Multiple Python versions on Windows: use py, venv and absolute paths

Verified 2026-10-291 min read

Keep Python 3.11 and 3.12 side by side, create a venv per app, and pin the interpreter instead of relying on PATH or the Store alias.

Tested onStandard / Datacenter · Build 26100.33650 · x64

Build: 26100.33650
Edition: Standard / Datacenter
Architecture: x64
Support: Documentation (E1)
Evidence: E1Official documentation
Verified: 2026-10-29
Page type: Method

The short answer

Use the py launcher to select a version and a venv per app to pin dependencies:

py -0p
py -3.11 -m venv C:\apps\api-a\.venv
C:\apps\api-a\.venv\Scripts\python.exe -m pip install -r requirements.txt

The venv’s python.exe is the production selector. Do not rely on python from PATH, and do not install app dependencies globally.

See what is installed

py -0p
py -3.11 --version
py -3.12 --version
where.exe python

If python opens the Microsoft Store, disable the App Execution Alias or use py/absolute paths. The Store alias is a common source of “wrong Python” confusion.

Per-app venv

py -3.11 -m venv C:\apps\api-a\.venv
C:\apps\api-a\.venv\Scripts\python.exe -m pip install --upgrade pip
C:\apps\api-a\.venv\Scripts\python.exe -m pip install -r C:\apps\api-a\requirements.txt

The venv records its base interpreter. Recreating it is cheaper than debugging a mixed global site-packages tree.

For a service, use the absolute venv python:

C:\apps\api-a\.venv\Scripts\python.exe -m waitress --listen=127.0.0.1:8000 app:app

Verify

C:\apps\api-a\.venv\Scripts\python.exe -c "import sys; print(sys.executable); print(sys.version)"
C:\apps\api-a\.venv\Scripts\python.exe -m pip list

Check sys.executable points at the app venv, not C:\Python311\python.exe.

Upgrade path

Install the new Python beside the old.
Create a new venv for the app (py -3.12 -m venv .venv312).
Install from the lock/requirements; run tests.
Switch the service path to the new venv.
Keep the old venv until rollback is no longer needed.

Lab boundary

E1. The lab did not install multiple Python versions or run a WSGI service; the commands are the selector pattern to verify on Server 2025.