A simple example of controlling a servo motor with RPi.GPIO

servo.py

import RPi.GPIO as GPIO
import time

servoPIN = 21
GPIO.setmode(GPIO.BCM)
GPIO.setup(servoPIN, GPIO.OUT)

p = GPIO.PWM(servoPIN, 50) # GPIO 21 for PWM with 50Hz
p.start(0) # Initialization

# Pulled minimum and maximum pulse from the datasheet for HS-322HD Servo: https://www.servocity.com/hs-322hd-servo
pulseMin = 553/1000.0
pulseMax = (2450 + 300)/1000.0 # Note that the datasheet indicated 2.450ms was the maximum range.  But that wasn't giving us 180 degrees, so we bumped it up and got 180 with another 0.3ms of pulse width

period = 20 # The "default" period (or cycle time) of 50Hz = 1/50th of a second = 1000ms/50 = 20ms
pulseRange = pulseMax - pulseMin # The range in milliseconds of our shortest to longest pulse

def turnTo(degrees): # A function to turn to a certain number of degrees
    if (degrees >= 0) and (degrees <= 180): # Check that the requested degrees is between 0 and 180
        pulseWidth = pulseMin + degrees/180*pulseRange # Compute our pulseWidth as our minPulse + percentage of our pulseRange— 180 is 100%, 0 is 0%
        dutyCycle = pulseWidth/period*100 # Compute our duty cycle frmo our pulse width (as a %)
        p.ChangeDutyCycle(dutyCycle) # Set the duty cycle
    else: # If we didn't get a valid degree number, tell people
        print degrees + " out of bounds"

try:
    while True: # Repeat forever
        turnTo(0) # Turn to 0 degrees
        time.sleep(1) # Wait one second
        turnTo(180) # Turn to 180 degrees
        time.sleep(1) # Wait one second

except KeyboardInterrupt: # When someone hits Ctrl+c
  p.stop() # Stop my PWM pin
  GPIO.cleanup() # And de-register all my GPIO pins