How to launch python Idle from a virtual environment (virtualenv)

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

On Windows, a Python script run from command line like this some_script.py might be run by other Python interpreter than the one used when using python some_script.py command (it depends on py files association). If one wants to avoid this problem it's best to create a batch file idle.bat with the content python -c "from idlelib.PyShell import main; main()" and place it in the Scripts folder in the virtualenv. Also, like others noted idle needs both tcl and tk folders to work. The simplest solution is to create symbolic links from virtualenv to the base Python installation like this

(2.7) c:\python\virtualenv\2.7\Lib>mklink /d tcl8.5 "c:\Program Files\Python\2.7\tcl\tcl8.5"
symbolic link created for tcl8.5 <<===>> c:\Program Files\Python\2.7\tcl\tcl8.5
(2.7) c:\python\virtualenv\2.7\Lib>mklink /d tk8.5 "c:\Program Files\Python\2.7\tcl\tk8.5"
symbolic link created for tk8.5 <<===>> c:\Program Files\Python\2.7\tcl\tk8.5

Python 3.6 modernized and refactored idlelib. This change included the renaming of several methods. Because of this, idlelib.PyShell must now be accessed with idlelib.pyshell. The following snippet is based on the accepted answer and should work for any Python version:

#!/usr/bin/env python
"""Simple script to run Idle from a venv in PyCharm."""

try:
    # Import for Python pre 3.6
    from idlelib.PyShell import main
except ModuleNotFoundError:
    # Import for Python version 3.6 and later
    from idlelib.pyshell import main

if __name__ == '__main__':
    main()

For Python 3.6+, please see Paul Wicking's answer below.

In Python prior to 3.6, IDLE is essentially

from idlelib.PyShell import main
if __name__ == '__main__':
  main()

So you can launch it yourself unless you built the virtualenv without default packages.