Kivy: screen defined in kvlang, how to add to screenmanager in python?

Question

When you have a screenmanager with one screen, in kvlang:

<ScreenManagement>:
    ScreenOne:

<ScreenOne>:
    name: 'First'
    id: screen1


<ScreenTwo>:
    name: 'Second'
    id: screen2

And as can be seen, you also have a second screen defined, but it is not added to the screenmanager.

How do you add it with python? (I want to do it based on some condition from a config file)

I know I can add a widget to the screenmanager with add_widget() but I don't know how to reference the ScreenTwo from python.

I tried with ids but I get an keyError with this code:

class ScreenManagement(ScreenManager):
    def __init__(self,**kwargs):
        super().__init__(**kwargs)

        def setup(*args):
            if True:    #under some condition, I want to add ScreenTwo
                #this does not work: KeyError
                screen_widget = App.get_running_app().root.ids['screen2']   
                self.add_widget(screen_widget)

        Clock.schedule_once(setup)

Minimal working example code

Answer 1

First, in your kv file, make sure ScreenTwo is a subclass of Screen : <ScreenTwo@Screen>: . Second, the angle brackets indicate a rule rather than an actual instantiation. It just means that if an object matching the rule is instantiated, it will get these properties. To instantiate an instance of ScreenTwo in your Python code, use Factory :

screen_widget = Factory.ScreenTwo()

Answer 2

You are doing a few things wrong. First the things in < and > are called rules. These rules govern how the class is made whenever you invoke ScreenOne() in Python or in kv. Second, you are not able to add an id to a rule then access that id from a root widget. The correct way to do it is in my kv file. You move the id into the <ScreenManagement> rule. Lastly, you can always see what is going on with your kv file by doing a print self.children .

kvlang = '''
<ScreenManagement>:
    ScreenOne:
        id: screen1


<ScreenOne>:
    name: 'First'


<ScreenTwo>:
    name: 'Second'

'''


class ScreenManagement(ScreenManager):
    def __init__(self,**kwargs):
        super(ScreenManagement,self).__init__(**kwargs)

        Clock.schedule_once(self.setup)

    def setup(self,*args):
        if True:    #under some condition, I want to add ScreenTwo
            print self.children, self.current
            self.add_widget(ScreenTwo())
            self.current = 'Second'
            print self.children, self.current
            print self.ids