Keep Both Python 2 and Python 3 on Windows

Here is notes to keep both Python 2 and Python 3 working on Windows.

New in Python 3.3

Start from Python 3.3, there is a py.exe installed to %SystemRoot% directory, which is helpful when multiple versions of Python installed.

• py -2 and py starts Python 2.x
• py -3 starts Python 3.x

Use shebang in Python scripts

In favor of py.exe, shebang in Python scripts can be recognized on Windows.

• #! python 3 for Python 3.x
• #! python 2 for Python 2.x

Installation order matters

If you install python3 first and then Python 2, the setup program of Python 2 will overwrite the .py file association, such that a Python 3 script with shebang #!python 3 will be executed by Python 2. So the better order is install Python 2 first and then Python 3.

Don’t need to add python to PATH

If both Python 2 and Python 3 executables are on PATH, then there will be confusion. Instead of add Python to PATH and call python your-script.py, the following command also work your-script.py.