IDLE

IDLE is an integrated development environment for Python, which has been bundled with the default implementation of the language since 1.5.2b1. It is packaged as an optional part of the Python packaging with many Linux distributions. It is completely written in Python and the Tkinter GUI toolkit.
IDLE is intended to be a simple IDE and suitable for beginners, especially in an educational environment. To that end, it is cross-platform, and avoids feature clutter.
According to the included README and the online IDLE documentation, its main features are:

Multi-window text editor with syntax highlighting, autocompletion, smart indent and other features.
Python shell with syntax highlighting.
Integrated debugger with stepping, persistent breakpoints, and call stack visibility.

Author Guido van Rossum says IDLE stands for "Integrated Development and Learning Environment", and since Van Rossum named the language Python after the British comedy group Monty Python, the name IDLE was probably also chosen partly to honor Eric Idle, one of Monty Python's founding members.