Communicating to Scratch via Python

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This article or section documents something not included in the current version of Scratch (3.0). It is only useful from a historical perspective.

Here are some instructions on how to use Python 3.x to communicate with Scratch 1.x via Remote Sensor Connections. These instructions will create a minimal program that sends commands.

Install Python

• Install Python 3
  • macOS has Python 2 pre-installed.
  • The latest release version can be obtained from here.

Sample Program "Hello"

Make a new project in Scratch 1.4. Find the Sensor Value block in the palette, right click on it, and select "enable remote sensor connections". Then drag out a When I Receive block and create the broadcast "hello".

Open a text editor and create a new file. Save it as communication.py and put this code in it:

import socket

HOST = 'localhost'
PORT = 42001

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((HOST, PORT))

def sendCMD(cmd):
    sock.send(len(cmd).to_bytes(4, 'big'))
    sock.send(bytes(cmd, 'UTF-8'))

sendCMD('broadcast "hello"')

Then, run the program with Python. This can be done from IDLE, or from the command line: on Microsoft Windows, type this into Command Prompt, followed by Enter; on macOS or Linux, use the Terminal.

python communication.py

(On macOS and some (GNU/)Linux distributions, python means Python 2.7, so instead type python3.) Data can also be sent to Scratch by sending sensor-update, followed by the name of a variable, followed by its new value. Then use Sensor Value to obtain the value. (Refer to Remote Sensors Protocol for details.)