raw_input in python without pressing enter

Under Windows, you need the msvcrt module, specifically, it seems from the way you describe your problem, the function msvcrt.getch:

Read a keypress and return the resulting character. Nothing is echoed to the console. This call will block if a keypress is not already available, but will not wait for Enter to be pressed.

(etc -- see the docs I just pointed to). For Unix, see e.g. this recipe for a simple way to build a similar getch function (see also several alternatives &c in the comment thread of that recipe).

Python does not provide a multiplatform solution out of the box.
If you are on Windows you could try msvcrt with:

import msvcrt
print 'Press s or n to continue:\n'
input_char = msvcrt.getch()
if input_char.upper() == 'S': 
   print 'YES'

curses can do that as well :

import curses, time

def input_char(message):
    try:
        win = curses.initscr()
        win.addstr(0, 0, message)
        while True: 
            ch = win.getch()
            if ch in range(32, 127): 
                break
            time.sleep(0.05)
    finally:
        curses.endwin()
    return chr(ch)

c = input_char('Do you want to continue? y/[n]')
if c.lower() in ['y', 'yes']:
    print('yes')
else:
    print('no (got {})'.format(c))