The REPL won't start in interactive mode, it only starts when explicitly given the `-i` option. 

The problems IPython is having, e.g. missing syntax highlighting and no tab completion should have the same root.

See this issue for mingw:
https://github.com/Alexpux/MINGW-packages/issues/730

They fixed it by patching python using this patch:
https://github.com/Alexpux/MINGW-packages/blob/master/mingw-w64-python3/1700-cygpty-isatty.patch

And this issue at msys2:
https://sourceforge.net/p/msys2/tickets/32/

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.

Tab completion does not work in the REPL even when started with `-i -u`, so there might be a deeper problem than just the PIPE that's preventing python from starting in interactive mode.

I'd assume this is using ConEmu's ChildGui which means it hacks up a MinTTY window to appear inside ConEmu's GUI. MinTTY doesn't use a real terminal but rather pipes as the processes stdio. Any native (non-MSYS2) Windows program would have a problem with this.

The known workaround is to launch such programs using WinPTY (Available in the MSYS2 pacman), which creates a real console for the program in the background and communicates it's input/output so that it appear in the original console.

Sadly that is a limitation of Windows, as Windows does not supply a proper PTY interface and MSYS2/MinTTY choose not to use something like WinPTY by default.