How to prevent multiple instances of the Root-Widget in Kivy - python-3.x

Question

OK, I try to make my first App in Kivy and I have a Problem with multiple Instances of my Root Widget.
I need to add widgets to my app while running. (to display a spreadsheet)
But because of the multiple instances of the Root Widget, I can't access the widgets with the kivy ids lookup object.
https://kivy.org/docs/guide/lang.html
I made three Buttons to display the problem in the console
Button -1-, -2-, and -3-
Please explain me why there are multiple instances and how I can prevent it, so the code will work.
Thanks for your help!
File --> Match.py
#!python
from __future__ import print_function
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty, StringProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.app import App
import gc
class my_gc():
def search_g(self):
for obj in gc.get_objects():
if isinstance(obj, Root):
try:
print(obj.ids)
except:
print('fail')
#Root Widget
class Root(BoxLayout):
sm = ObjectProperty(None)
sc0 = ObjectProperty(None)
sc1 = ObjectProperty(None)
sc2 = ObjectProperty(None)
sc3 = ObjectProperty(None)
mylbl0 = StringProperty('screen 0')
mylbl1 = StringProperty('screen 1')
mylbl2 = StringProperty('screen 2')
mylbl3 = StringProperty('screen 3')
def find_ids(self):
print(self.ids)
class Screen_Manager(ScreenManager):
pass
class Screen_0(Screen):
pass
class Screen_1(Screen):
pass
class Screen_2(Screen):
pass
class Screen_3(Screen):
pass
class Match(App):
rootwidget = Root()
sm = Screen_Manager()
gc = my_gc()
def build(self):
pass
Match().run()
File --> Match.kv
#Root Widget (BoxLayout)
Root:
sm: sm
sc1: sc0
sc1: sc1
sc1: sc2
sc1: sc3
BoxLayout:
spacing: '20dp'
orientation: 'vertical'
Screen_Manager:
id: sm
Screen_0:
id: sc0
name: 'sc0'
manager: 'sm'
BoxLayout:
id: box_0-0
Label:
id: lbl_0
text: app.rootwidget.mylbl0
Label:
id: lbl_0-1
text: root.mylbl0
Screen_1:
id: sc1
name: 'sc1'
manager: 'sm'
Label:
id: lbl1
text: root.mylbl1
Screen_2:
id: sc2
name: 'sc2'
manager: 'sm'
Label:
id: lbl2
text: root.mylbl2
Screen_3:
id: sc3
name: 'sc3'
manager: 'sm'
Label:
id: lbl3
text: root.mylbl3
#Tab-Buttons
BoxLayout:
size_hint: 1 , None
height: '60dp'
orientation: 'horizontal'
Button:
text: '-0-'
on_press: root.sm.current = sc0.name
Button:
text: '-1-'
on_press: root.sm.current = sc1.name
Button:
text: '-2-'
on_press: root.sm.current = sc2.name
Button:
text: '-3-'
on_press: root.sm.current = sc3.name
Button:
text: '<'
size_hint: None , 1
width: '60dp'
on_press: root.sm.current = root.sm.previous()
Button:
text: '>'
size_hint: None , 1
width: '60dp'
on_press: root.sm.current = root.sm.next()
Button:
text: '-b1-'
size_hint: None , 1
width: '60dp'
#on_press: root.search_g() #<-- doesn't work
on_press: app.gc.search_g() #<-- works
Button:
text: '-b2-'
size_hint: None , 1
width: '60dp'
on_press: root.find_ids() #<-- doesn't work
on_press: app.rootwidget.find_ids() #<-- works
Button:
text: '-b3-'
size_hint: None , 1
width: '60dp'
on_press: print(root.mylbl1) #<-- works
on_press: print(app.rootwidget.mylbl1) #<-- works

Answer 1

Mutilple Instance - Root Widget
As per your code, you don't have mutiple instances of the root widget. Your root widget is Root: as per the kv file, match.kv.
root.search_g()
It doesn't work because your root which is the class Root(BoxLayout) does not have the search_g() method.
root.find_ids - works
When I commented off app.rootwidget.find_ids, the call to root.find_ids works i.e. it printed the ids. The keyword root is available only in rule definitions and represents the root widget of the rule (the first instance of the rule). When your kv file is parsed, kivy collects all the widgets tagged with id’s and places them in self.ids dictionary type property.
app.rootwidget.find_ids
It works but there were no ids because it is referencing to the method find_ids in the initiated object, class Root. The rootwidget is just a variable by name and it is a different object than the one from Root:
From the attached screen shot, you will notice that the object locations for root.find_ids and app.rootwidget.find_ids are different.
Adding Widgets Dynamically
To add widgets dynamically into your app while it is running, you have to do the following. For an example, please refer to my other post at Kivy/python : when i click row then it shows error IndexError: list index out of range
Python Script - match.py
Add an import statement as follow:
from kivy.properties import ObjectProperty
Declare a variable (e.g. container) or ObjectProperty type e.g. in the class Root
class Root(BoxLayout):
container = ObjectProperty(None)
Remove all widgets added dynamically before adding
Add widgets dynamically.
kv File - match.kv
Define a layout e.g. BoxLayout or GridLayout in your kv file.
Give an id to the layout e.g. container in your kv file.
Hook up the ObjectProperty to the id
Root:
container: container
...
# Container for adding widgets dynamically
BoxLayout:
id: container
Solution
The solution is to declare an ObjectProperty, rootwidget as follow:
snippet
class Match(App):
rootwidget = ObjectProperty(None)
title = "Solution App"
gc = my_gc()
def build(self):
self.rootwidget = Root()
return self.rootwidget
match.py
#!python
from __future__ import print_function
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty, StringProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.app import App
import gc
class my_gc():
def search_g(self):
print("my_gc.search_g called")
for obj in gc.get_objects():
if isinstance(obj, Root):
try:
print("obj={0}, ids={1}".format(obj, obj.ids))
for key, value in obj.ids.items():
print(" key={0}, value={1}".format(key, value))
except:
print('fail')
# Root Widget
class Root(BoxLayout):
sm = ObjectProperty(None)
sc0 = ObjectProperty(None)
sc1 = ObjectProperty(None)
sc2 = ObjectProperty(None)
sc3 = ObjectProperty(None)
mylbl0 = StringProperty('screen 0')
mylbl1 = StringProperty('screen 1')
mylbl2 = StringProperty('screen 2')
mylbl3 = StringProperty('screen 3')
def find_ids(self):
print("Root.find_ids called")
print("self={0}, self.ids={1}".format(self, self.ids))
for key, value in self.ids.items():
print(" key={0}, value={1}".format(key, value))
class Screen_Manager(ScreenManager):
pass
class Screen_0(Screen):
pass
class Screen_1(Screen):
pass
class Screen_2(Screen):
pass
class Screen_3(Screen):
pass
class Match(App):
rootwidget = ObjectProperty(None)
title = "Solution App"
gc = my_gc()
def build(self):
self.rootwidget = Root()
return self.rootwidget
if __name__ == "__main__":
Match().run()
match.kv
#Root Widget (BoxLayout)
<Root>:
sm: sm
sc1: sc0
sc1: sc1
sc1: sc2
sc1: sc3
BoxLayout:
spacing: '20dp'
orientation: 'vertical'
Screen_Manager:
id: sm
Screen_0:
id: sc0
name: 'sc0'
manager: 'sm'
BoxLayout:
id: box_0-0
Label:
id: lbl_0
text: root.mylbl0
Label:
id: lbl_0-1
text: root.mylbl0
Screen_1:
id: sc1
name: 'sc1'
manager: 'sm'
Label:
id: lbl1
text: root.mylbl1
Screen_2:
id: sc2
name: 'sc2'
manager: 'sm'
Label:
id: lbl2
text: root.mylbl2
Screen_3:
id: sc3
name: 'sc3'
manager: 'sm'
Label:
id: lbl3
text: root.mylbl3
#Tab-Buttons
BoxLayout:
id: secondBoxLayout
size_hint: 1 , None
height: '60dp'
orientation: 'horizontal'
Button:
text: '-0-'
on_press: root.sm.current = sc0.name
Button:
text: '-1-'
on_press: root.sm.current = sc1.name
Button:
text: '-2-'
on_press: root.sm.current = sc2.name
Button:
text: '-3-'
on_press: root.sm.current = sc3.name
Button:
text: '<'
size_hint: None , 1
width: '60dp'
on_press: root.sm.current = root.sm.previous()
Button:
text: '>'
size_hint: None , 1
width: '60dp'
on_press: root.sm.current = root.sm.next()
Button:
text: '-b1-'
size_hint: None , 1
width: '60dp'
on_press:
print("\non_press: {}".format(self.text))
# root.search_g() #<-- doesn't work
print(" app.gc_search_g()")
app.gc.search_g() #<-- works
Button:
text: '-b2-'
size_hint: None , 1
width: '60dp'
on_press:
print("\non_press: {}".format(self.text))
print(" root.find_ids()")
root.find_ids() #<-- works
print("\n app.root.find_ids()")
app.rootwidget.find_ids() #<-- works
Button:
text: '-b3-'
size_hint: None , 1
width: '60dp'
on_press:
print("\non_press: {}".format(self.text))
print(root.mylbl1) #<-- works
print(app.rootwidget.mylbl1) #<-- works
Output