Kivy change label text with python

Question

I'm semi-OK with Python but brand new to Kivy, I know my problem is referencing the label ID but I can't seem to solve it and searching doesn't seem to give me what I need.

I'm trying to get a label to display the current time, so I know I have the right framework in terms of updating etc but I'm sure its all down to referencing the label ID somehow and its that I'm struggling with?

The following code runs fine, displays the labels etc until I try to update the label_text.text at which point i get an error: AttributeError: 'float' object has no attribute 'lbl_time'. I've tried str(time.strftime("%H:%M:%S")) but that doesn't solve it.

from kivy.app import App
from kivy.base import runTouchApp
from kivy.lang import Builder
from kivy.properties import ListProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ObjectProperty
from kivy.uix.screenmanager import ScreenManager, Screen, WipeTransition, FadeTransition
from kivy.uix.anchorlayout import AnchorLayout
from kivy.properties import ObjectProperty, StringProperty
from kivy.clock import Clock
import time
from datetime import datetime

class MainScreen(Screen):

def update_time(self):
    lbl_time = ObjectProperty()
    MyTime = time.strftime("%H:%M:%S")
    self.lbl_time.text = MyTime

class DetailScreen(Screen):
    pass

class SettingsScreen(Screen):
    pass

class MyScreenManager(ScreenManager):
    pass



root_widget = Builder.load_string('''
    MyScreenManager:
    MainScreen:
    DetailScreen:
    SettingsScreen:

<MainScreen>:
    name: 'main'
    BoxLayout:
        orientation: 'vertical'
        Label:
            id: lbl_time
            text: 'Time' 
            font_size: 60
        Label:
            text: 'Main2'
            font_size: 30
        GridLayout:
            cols: 2
            Label:
                text: 'Bottom'
                font_size: 30
            Label:
                text: 'Bottom1'
                font_size: 30

 <DetailScreen>:
     name: 'details'

 <SettingsScreen>:
     name: 'settings'

 ''')
 class ScreenManagerApp(App):

     def build(self):
         return root_widget

     def on_start(self):
         Clock.schedule_interval(MainScreen.update_time, 1)

 ScreenManagerApp().run()

Answer 1

This was more of a Python problem rather than a Kivy one. You were calling the update_time of the class MainScreen class, not of the object/instance of the MainScreen. Basically, you would need to save a reference to the object (self.main_screen) in the build method, and then use it in the on_start.

class ScreenManagerApp(App):

    def build(self):
        self.main_screen = MainScreen()
        return self.main_screen

    def on_start(self):
        Clock.schedule_interval(self.main_screen.update_time, 1)

Also you cannot access id outside of the kv language, i.e. in the python. You have to reference the id by adding a property, e.g. the_time:

<MainScreen>:
    name: 'main'
    the_time: _id_lbl_time
    BoxLayout:
        orientation: 'vertical'
        Label:
            id: _id_lbl_time

A minor problem is that the update_time() receives to parameters. Also, there were quite a few weird things in your code, so I couldn't run the code. I fixed all the above in the code below:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import Screen
from kivy.properties import ObjectProperty, StringProperty
from kivy.clock import Clock
import time
from datetime import datetime

Builder.load_string('''
<MainScreen>:
    name: 'main'
    the_time: _id_lbl_time
    BoxLayout:
        orientation: 'vertical'
        Label:
            id: _id_lbl_time
            text: 'Time' 
            font_size: 60
 ''')

class MainScreen(Screen):

    def update_time(self, sec):
        MyTime = time.strftime("%H:%M:%S")
        self.the_time.text = MyTime


class ScreenManagerApp(App):

    def build(self):
        self.main_screen = MainScreen()
        return self.main_screen

    def on_start(self):
        Clock.schedule_interval(self.main_screen.update_time, 1)

ScreenManagerApp().run()