How to access event data? - bokeh

Question

Click handlers in bokeh 1.0.3 used to have the signature attr, old, new. Now the are passed a single event object. How can I access its values?
menu = [('a', 'a'), ('b', 'b')]
dropdown = Dropdown(label='clickme', menu=menu)
def click_handler(event):
print(event)
returns
bokeh.events.MenuItemClick object at 0x7ff7de1cc208
EDIT: Where in the documentation is access to values of events described? I could not find anything on https://docs.bokeh.org/en/latest/docs/reference/events.html

Answer 1

I'm not sure where you got this information, but it's not quite correct. Dropdown was refined as a type of Button recently, so the ability to respond to the same kind of click events that other buttons do was added. But nothing was replaced. Callbacks for property changes work for any Bokeh object property, including Dropdown.value, and this has not changed:
from bokeh.io import curdoc
from bokeh.models import Dropdown
menu = [('a', 'a'), ('b', 'b')]
dropdown = Dropdown(label='clickme', menu=menu)
def cb(attr, old, new):
print(attr, old, new)
dropdown.on_change('value', cb)
curdoc().add_root(dropdown)

Answer 2

I'm coming in late, but I had the same issue as Leevi, and bigreddot's answer did not work for me (Bokeh 2.4.2, Python 3.10.2). Short answer is that a Bokeh Dropdown does not have a value attribute, so on_change('value, handler) will not work. The MenuItemClick object, has an item attribute, which is what you want.
menu = [('a', 'a'), ('b', 'b')]
dropdown = Dropdown(label='clickme', menu=menu)
def click_handler(event):
print(event.item)
dropdown.on_click(click_handler)
Long answer: I had the following code:
def choose_pipeline(event):
"""Change to the chosen pipeline"""
print(event)
pipeline_dropdown = Dropdown(label='Available Pipelines',
menu=[("Pipeline1", "pipeline1_value"),
("Pipeline2", "pipeline2_value")])
pipeline_dropdown.on_change('value', choose_pipeline)
and got this error:
File "C:\Users\me\PycharmProjects\DHLLDV\Scripts\SystemTab.py", line 69, in system_panel
pipeline_dropdown.on_change('value', choose_pipeline)
File "C:\Users\me\PycharmProjects\DHLLDV\venv\lib\site-packages\bokeh\model\model.py", line 434, in on_change
descriptor = self.lookup(attr)
File "C:\Users\me\PycharmProjects\DHLLDV\venv\lib\site-packages\bokeh\core\has_props.py", line 469, in lookup
raise AttributeError(f"{cls.__name__}.{name} property descriptor does not exist")
AttributeError: Dropdown.value property descriptor does not exist
I wrote this:
def choose_pipeline(event):
"""Change to the chosen pipeline"""
print(event.__dict__)
pipeline_dropdown = Dropdown(label='Available Pipelines', menu=[("Pipeline1", "pipeline1_value"),
("Pipeline2", "pipeline2_value")])
pipeline_dropdown.on_click(choose_pipeline)
And saw the result when I picked Pipeline2:
{'item': 'pipeline2_value', '_model_id': '1365'}