
+
+    def test_insert(self):
+        text = self.editwin.text
+        get = self.linenumber.sidebar_text.get
+        config = self.linenumber.sidebar_text.config
+        equal = self.assertEqual
+
+        text.insert('insert', 'foobar')
+        equal(get('1.0', 'end'), '1\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+        text.insert('insert', '\nfoo')
+        equal(get('1.0', 'end'), '1\n2\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+        text.insert('insert', 'hello\n'*2)
+        equal(get('1.0', 'end'), '1\n2\n3\n4\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+        text.insert('insert', '\nworld')
+        equal(get('1.0', 'end'), '1\n2\n3\n4\n5\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+    def test_delete(self):
+        text = self.editwin.text
+        get = self.linenumber.sidebar_text.get
+        config = self.linenumber.sidebar_text.config
+        equal = self.assertEqual
+
+        text.insert('insert', 'foobar')
+        text.delete('1.1', '1.3')
+        equal(get('1.0', 'end'), '1\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+        text.insert('insert', 'foo\n'*2)
+        text.delete('3.1')
+        text.delete('2.0', '2.end')
+        equal(get('1.0', 'end'), '1\n2\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+        text.delete('1.3', 'end')
+        equal(get('1.0', 'end'), '1\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+        text.delete('1.0', 'end')
+        equal(get('1.0', 'end'), '1\n')
+        equal(config()['state'][-1], tk.DISABLED)
+
+    def test_sidebar_text_width(self):
+        """
+        Test that linenumber text widget is always at the minimum
+        width
+        """
+        def get_width():
+            return self.linenumber.sidebar_text.config()['width'][-1]
+        text = self.text
+        equal = self.assertEqual
+
+        equal(get_width(), 1)
+
+        text.insert('insert', 'foo')
+        equal(get_width(), 1)
+
+        text.insert('insert', 'foo\n'*10)
+        equal(get_width(), 2)
+
+        text.insert('insert', 'foo\n'*100)
+        equal(get_width(), 3)
+
+        text.delete('50.0', 'end')
+        equal(get_width(), 2)
+
+        text.delete('5.0', 'end')
+        equal(get_width(), 1)
+
+        text.delete('1.0', 'end')
+        equal(get_width(), 1)
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)