Can't PWM LED on pin 18

Question

Using this code, I can't use PWM on the pi. It's modified off of a Linux user article.

import time
import RPi.GPIO as GPIO
led = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(led, GPIO.OUT)
GPIO.output(led,GPIO.LOW)
time.sleep(0.5)
pwm = GPIO.PWM(led, 50)
time.sleep(0.5)
pwm.ChangeDutyCycle(25)
time.sleep(0.5)

The LED stays off, or if I change the line GPIO.output(led,GPIO.LOW) to GPIO.output(led,GPIO. HIGH) it stays on. I receive no error or warning messages.

EDIT: Here is the code I modified it off, when I run this (sudo python candle_light.py) it flickers as described.

#!/usr/bin/env python

# Import the modules used in the script
import random, time
import RPi.GPIO as GPIO

# Assign the hardware PWM pin and name it
led = 18

# Configure the GPIO to BCM and set it to output mode
GPIO.setmode(GPIO.BCM)
GPIO.setup(led, GPIO.OUT)

# Set PWM and create some constants we'll be using
pwm = GPIO.PWM(led, 100)
RUNNING = True
WIND = 9

def brightness():
   """Function to randomly set the brightness of the LED between 5 per cent and 100 per cent power"""
   return random.randint(5, 100)

def flicker():
   """Function to randomly set the regularity of the'flicker effect'"""
   return random.random() / WIND

print "Candle Light. Press CTRL + C to quit"

# The main program loop follows.
# Use 'try', 'except' and 'finally' to ensure the program
# quits cleanly when CTRL+C is pressed to stop it.

try:
   while RUNNING:
      # Start PWM with the LED off
      pwm.start(0)
      # Randomly change the brightness of the LED
      pwm.ChangeDutyCycle(brightness())
      # Randomly pause on a brightness to simulate flickering
      time.sleep(flicker())

# If CTRL+C is pressed the main loop is broken
except KeyboardInterrupt:
   running = False
   print "\nQuitting Candle Light"

# Actions under 'finally' will always be called, regardless of
# what stopped the program (be it an error or an interrupt)
finally:
   # Stop and cleanup to finish cleanly so the pins
   # are available to be used again
   pwm.stop()
   GPIO.cleanup()

Answer 1

I found the answer, simple mistake - I was missing a pwm.start(int) before using the pwm variable. The working code should look like this:

GPIO.setmode(GPIO.BCM)
GPIO.setup(led, GPIO.OUT)
GPIO.output(led,GPIO.LOW)
time.sleep(0.5)

pwm = GPIO.PWM(led, 50)
pwm.start(50)#d'oh!

time.sleep(0.5)
pwm.ChangeDutyCycle(25)
time.sleep(0.5)   
pwm.stop()
GPIO.cleanup()

This starts the LED at half brightness, waits half a second, reduces it to a quarter brightness, waits half a second, then cleans up and stops.