code: def callback(timestep):
      z = get_z(timestep)
      new_contour_data = contour_data(x, y, z, levels)
      contour_renderer.set_data(new_contour_data)
  
imports: import numpy
  from bokeh.driving import count
  from bokeh.palettes import PiYG
  from bokeh.plotting import curdoc, figure
  from bokeh.plotting.contour import contour_data
label_desc: Use image node_id 2 for calling a global function with example usage: callback(timestep) without return types