Counter.py

import pglet
from pglet import Page, Text, Button, Stack, Textbox

page = pglet.page("index")
page.update(Page(title="Counter"))
page.clean()

def on_click(e):
    try:
        count = int(page.get_value('number'))
        #if we get here the number is int
        page.send('set number errorMessage=""')

        if e.data == '+':
            page.set_value('number', count + 1)

        elif e.data =='-':
            page.set_value('number', count - 1)

    except ValueError:
        page.send('set number errorMessage="Please enter a number"')  

page.add(
    Stack(horizontal = True, controls=[
        Button(text='-', onclick=on_click, data='-'),
        Textbox(id='number', value = '0', align = 'right'),
        Button(text='+', onclick=on_click, data='+'),
    ])
)

page.wait_close()