Calculation of the number of symbols in TextInput

[Mystic]

If I enter the textInput numbers and then they are immediately or removed, then textInput does not become an empty line and does not show the number of symbols equal to 0.

How, when the last symbol (or all at once) is removed, is the length of the line 0?

from kivy.app import App
from kivy.uix.textinput import TextInput
from kivy.uix.label import Label
from kivy.uix.boxlayout import BoxLayout
class MainApp(App):

def build(self):

bl = BoxLayout(orientation = 'vertical')

self.txt = TextInput()

self.txt.bind(text=self.on_focusa)

bl.add_widget(self.txt)

self.lab = Label()

bl.add_widget(self.lab)
    return bl

def on_focusa(self, instance, value):
    try:
        txt = int(self.txt.text)*2
        self.lab.text = str(txt)
        print(len(str(txt)))
    except ValueError:
        pass

if name == 'main':

MainApp().run()

[rielyn]

The problem is, when you remove all the signs from the text field, the value doesn't come on_focusa, the function doesn't work as you wish.

Such a code can solve the problem, it can be optimized if the repetitive code is placed in a separate function:

    def on_focusa(self, instance, value):
    if value: # Проверяем значение value
        try:
            txt = int(self.txt.text)*2
            self.lab.text = str(txt)
            print(len(str(txt)))
        except ValueError:
            pass
    else: # Если value отсутствует, то выводим нули
        txt = 0
        self.lab.text = str(txt)
        print(len(str(txt)))