Here's how to get Raspberry Pi and Arduino to talk to each other.

General Purpose Input Output (GPIO)

Control the GPIO pins on Rasberry PI using Python

 sudo apt-get update

 sudo apt-get install python-rpi.gpio

Font Inconsolata

Improve the user experience of your Arduino editor by installing Inconsolata

 sudo apt-get install ttf-inconsolata

Then edit the Arduino preferences:

 nano ~/.arduino/preferences.txt

and change the following lines to:

 editor.font=Inconsolata,medium,14
 editor.antialias=true

Arduino module

sudo apt-get update

sudo apt-get install arduino

Rasberry Pi & Arduino over serial

You can get Raspberry Pi and Arduino to talk to each over a serial connection

sudo apt-get install python-serial python3-serial

Open Arduino and upload this script:

 void setup(){
   Serial.begin(9600)
 }

Count upwards and send each number over the serial connection

void loop(){
  for(byte n=0;n<255;n++){
    Serial.write(n);
    delay(50);
  }
}

Open Python editor and execute

import serial

port = "/dev/ttyACM0"
serialFromArduino = serial.Serial( port, 9600 )
serialFromArduino.flushInput()
while True:
  if( serialFromArduino.inWaiting() > 0 ):
    input = serialFromArduino.read(1)
    print( ord(input) )