Short answer

1. Start the virtual environment
2. Run python -m idlelib.idle

From this answer.

Long answer

This answer assumes Python 3.

There are a few different virtual environment managers, each of which has a slightly different way of handling where python is installed and how it’s run, as detailed in this answer.

This answer assumes the venv module is used, and that it was installed following the docs.

Note: Some Linux distributions package the venv module into a separate package: Ubuntu and Debian

If the virtual environment was installed in a folder called my_project-venv by running python -m venv my_project-venv from inside the folder my_project, the virtual environment will be inside a new folder created by the module:

my_project_dir
      │
      ┝━ my_project-venv

On Windows, with Python 3.7.1, the files inside the my_project-venv folder will probably look like this:

my_project-venv
      │
      ┝━ Include
      ┝━ Lib
      ┝━ Scripts
      │     ┝━ ...
      │     ┝━ activate.bat
      │     ┝━ Activate.ps1
      │     ┝━ deactivate.bat
      │     ┕━ ...
      │
      ┕━ pyvenv.cfg

The virtual environment can be started by running either the activate.bat or Activate.ps1 script, depending on whether cmd or PowerShell is used:

:: Using cmd.exe
cd my_project_dir
.\my_project-venv\Scripts\activate.bat

# Using PowerShell
cd my_project_dir
.\my_project-venv\Scripts\Activate.ps1

Note: These scripts don’t keep the shell open if run by double-clicking them. Start a shell, then run them by typing the above commands, with the folder names changed for your project

On most other operating systems, the virtual environment folder will look like this:

my_project-venv
      │
      ┝━ bin
      │     ┝━ ...
      │     ┝━ activate
      │     ┝━ activate.csh
      │     ┝━ activate.fish
      │     ┕━ ...
      │
      ┝━ include
      ┝━ lib
      ┝━ lib64
      ┕━ pyvenv.cfg

Then, from any shell other than csh or fish, activate the environment by:

# Most operating systems
cd my_project_dir
. my_project-venv/bin/activate

For csh and fish there are shell-specific scripts for activating the virtual environment (activate.csh and activate.fish, respectively) and they can be run like the activate script.

Once the virtual environment has been activated on all operating systems, running the following will start IDLE with access to the packages installed into the virtual environment:

python -m idlelib.idle