Controlling an Arduino UNO from Raspberry Pi 3 (Controlling not programming)

Question

I am working on an autonomous robot, which uses a python script running on my RPi 3 to detect traffic lights. I want that as soon as the RPi detects that the traffic light has gone off, it would signal the Arduino to do its thing (i.e run the sketch that I have made for the robot).

Now the thing is, all I can do is to run the Arduino IDE on my RPi, I can upload code to it, I can see the output on the Serial monitor, but how do I get my python script to interface with the Arduino. Let's say upon detecting the traffic light switching off, the python script changes a variable arduinogo from 0 to 1, and I set up my Arduino accordingly to receieve the variable and if its value is 1, do its job. So how can I achieve this communication?

EDIT:
I got it to work, I followed a tutorial here - http://www.instructables.com/id/Interface-Python-and-Arduino-with-pySerial/ . This uses pySerial to communicate with the Arduino's serial port. I was able to make the Arduino's onboard LED light up by passing a variable having an ASCII character from my python script written on my PC (Win 8.1) on the Netbeans 8.1 IDE. This works flawlessly on my PC, I have downloaded the tar package to install on my Raspberry Pi, lets see if this works this well on the RPi as well.

Answer 1

If your sketch on arduino is taking input from serial interface, you should be looking for a serial I/O library in python and used it to send the commands over the serial interface.

Answer 2

If you want the Pi to cause the Arduino to do "complicated" tasks, then go with Tahir's answer. If it's simpler than that, why not just attach a GPIO wire from the pi to the arduino? Failing that, this is the kind of task that I2C was absolutely made for.

Answer 3

There are many different ways of communicating.

You can use serial over USB cable or directly.

You can use direct connection of GPIO pins and just use digitalRead digitalWrite or a protocol like SPI ORI²C.

I have used all of these depending on the project.

Whatever you do make sure that you ensure the Pi 3.3V logic and Uno 5V do not mix. There are many different ways of doing this.