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Scratch Networking Sensor Client API for Python

15.01.2013 Stefan Wendler - [] (


This document describes how to use the Python API for creating a Scratch Networking Sensor Client. The main thing provided by the API is a simple way to send and receive sensor value updates and broadcast messages.

For communicating with Scratch, the API implements the remote sensor protocol [as described here] (

To see, how to install the API see the [README] (../ that is included.

Basic API Usage


Before you start using the API, make sure, Scratch is running, and the remote sensor protocol is activated (by using the right click menu on one of the sensor blocks). For a more detailed description of how to [enable/use remote sensors] ( see the Scratch Wiki.

Create a new RemoteSensor

First, the remote sensor module needs to be imported:

from scratch.remote sensor import RemoteSensor 

Now, a new RemoteSensor instance needs to be created. If you intend to connect to a Scratch instance running on the same machine as your API program, the host and port parameters could be omitted:

# Remote sensor connected to default host/port (localhost:42001)
rs = RemoteSensor()

To receive updates and messages, the receiver thread needs to be started:

# Start receiver thread

Using Sensor Values

Each sensor value is represented by a variable. To introduce a new variable, or assign a new value to a already introduced variable, just assign the desired value to it in the same way as you would do for a class attribute:

# Create new sensor variable 'a', set value to 1. This will result
# in a 'sensor-update' message sent to Scratch sensor server. 
rs.values.a = 1 

# Create an other variable
rs.values.b = 0.2 

# An yet an other ...
rs.values.x = "dynamic sensor-update"

Note: one could use int, float and string as a right-value for a sensor variable.

At the moment you introduce a new variable it is known to the Scratch sensor server. If a variable known to the server is modified form within Scratch, an sensor-update message is sent to all the connected clients. The python API listens for this messages, and updates the values for already known variables, or creates new variables for not known variables. This, when accessing the value of a variable, it will nor necessarily contain the value you wrote in earlier, but the value last published by the server.

Broadcast Messages

To sent a broadcast message use the following call:

# Broadcast a message ...

Register Call-back Handlers for incoming Messages

To get notified about incoming broadcast messages or variable updates, register call back handlers:

from scratch.remote sensor import RemoteSensor 

def updateHandler(var, val):
	Handler called for incoming sensor-updates ...
	print("-> received update: %s = %s" % (var, val))

def messageHandler(t, msg):
	Handler called for incoming broadcast messages ... 
	print("-> received message: %s" % msg)

# Remote sensor connected to default host/port (localhost:42001)
rs = RemoteSensor()

# Register call back handler for sensor-updates
rs.updateHandler  = updateHandler

# Register call back handler for broadcast messages
rs.messageHandler = messageHandler

# Start receiver thread

# Receiver thread is a daemon thread, thus we need to make sure that the main 
# program does not exit while we are note done ...
raw_input('Press Enter to quit...\n')

Now, every time a variable gets updated or a new message is received, the corresponding handler is called.

Using the Wrapper Framework

Additionally to the already mentioned API functions, a wrapper framework is provided. This framework helps creating a remote sonsor client which easily runs as daemon. The framework takes care of starting/stopping the sensor in the background, writing to a logfile, managing the loglevel, connecting (and reconnecting) to the sensor server.

To write a sensor that runs as a daemon, simple wite a class that is derived from RemoteSensor, overwrite __init__ and provide a worker method:

import logging

from scratch.remotesensor import RemoteSensor, DEFAULT_HOST, DEFAULT_PORT 

class WrappedRemoteSensor(RemoteSensor):

	# Give that sensor a name (used e.g. with hearbeat messages) 
	name = "wrap"

	def __init__(self, myArgs = {}):
		Create a new instance of the monitoring remote sensor. 

		@param  myArgs  arguments for the sensor: host and port.

		RemoteSensor.__init__(self, args = myArgs)

	# name used e.g. for heartbeat
	name = "wrap"

	def __init__(self, myArgs = {}):
		Create a new instance of the remote sensor. 

		@param  myArgs  arguments for the sensor: host and port.

		RemoteSensor.__init__(self, args = myArgs)

	def worker(self):
		This is the worker method which is called over and over again in the server loop. 
		Do all yout sensor stuff here.
		i = 0


			# change variable (will send sensor update)
			self.values.dummyValue = i

			# notfiy that we had some input changes


		except Exception as e:

To run a wrapped sensor client, execute the daemon wrapper and pass your sensor module/class in as parameter. E.g. if you saved the above examples in a file, and the name of your derived class is WrappedRemoteSensor you could start it like this (assuming you installed the PynetsenseAPI):

  1. get the location of your Python libraries:

    export PD=$(python -c "from distutils.sysconfig import get_python_lib; print get_python_lib(False, False, '/usr/local')")

  2. start the daemon in background:

    python $PD/scratch/wrappers/ --pid /tmp/ --log /tmp/mysensor.log --wrap mysensor#WrappedRemoteSensor start

To watch the output of your sensor, you could tail the logfile:

tail -f /tmp/mysensor.log

To stop the daemon:

python $PD/scratch/wrappers/ --pid /tmp/ --log /tmp/mysensor.log --wrap mysensor#WrappedRemoteSensor stop

Or to restart:

python $PD/scratch/wrappers/ --pid /tmp/ --log /tmp/mysensor.log --wrap mysensor#WrappedRemoteSensor restart

Note: By default, "--pid" is set to "/var/run/" and "--log" to "/var/log/srsd.log". Thus, if you run the daemon as non-root user, it is likely that you are not allowed to write to that location.

To pass arguments to the wrapped sensor (e.g. host/port), use the "--wrapargs" parameter:

    python $PD/scratch/wrappers/ --pid /tmp/ --log /tmp/mysensor.log --wrap mysensor#WrappedRemoteSensor --wrapargs "host=" start

If you like to run your sensor in the foreground:

    python $PD/scratch/wrappers/ --foreground --wrap mysensor#WrappedRemoteSensor start

For "real-life" applications it might be a good idea to write a litte shell-script helper. As an example you couls have a look at the Raspberry Pi wrapper [here] (