Nuke Threading Example with Python Panel

import nuke
import nukescripts
import threading


class renderWrites(threading.Thread):

    def __init__(self, writenode):
        # Start threading...
        threading.Thread.__init__(self)
        # Get the currently selected Write node from the dropdown knob...
        self.writeNode = nuke.toNode(writenode)

    def run(self):
        try:
            nuke.executeInMainThread(nuke.execute, args=self.writeNode, kwargs={'continueOnError':True} )
        except Exception as e:
            print "ERROR: %s" % e


class RenderWritesPanel(nukescripts.PythonPanel):

    def __init__(self):

        self.write_nodes = []
        for n in nuke.allNodes():
            if n.Class() == 'Write':
                name = n.name()
                self.write_nodes.append(name)

        nukescripts.PythonPanel.__init__(self, "Render Writes Panel")
        self.writes = nuke.Enumeration_Knob("writes", "writes", self.write_nodes)
        self.render = nuke.PyScript_Knob("render", "render")
        for i in [self.writes, self.render]:
            self.addKnob(i)

    def knobChanged(self, knob):
        if knob is self.render:
            t = renderWrites(self.writes.value())
            t.start()


RenderWritesPanel().show()