import nuke

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

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

# Store function on node's "returner" knob
function_knob = nuke.Multiline_Eval_String_Knob('returner')
node.addKnob(function_knob)
node.knob('returner').setText(function_to_store)

# Retrieve the function from the node's knob and make it available
function_retrieved = node.knob('returner').getText()
exec(function_retrieved)

# Execute it
returned_value = returner('something')  # Use the function's name
print(returned_value)  # Prints 'something'