"Saimadhav, please try class_="Idle" (title case). I suppose that IDLE's activity bar entry will be named "IDLE" (upper case), because Gnome Shell will get the name from IDLE's desktop file (/usr/share/applications/idle.desktop)."

No. It stays "Idle". Also there was no file for me in /usr/share/applications/idle.desktop. An idle.desktop file was found in /usr/share/app-install/desktop/idle.desktop, whose content is
[Desktop Entry]
X-AppInstall-Package=idle
X-AppInstall-Popcon=132
X-AppInstall-Section=main

Name=IDLE
Comment=Integrated Development Environment for Python
Exec=/usr/bin/idle
Icon=/usr/share/pixmaps/idle.xpm
Terminal=false
MultipleArgs=false
Type=Application
Categories=Application;Development;
StartupNotify=true

I don't have any icon file at /usr/share/pixmaps/idle.xpm

After copying the above idle.desktop to /usr/share/applications/, the activity bar becomes "IDLE". There is NO icon.
If I set class_='Idle3', it works as expected with icon and text.

With 2.7, 'Idle2', displays same behavior with proper icon and text.
With 2.7, 'Idle' it displays "IDLE" and no icon.