import nuke
import cPickle

# Function which I'd like to store on node (example)
def returner(string):
    return string

# Create node
node = nuke.createNode('NoOp')

# Store function on node
pickled_returner = cPickle.dumps(returner)
function_knob = nuke.Multiline_Eval_String_Knob('returner')
node.addKnob(function_knob)
node.knob('returner').setText(pickled_returner)

# Retrieve the function from the node's knob and execute it
unpickled_returner = node.knob('returner').getText()
retrieved_function = cPickle.loads(unpickled_returner)
return_value = retrieved_function('hello world')  # Result
print(return_value)