How to get value from TextInput

I am trying to get a value from input box using TextInput.
When the user enter a number, that number can be passed to another variable, tf, which will be used later.
Right now I have something like this.

from bokeh.models import TextInput, Paragraph
from bokeh.plotting import curdoc, show
from bokeh.layouts import layout, column

text_input = TextInput(name='Text Input', placeholder='Enter a string here...')
tf = text_input.value
curdoc().add_root(text_input)
curdoc().title = ""

Thank you so much!