Cannot display arduino data into kivy window - python-3.x

Question

Im am trying to display my arduino data into my kivy window under label but it says an error " ValueError: Label.text accept only str". Anyone able to help?
Below is my python kivy file
test.py file
import serial
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.uix.button import Button
class MainWindow(Screen):
def tm(self):
while (1):
with serial.Serial('COM4', 9600) as ser:
x = ser.readline()
return str(x)
ser.close()
class WindowManager(ScreenManager):
pass
hi = Builder.load_file("test.kv")
class MyMainApp(App):
def build(self):
return hi
if __name__ == "__main__":
MyMainApp().run()
test.kv file
WindowManager:
MainWindow:
<MainWindow>:
Label:
size: 75, 50
size_hint: None, None
pos_hint: {'right': 1, 'bottom': 1}
background_color: (1.0, 0.0, 0.0, 1.0)
text: root.x

Answer 1

I see a few problems in your example.
Be careful naming your variables. x for example, is almost always a NumericProperty of the widget already.
When you reference an attribute in python, use self, or the widgets x in your case wil not change.
When you have a loop in kivy, you will block the application. So you either use Clock or threading. I will suggest threading instead of Clock when using serial, because readline() will also block the app if waiting for an input.
You never actually call tm()
So, heres what I would change in MainWindow class, and a few more imports.
import serial
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.uix.button import Button
from kivy.properties import StringProperty
from kivy.clock import mainthread
import threading
KV = """
WindowManager:
MainWindow:
<MainWindow>:
Label:
size: 75, 50
size_hint: None, None
pos_hint: {'right': 1, 'bottom': 1}
background_color: (1.0, 0.0, 0.0, 1.0)
text: root.label_text
"""
class MainWindow(Screen):
label_text = StringProperty("")
def __init__(self, **kwargs):
super(MainWindow, self).__init__(**kwargs)
threading.Thread(target=self.tm).start()
def tm(self):
while (1):
with serial.Serial('COM4', 9600) as ser:
value = ser.readline()
ser.close()
self.update_label(value.decode("ascii").strip())
#mainthread
def update_label(self, value):
self.label_text = value
class WindowManager(ScreenManager):
pass
class MyMainApp(App):
def build(self):
return Builder.load_string(KV)
if __name__ == "__main__":
MyMainApp().run()
Be sure to use mainthread when changing any ui element in another thread. In this case when you change a stringproperty that will change the display.