def createWriteDir():
  import nuke, os, errno
  file = nuke.filename(nuke.thisNode())
  dir = os.path.dirname( file )
  osdir = nuke.callbacks.filenameFilter( dir )
  # cope with the directory existing already by ignoring that exception
  try:
    os.makedirs( osdir )
  except OSError, e:
    if e.errno != errno.EEXIST:
      raise
nuke.addBeforeRender(createWriteDir)