使用 Kivy 切换屏幕：AttributeError: 'NoneType' object has no attribute 'transition'

Question

I've run into an AttributionError when trying to call a "screen switch" function defined in one class from another using kivy library in python.

The structure of my code is as follows:

from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.app import App

def init_grid(width, height, mines):
    global grid_field
    global flagged
    # Initiate the gird field with empty grids
    grid_field = [[ClassA() for i in range(width)] for j in range(height)]
    flagged = 0
    ...


class Main(App):
   def build(self):
      sm = ScreenManager()

      current = Current(name="current")
      screenA = ScreenA(name="screenA")
      screenB = ScreenB(name="screenB")
      ...
      ...

      sm.add_widget(current)
      sm.add_widget(screenA)
      sm.add_widget(screenB)
      ...
      ...

      sm.current = "current"
      return sm

...

class ClassA():
    # a class my code is at when the error arises, 
    # which is not the starting "current" screen class 
    # but a different class that serves a certain purpose to my program
    def __init__(self, **kwargs):
        ...
        ...
        self.button = Button()
        self.button.bind(on_touch_down=self.method)


    def method(self, instance, touch):
        if (a condition):
            ScreenA().change_to_B()

    ...




...


class ScreenA(Screen):
    def __init__(self, **kwargs):
        Screen.__init__(self, **kwargs)
        ...
        ...
        init_grid(10, 10, 10)

    def change_to_B(self):
        self.manager.transition.direction = "left"
        self.manager.current = "screenB"


...


class ScreenB(Screen):
    def __init__(self, **kwargs):
        Screen.__init__(self, **kwargs)
        ...
        ...


if __name__ == '__main__':
    Main().run()

The error message is:

in change_to_B
     self.manager.transition.direction = "left"
 AttributeError: 'NoneType' object has no attribute 'transition'

From my code above, I defined the screen-switch function change_to_B in class ScreenA , and I called this function in class ClassA . I have inherited the Screen parent class in the both classes ScreenA and ScreenB I have created. But strangely enough, the error message seems to suggest that the object screenA is a NoneType object, which shouldn't be the case as I have declared that to be screen object when creating the class by inheriting the Screen parent class.

Can anyone point out what is the mistake I am missing out? Thank you in advance!!!

EDIT:

I think I need to make clear of my code. So I'm basically trying to write the minesweeper game and the ClassA is the grid that the gameboard of minesweeper consists of, and it has a Button attribute to implement the clicking function method (shown in the code above). In the clicking function method , I was trying to call the screen-switch function change_to_B() defined in class ScreenA , which is a screen object declared in the build method of the ScreenManager (I think).

The flow of the code is: Current(name="current") (which is irrelavent to this question) --> ScreenA(name="screenA"), where ClassA() is created in the global list variable from function init_grid() --> ClassA(), where clicking on the UI calls the function binded to the Button attributes of the ClassA() objects.

The ClassA object is created in a function init_grid , where a global variable grid_field is created to store the ClassA objects in a 2D list.

Answer 1

Common mistake - you use more than one object of each screens, so you add one object to ScreenManager and that object has property manager . But then you create another object of ScreenA class here:

def __init__(self, **kwargs):
        ...
        ...
        ScreenA().change_to_B()

Screen Manager knows nothing about that object. So the thing you should keep in mind is that you add objects to screen manager, not classes . You didn't provide the whole code so I don't know where you define the object of ClassA class, but you should pass object from build method there to make it work. Something like this:

class ScreenA(Screen):
    def __init__(self, **kwargs):
        Screen.__init__(self, **kwargs)
        ...
        ...
        # here we pass the object of the ScreenA class (which is self) to init grid to be able to pass it to ClassA
        init_grid(10, 10, 10, self)

# add object to arguments
def init_grid(width, height, mines, screenA):
    global grid_field
    global flagged
    # pass the object to ClassA
    grid_field = [[ClassA(screenA) for i in range(width)] for j in range(height)]

class classA():
    # don't forget to add object to arguments
    def __init__(self, screenA, **kwargs):
        ...
        ...
        self.screenA = screenA

    def method(self, instance, touch):
        if (a condition):
            self.screenA.change_to_B()

Or with global variables:

current = Current(name="current")
screenA = ScreenA(name="screenA")
screenB = ScreenB(name="screenB")

class Main(App):
   def build(self):
      sm = ScreenManager()

      global current
      global screenA
      global screenB
      ...
      ...

      sm.add_widget(current)
      sm.add_widget(screenA)
      sm.add_widget(screenB)
      ...
      ...

      sm.current = "current"
      return sm

class ClassA():
    def method(self, instance, touch):
        global screenA
        if (a condition):
            screenA.change_to_B()