Source code: Lib/idlelib/
IDLE is Python’s Integrated Development and Learning Environment.
IDLE has the following features:
coded in 100% pure Python, using the tkinter GUI toolkit cross-platform: works mostly the same on Windows, Unix, and Mac OS X Python shell window (interactive interpreter) with colorizing of code input, output, and error messages multi-window text editor with multiple undo, Python colorizing, smart indent, call tips, auto completion, and other features search within any w