Using Python Launcher and Shebang to Manage Multiple Python Versions on Windows and Linux

The author introduces the common problem of needing different Python interpreter versions (Python 2.x vs Python 3.x) for different scripts and the confusion caused by the default python command that follows the system PATH.

On Unix‑like systems the solution is to use a shebang line (e.g., #!/usr/bin/python2) combined with a symbolic link that points the generic python command to the desired interpreter.

On Windows, PEP 397 introduced the Python Launcher (executables py.exe and pyw.exe) which reads the shebang in a script and automatically selects the appropriate interpreter.

Installation of the launcher is optional during the Python installer setup (starting with Python 3.3) or can be performed separately; once installed it resides in the system PATH.

The launcher provides two executables: py.exe for console programs and pyw.exe for GUI programs. Their locations can be verified with:

where py
C:\Windows\py.exe

where pyw.exe
C:\Windows\pyw.exe

Typical usage examples:

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

# Execution
M:\>py test.py   # runs with Python 2.7

Changing the shebang to #! python3 makes the same script run with the latest Python 3 interpreter:

#! python3
import sys
print(sys.version)

M:\>py test.py   # runs with Python 3.7

If a script lacks a shebang, the launcher defaults to the newest installed interpreter:

import sys
print(sys.version)

M:\>py test.py   # runs with Python 3.7

Explicit version selection is also possible via command‑line switches, e.g.:

M:\>py -3 test.py      # force Python 3
M:\>py -3.5 test.py    # force Python 3.5

In summary, the Python Launcher simplifies version management on Windows, while shebang lines provide a portable way to specify interpreter versions on both Windows and Linux, eliminating the need for manual PATH adjustments.