Control a web radio stream on a Raspi with GPIO, a button and an LED.

wfmugpio.py

#!/usr/bin/env python

import subprocess as sp
import RPi.GPIO as g
import time
import os

# see http://cyfta.com/?p=8480
# & listen to WFMU! http://wfmu.org

# "constants", change these.
# raspi pinout
pin_button = 4
pin_led = 17
# seconds before we accept another input
# (give mplayer time to terminate)
input_delay = 5 
# run 'id' to see these
user_name = 'pi'
user_id = 1000
user_gid = 1000
user_home_dir = '/home/pi'

# do not change the URL.
player_cmd = ["mplayer", "-quiet", "http://stream0.wfmu.org/freeform-high.aac"]

print "setting up pins..."
g.setmode(g.BCM)
g.setup(pin_button, g.IN)
g.setup(pin_led, g.OUT)

# run subprocess as different, non-root user
# https://stackoverflow.com/a/6037494
def demote ():
    def result ():
        global user_name, user_id, user_gid
        #os.setgid(1001)
        # we need initgroups to set all groups, including 'audio'
        os.initgroups(user_name, user_gid)
        os.setuid(user_id)
        print 'demoted %d %d' % (os.getgid(), os.getuid())
    return result

env = os.environ.copy()
env[ 'HOME'     ]  = user_home_dir
env[ 'LOGNAME'  ]  = user_name
env[ 'PWD'      ]  = user_home_dir
env[ 'USER'     ]  = user_name

p = None 
blink = False
wait = 0

def play ():
    global p, blink, player_cmd, user_home_dir
    if p is not None and p.poll() is None:
        print "seems to be playing. terminating!"
        p.terminate()
    if p is None or p.poll() is not None:
        print "play!"
        p = sp.Popen(
            player_cmd,
            preexec_fn = demote(),
            cwd = user_home_dir,
            env = env
        )

def stop ():
    global p
    print "terminating!"
    p.terminate()

# let the led flutter!
def flutter (length, delay):
    for x in range(0, length):
        g.output(pin_led, g.LOW if x % 2 == 0 else g.HIGH)
        time.sleep(delay)

flutter(6, 0.05) # hi!
print "starting polling..."
while True:
    poll = p.poll() if p is not None else -1
    # we could do this using events...
    if g.input(pin_button) == g.HIGH:
        # if we are not blocked by a previous input...
        if wait == 0:
            flutter(6, 0.05)
            wait = input_delay
            if poll is None:
                stop()
            else:
                play()
        else:
            # if we're blocked, give visual feedback
            flutter(50, 0.02)
    blink = not blink if poll is None else False
    g.output(pin_led, g.LOW if blink else g.HIGH)
    wait = wait - 1 if wait > 0 else 0
    time.sleep(1) # adjust this to poll for input faster