References in Python

Question

I have a multicasting network that needs to continuously send data to all other users. This data will be changing constantly so I do not want the programmer to have to deal with the sending of packets to users. Because of this, I am trying to find out how I can make a reference to any object or variable in Python (I am new to Python) so it can be modified by the user and changes what is sent in the multicasting packets.

Here is an example of what I want:

>>> test = "test"
>>> mdc = MulticastDataClient()
>>> mdc.add(test) # added into an internal list that is sent to all users

# here we can see that we are successfully receiving the data
>>> print mdc.receive() 
{'192.168.1.10_0': 'test'}

# now we try to change the value of test
>>> test = "this should change"
>>> print mdc.receive()
{'192.168.1.10_0': 'test'}   # want 'test' to change to -> 'this should change'

Any help on how I can fix this would be very much appreciated.

UPDATE:

I have tried it this way as well:

>>> test = [1, "test"]
>>> mdc = MulticastDataClient()
>>> mdc.add(test)
>>> mdc.receive()
{'192.168.1.10_1': 'test'}
>>> test[1] = "change!"
>>> mdc.receive()
{'192.168.1.10_1': 'change!'}

This did work. However,

>>> val = "ftw!"
>>> nextTest = [4, val]
>>> mdc.add(nextTest)
>>> mdc.receive()
{'192.168.1.10_1': 'change!', '192.168.1.10_4': 'ftw!'}
>>> val = "different."
>>> mdc.receive()
{'192.168.1.10_1': 'change!', '192.168.1.10_4': 'ftw!'}

This does not work. I need 'ftw!' to become 'different.' in this case. I am using strings for testing and am used to strings being objects from other languages. I will only be editing the contents inside of an object so would this end up working?

Answer 1

In python everything is a reference, but strings are not mutable. So test is holding a reference to "test". If you assign "this should change" to test you just change it to another reference. But your clients still have the reference to "test". Or shorter: It does not work that way in python! ;-)

A solution might be to put the data into an object:

data = {'someKey':"test"}
mdc.add(data)

Now your clients hold a reference to the dictionary. If you update the dictionary like this, your clients will see the changes:

data['someKey'] = "this should change"