如何将坐标从Bokeh Tap事件追加到python对象？

from bokeh.plotting import figure, show
from bokeh.models import TapTool, ColumnDataSource
from bokeh.io import curdoc

p = figure(plot_width=400, plot_height=400, tools='tap')

source = ColumnDataSource(data=dict(x=[], y=[]))

p.circle('x', 'y', size=10, source=source)

def callback(event):
    x, y = event.x, event.y
    source.data['x'].append(x)
    source.data['y'].append(y)
    source.trigger('data', source.data, source.data)

tap_tool = TapTool(callback=callback)
p.add_tools(tap_tool)

curdoc().add_root(p)
show(p)