Windows command resolution has distinct layers

Inspect which executable will run

PowerShell aliases, functions and commands can shadow binaries with the same short name. The Python launcher (py) may select a different installed interpreter than the python command on PATH. A package installed with one interpreter will not necessarily be available through the other. Check the command type and executable path before changing environment variables or reinstalling Python.

Get-Command python -All
Get-Command py -All
where.exe python
py -0p
python -c "import sys; print(sys.executable)"

When a virtual environment appears broken

A venv created on one computer may contain interpreter paths that do not exist on another; recreate it from the dependency manifest rather than copying its whole directory. PowerShell activation can also be affected by execution policy, but launching .venv\Scripts\python.exe directly often avoids activation entirely. Do not disable Windows security features globally merely to run one script.

Different errors, different responses

“Python not found” suggests command discovery or installation. “ModuleNotFoundError” after Python starts suggests packages installed into the wrong interpreter, a shadowed module name or missing dependency. An “Access denied” on a downloaded executable involves rights or file blocking rather than a Python syntax error. Inspect the full error and the path involved.

End-to-end exercise

Create a disposable Python project with a requirements file and a fresh venv. Print sys.executable, install one harmless dependency using that binary's -m pip, and run a simple import test. Record Windows version, architecture and exact launcher choice. Validate without administrator rights whenever the project doesn't require them.

Run the chosen interpreter without activation

& .\.venv\Scripts\python.exe -c "import sys; print(sys.executable); print(sys.prefix)"
& .\.venv\Scripts\python.exe -m pip --version

PowerShell's call operator & is especially useful for an executable path held in a variable or containing spaces. Activating a venv changes the current shell environment; it is not required to invoke that interpreter directly. The legacy py -0p listing is not supported by every launcher implementation, so inspect py --help if that option is rejected.

Change the narrowest scope and verify a new shell

$env:Path changes the current process environment. User-level and machine-level environment settings have different persistence and authority. Existing shells do not automatically receive all changes made elsewhere. After a deliberate PATH edit, open a fresh shell, run Get-Command python -All, then print sys.executable again. Preserve unrelated PATH entries and never replace the entire value merely to add a project tool.

Test a path containing spaces and a local file named json.py: the latter can shadow Python's standard library independently of PATH. The resolution chain includes both shell command lookup and Python module lookup; recording only the first leaves half the diagnosis unexplained.