How Python Launcher Simplifies Multi‑Version Python Execution on Windows and Linux

1. Problem Statement

You may have encountered the issue where the python command runs an unexpected interpreter version, especially when a script requires a specific Python 2.x interpreter while the default python points to Python 3.x. python test.py Because the system resolves python via the PATH, the interpreter that appears first in PATH is used, which can lead to running the wrong version.

In Linux

On Unix‑like systems, a symbolic link can bind a generic python command to a concrete interpreter path. The shebang line (e.g., #!/usr/bin/python2) then causes the script to invoke the linked interpreter.

#!/usr/bin/python2

In Windows

Windows faces the same issue, and the solution is provided by the Python Launcher introduced in PEP 397 and shipped with Python 3.3 and later.

2. Solution

The Python Launcher ( py.exe for console, pyw.exe for GUI) can locate and execute the appropriate Python version based on the script’s shebang or command‑line options.

How to Install Python Launcher

Starting with Python 3.3, the launcher is an optional component during installation. Selecting it adds py.exe to the system PATH. It needs to be installed only once and works with all later Python versions.

How to Use Python Launcher

After installation, you can run py from any directory. By default, it launches the newest installed interpreter. If multiple versions exist, you can specify the desired one:

py -2 test.py   # use the latest Python 2.x

py -3.7 test.py # use Python 3.7

Shebang lines also guide the launcher. For example, a script starting with #! python2.7 will be executed with Python 2.7, while #! python3 selects the latest Python 3.x.

#! python2.7
import sys
print(sys.version)

Running py test.py will automatically invoke the interpreter indicated by the shebang. If no shebang is present, the launcher defaults to the newest version, but you can override it with -2, -3, or a specific version number.