IDLE is the Python IDE built with the Tkinter GUI toolkit.
IDLE has the following features:
- coded in 100% pure Python, using the Tkinter GUI toolkit
 - cross-platform: works on Windows and Unix 
Included with http://homepages.cwi.nl/~jack/macpython/ for Mac OS X
 - multi-window text editor with multiple undo, Python colorizing and many other features, e.g. smart indent and call tips
 - Python shell window (a.k.a. interactive interpreter)
 - debugger (not complete, but you can set breakpoints, view and step)
 
It is a part of the standard library and is described in the reference manual: https://docs.python.org/library/idle.html
The source code for IDLE is included in the standard library. More information on developing IDLE can be found at http://docs.python.org/devguide/
See also: IDLEfork, IDLEextensions
