The problem as I understand it is that Msys uses a pipe, but Python by default limits interactive REPL mode to terminals only. The same thing happens if you start Python on a pipe some other way, for instance “cat | python” vs “cat | python -i”.

I would prefer if Python decided to use interactive mode when no file name is given, rather than by checking if the file is a terminal. But that would break compatibility.

Another tip with Msys: enable unbuffered output with “python -u”. Otherwise output gets buffered, won’t be seen immediately, and may be lost if there is an unclean exit.