История изменений

Код CPP это пример.

я не знаю что такое man

Ты про это?
man popen

Since the standard input of a command opened for reading shares its seek offset with the process that called popen(), if the original process has done a buffered read, the command's input position may not be as expected. Similarly, the output from a command opened for writing may become intermingled with that of the original process. The latter can be avoided by calling fflush(3) before popen().

Сделал:

handle = Popen(CMD, shell=True, stdin=PIPE) 
time.sleep(2)
handle.stdin.flush()
handle.stdin.write("123")
time.sleep(2)

Код CPP это пример.

я не знаю что такое man

Ты про это?
man popen

Since the standard input of a command opened for reading shares its seek offset with the process that called popen(), if the original process has done a buffered read, the command's input position may not be as expected. Similarly, the output from a command opened for writing may become intermingled with that of the original process. The latter can be avoided by calling fflush(3) before popen().