idle3 software package provides IDE for Python using Tkinter (default version), you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install idle3
idle3 is installed in your system.
Make ensure the idle3 package were installed using the commands given below,
$ sudo dpkg-query -l | grep idle3 *
You will get with idle3 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Python |
| Package | idle3 |
| Version | 3.5.1-4 |
| Description | IDE for Python using Tkinter (default version) |
| Sub Section | Python |