How to capture value of dropdown widget in bokeh python?

EDIT This answer does not apply for Bokeh Versions 2.X.X anymore. See comment and the other answer below.

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

In Bokeh 2.0.0, Dropdown.value was removed. The correct way to get what item has been clicked is:

from bokeh.models import Dropdown

d = Dropdown(label='Click me', menu=['a', 'b', 'c'])


def handler(event):
    print(event.item)


d.on_click(handler)