glorp | A CLI-based HTTP intercept and replay proxy | Proxy library

The main problem here is that the Python interpreter is not in interactive mode when we start it as a subprocess. As the Python documentation notes:

When called with standard input connected to a tty device, it prompts for commands and executes them

In this case, the subprocess's stdin is not connected to a terminal ("tty device"). And

In non-interactive mode, the entire input is parsed before it is executed.

So Python just sits there and does nothing until stdin is closed. But we can force it into interactive mode with the -i command-line option. Then:

When a script is passed as first argument or the -c option is used, enter interactive mode after executing the script or the command, even when sys.stdin does not appear to be a terminal.

It's also useful to pass the -q option

Don’t display the copyright and version messages even in interactive mode.

and communicate with the subprocess in unbuffered mode by setting bufsize=0. Then we are able to have the interpreter run multiple commands and we can inspect the output in between: