if statment not working with python widget

I am working on a Jupyter Notebook in which I am creating a tab widget in python using the following code:

import ipywidgets as widgets

tab_contents = ['P0', 'P1']
children = [widgets.Text(description=name) for name in tab_contents]
tab = widgets.Tab()
tab.children = children
for i in range(len(children)):
    tab.set_title(i, str(i))

if tab.selected_index ==0:
    vartest = 0
else:
    vartest = 1

display(tab)

In a different cell, I check the value of vartest by calling it

vartest

The problem is that independently of the tab I have selected, vartest is always 0. What I want is that when I am in tab P0 (tab.selected_index ==0), vartest should be 0, when I am in P1 (tab.selected_index ==1) vartest should be 1.

What am I missing?