Python - Kivy: AttributeError: 'super' object has no attribute '__getattr__' when trying to get self.ids

Question

I wrote a code for a kind of android lock thing, whenever I try to get an specific ClickableImage using the id it raises the following error:

AttributeError: 'super' object has no attribute '__getattr__'

I've spent hours trying to look for a solution for this problem, I looked other people with the same issue, and people told them to change the site of the builder, because it needed to be called first to get the ids attribute or something like that, but everytime I move the builder, it raises the error "class not defined". Any clues?

Here is my code:

from kivy.app import App
from kivy.config import Config
from kivy.lang import Builder
from kivy.graphics import Line
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.widget import Widget
from kivy.uix.image import Image
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.behaviors import ButtonBehavior

#Variables
cords = ()
bld = Builder.load_file('conf.kv')

class Manager(ScreenManager): pass
class Principal(Screen): pass

class ClickableImage(ButtonBehavior, Image):
    def on_press(self):
        self.source = 'button_press.png'

    def on_release(self):
        self.source = 'button.png'
        self.ids.uno.source = 'button_press.png'

class canva(Widget):
    def on_touch_down(self, touch):
        global cords
        with self.canvas:
            touch.ud['line'] = Line(points=(touch.x, touch.y), width=1.5)
        cords = (touch.x, touch.y)

    def on_touch_move(self,touch):
        global cords
         touch.ud['line'].points = cords + (touch.x, touch.y)

    def on_touch_up(self,touch):
        self.canvas.clear()

class Api(App):    
    def build(self):
        return bld

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

and here is my .kv file:

# conf to file:  test.py

<Manager>:
    Principal:

<Principal>:
    GridLayout:
        size_hint_x: 0.5
        size_hint_y: 0.6
        width: self.minimum_width
        cols: 3
        ClickableImage:
            id: 'uno'
            size: 10,10
            source: 'button.png'
            allow_strech: True
        ClickableImage:
            id: 'dos'
            size: 30,30
            source: 'button.png'
            allow_strech: True
    canva:

Solution

Let's look at the output:

'super' object has no attribute '__getattr__'

In kv language id is set in a special way(up to 1.9.2 now), its value is not a string, because it's not a casual variable. You can't access it with <widget>.id.

I'd say it's similar to canvas, which is not a widget, yet it may look like that(which is why I was confused by your code :P). You've already noticed something: <some object> is like Python's something = <object> and that's (at least what I think) is the whole point of id's value not being a string(which to some is odd). If id was a string, there'd probably be needed a check to exclude it somehow from casual assigning values. Maybe it's because of performance or just simplicity.

Therefore let's say id is a keyword for a future keyword. In fact, it is, because the characters assigned to id will become a string key with a value of object got from WeakProxy, to the object WeakProxy points to. Or better said:

id: value

becomes

<some_root_widget>.ids[str(value)] = weakref.proxy(value)

where value becomes an object(what print(self) would return)

I suspect(not sure) that if you use string as the value for id, you'll end up with weakref / WeakProxy pointing to a string. I use the word point as it reminds me pointers, don't get confused with C pointers.

Now if you look again at the output:

• super gives you an access to a class you inherit from

• print('string id'.__getattr__) will give you the same error, but 'super' is substituted with the real value, because well... it doesn't have __getattr__

Therefore if you assign a string value to id, you'll get into this situation:

<some_root_widget>.ids[str('value')] = weakref.proxy('value')  # + little bit of magic

Although str('value') isn't necessarily wrong, by default you can't create weakref.proxy for a string. I'm not sure how Kivy handles this with WeakProxies, but if you assign a string to id, roughly this is what you get.

(Please correct me if I'm wrong)