How to capture value of dropdown widget in bokeh python?

Question

The official documentation of bokeh 0.12.1 in the link give the below code for creating a dropdown.

http://bokeh.pydata.org/en/latest/docs/user_guide/interaction/widgets.html#userguide-interaction-widgets

But its doesn't clearly mention how to capture the value of the dropdown widget when someone click and selects a value from the dropdown.

from bokeh.io import output_file, show
from bokeh.layouts import widgetbox
from bokeh.models.widgets import Dropdown

output_file("dropdown.html")

menu = [("Item 1", "item_1"), ("Item 2", "item_2"), None, ("Item 3", "item_3")]
dropdown = Dropdown(label="Dropdown button", button_type="warning", menu=menu)

show(widgetbox(dropdown))

Question

Is see that there are 2 methods called on_click() & on_change() but from the documentation couldn't figure out how to capture the value. How can we assign the selected value to a new variable?

EDIT

Based on input from @Ascurion i have updated my code as shown below. But when i select a value in dropdown nothing is printed in ipython console in Spyder. Please advise.

    from bokeh.io import output_file, show
    from bokeh.layouts import widgetbox
    from bokeh.models.widgets import Dropdown

    output_file("dropdown.html")


    menu = [("Item 1", "item_1"), ("Item 2", "item_2"), None, ("Item 3", "item_3")]
    dropdown = Dropdown(label="Dropdown button", button_type="warning", menu=menu)

    def function_to_call(attr, old, new):
        print dropdown.value

    dropdown.on_change('value', function_to_call)
    dropdown.on_click(function_to_call)
    show(widgetbox(dropdown))

Answer 1

If you set on_change e.g. as follows:

dropdown.on_change('value', function_to_call)

one can access the value of the selected item in function_to_call as follows:

def function_to_call(attr, old, new):
    print dropdown.value

For this to work dropdown has to be defined before function_to_call.

The documentation on how to access values set in widgets with on_click and on_change (bokeh version 12.1) can be found here at the top of the page:

http://bokeh.pydata.org/en/latest/docs/user_guide/interaction/widgets.html

EDIT

To get interactive feedback you have to run bokeh in server mode, so that the python code can be evaluated when you interact with a widget. I changed your example slightly to allow to be run with the

bokeh serve --show file_name.py

command. The code below then prints out the selected item in the terminal.

from bokeh.io import output_file, show
from bokeh.layouts import widgetbox
from bokeh.models.widgets import Dropdown
from bokeh.plotting import curdoc

menu = [("Quaterly", "time_windows"), ("Half Yearly", "time_windows"), None, ("Yearly", "time_windows")]
dropdown = Dropdown(label="Time Period", button_type="warning", menu=menu)

def function_to_call(attr, old, new):
    print dropdown.value

dropdown.on_change('value', function_to_call)

curdoc().add_root(dropdown)

See here for more information:

http://bokeh.pydata.org/en/latest/docs/user_guide/server.html