> Fully usable, portable

*and unsupported* - you apparently ignored that point from Steve's comment

> and working Python interpreter

That's (essentially) pure luck. I don't think I've ever seen any
suggestion that the /a flag for *any* MSI installer (Python or
otherwise) will produce a working install - it's designed (and
documented by Microsoft) as being to allow system admins to unpack the
MSI onto a network share from where it can be installed onto user
machines (for details see
https://msdn.microsoft.com/en-us/library/windows/desktop/aa367541(v=vs.85).aspx).