I would like to install Python in a new location, completely separate and not affecting an existing installation of the same version. 

Despite I use /TargetDir=newdir, the installer goes into the "Modify Setup" dialog. If I chose "Modify", it shows my TargetDir read-only in the Customize installation location, but then modifies the original installation. 

If I run it with /passive or /quiet, it also modifies the existing installation instead of creating a new one at /TargetDir (doing nothing, since there is nothing to modify).

If I choose repair at the prompt, it installs the Debug version (only!) into the TargetDir, with no library files except venv\scripts\nt (containing the debug .exe). 

It seems the installer checks whether there is an existing installation based on the registry (or by other means such peeking into the default installation directory). If there is an existing installation, the  /TargetDir seems to be ignored, except for the peculiar repair installation of the debug build (which I never asked for).

I believe this to be a bug in the installer; however if ignoring the /TargetDir once an existing installation is found (which I do not find desirable), it should be documented. 

The /TARGETDIR option of the old .msi installer didn't have this problem, it would reliably install into the target dir.