Switching kivy widgets
Question: I am using the Kivy python library.

I have two widgets defined.

When the program runs, I run the first widget.

When that widgets button is pressed, I want it to dissapear and be replaced
with the second widget.

Here is the .kv for the two widgets

    
    
    #uitest.kv
    <TestForm>:
        canvas:
            Rectangle:
                pos: self.center_x, 0
                size: 10, self.height
    
        BoxLayout:
            size: root.size
            padding: 40
            Button:
                text: 'Hello'
                on_release: root.testCallback()
    
    <TestForm2>:
        canvas:
            Rectangle:
                pos: self.center_x, 0
                size: self.height, 10
    

My main python file runs the app, and returns the first widget

    
    
    #main.py
    from testform import TestForm
    from kivy.app import App
    
    class UITestApp(App):
        def build(self):
            return TestForm()
    
    # Main launching point
    if __name__ in ('__main__', '__android__'):
        UITestApp().run()
    

My first widget has a callback. This is where the code-in-question belongs

    
    
    from testform2 import TestForm2
    from kivy.uix.widget import Widget
    
    class TestForm(Widget):
        def testCallback(self):
            TestForm2() # Code in question goes here. @TODO replace this widget with TestForm2 widget.
    

The idea here is to have a user interface manager. This manager doesn't run
the UI like a tree, but like a list and stack. The list holds instances of all
my UI Forms. The stack holds the traversal of said forms, whenever we jump to
a form we push it to the stack and "render" or whatever that one.

EDIT: I chose my answer, but in my searches I also found the Screen object:
<http://kivy.org/docs/api-kivy.uix.screenmanager.html> Personally, the clear()
and add() commands are more powerful, but the screen takes a lot of that out
of your hands if you're interested. Transition effects too.


Answer: My suggestion is to have an interface manager widget, then you can have
various widgets for your UI forms.

    
    
    import kivy
    from kivy.uix.label import Label
    from kivy.uix.button import Button
    from kivy.uix.boxlayout import BoxLayout
    from kivy.app import App
    
    class InterfaceManager(BoxLayout):
    
        def __init__(self, **kwargs):
            super(InterfaceManager, self).__init__(**kwargs)
    
            self.first = Button(text="First")
            self.first.bind(on_press=self.show_second)
    
            self.second = Button(text="Second")
            self.second.bind(on_press=self.show_final)
    
            self.final = Label(text="Hello World")
            self.add_widget(self.first)
    
        def show_second(self, button):
            self.clear_widgets()
            self.add_widget(self.second)
    
        def show_final(self, button):
            self.clear_widgets()
            self.add_widget(self.final)
    
    
    class MyApp(App):
        def build(self):
            return InterfaceManager(orientation='vertical')
    
    if __name__ == '__main__':
        MyApp().run()
    

You wouldn't structure it like that of course. You could hold all your forms
in a dictionary on the Container object and have a universal callback which
provides another form by key.

    
    
    class InterfaceManager(BoxLayout):
    
        def __init__(self, **kwargs):
            super(InterfaceManager, self).__init__(**kwargs)
            self.forms = {}
    
        def add_form(self, key, form):
            self.forms[key] = form
    
        def uniCallback(self, button):
            self.clear_widgets()
            # You could introduce a more elegant key
            # handling system here.
            self.add_widget(self.forms[button.text])
    

