`cmd.exe dir` ignores the unrecognized `dir` argument and runs the interactive shell. The shell is waiting for input on stdin. You can type "exit" and press enter to return to Python.  

When you run this from IDLE (i.e. via pythonw.exe), there's no StandardInput handle to inherit, so subprocess executes cmd.exe with its stdin set to a closed pipe. (I don't know why a closed pipe is used. I think "NUL" would be better.) The shell exits immediately because ReadFile fails with ERROR_BROKEN_PIPE.