Issue with weakref in dropdown - Kivy ids

[Shoumik Das]

Hi! I have a weird issue at hand. I have built a small app with multiple screens and a custom drop down implemented in each screen. The drop downs seem to work fine till the time I switch screens and reactivate any of the drop downs. I then get the following error:

ReferenceError: weakly-referenced object no longer exists

Drop down working fine

Navigate to a different screen. The time zone drop down also works fine.

Return back to the old screen and re-select a drop down

I initially thought that it might be because I am using the inspector tool which is still in an experimental phase but then I tested the scenario without launching the inspector tool and I still get the same weak reference error. I have set a different id for each of the drop downs that I am using in the Kivy file.

Sample Python code:

class ImageLabel(BoxLayout):

source = StringProperty('images/expand-arrow-48.png')

text = StringProperty('User')

class ImageLabelButton(ButtonBehavior, ImageLabel):

pass

class ImageLabelButtonTop(ButtonBehavior, ImageLabel):

pass

Kivy Code:

<SivaStatusScreen>:

name: 'status_screen'

canvas.before:

Color:

rgba: 255/255, 255/255, 255/255, 1

Rectangle:

pos: self.pos

size: self.size

AnchorLayout:

id: status_add

anchor_x: 'right'

anchor_y: 'bottom'

ImageButton:

id: status_addbtn

source: {'normal': 'images/plus-96.png', 'down': 'images/plusblue-96.png'} [self.state]

size_hint: 0.2, 0.2

on_release: root.new_status_entry()

BoxLayout:

id: status_layout

orientation: 'vertical'

BoxLayout:

id: actionbar_layout

size_hint_y: None # size the box layout to contain the action bar

height: 48 # width of the action bar in line with the minimum dimensions of the icons

ActionBar:

id: status_actionbar

pos_hint: {'top': 1}

background_image: ''

background_color: 195/255, 60/255, 35/255, 1

ActionView:

use_separator: True

ActionPrevious:

title: 'SIVA'

with_previous: False

ActionOverflow:

CustomActionButton:

id: status-button

important: True

source: {'normal': 'images/communicationgreen-48.png', 'down': 'images/communication-48.png'} [self.state]

on_release: root.to_statusscreen()

CustomActionButton:

id: accounts-button

important: True

source: {'normal': 'images/key-48.png', 'down': 'images/keygreen-48.png'} [self.state]

on_release: root.to_accountsscreen()

CustomActionButton:

id: bot-button

important: True

source: {'normal': 'images/bot-48.png', 'down': 'images/botgreen-48.png'} [self.state]

on_release: root.to_botscreen()

CustomActionButton:

id: logout-button

important: True

source: {'normal': 'images/shutdown-48.png', 'down': 'images/shutdowngreen-48.png'} [self.state]

on_release: root.to_logout()

BoxLayout:

id: status_display

# size_hint is not needed. This Layout shares space with the BoxLayout that has a fixed y size, it will grow to fill the space.

orientation: 'vertical'

BoxLayout:

id: status_label

size_hint_y: None

height: 40

orientation: 'horizontal'

BoxLayout:

orientation: 'horizontal'

Image:

keep_ratio: True

source: 'images/communication-48.png'

Label:

text: 'Status'

markup: True

color: 0, 0, 0, 1

ImageLabelButtonTop:

# conflict in the on_release defined for ImageLabelButton and this instance.

id: parent_button

source: 'images/expand-arrow-48.png'

text: 'User'

on_release:

# root.add_dd_values()

dropdown.open(self)

on_parent: dropdown.dismiss()

size_hint_y: None

height: '40dp'

DropDown:

id: dropdown

on_select:

# args is a reserved keyword which returns only two values: object alias (0) and data (1).

parent_button.text = args[1][0]

parent_button.source = args[1][1]

# Invoked inside dropdown

# root - Screen, self - DropDown object

ImageLabelButton:

source: 'images/twitter-circled-48.png'

text: 'Twitter'

ImageLabelButton:

source: 'images/linkedin-circled-48.png'

text: 'LinkedIn'

BoxLayout:

id: status_content

<ImageLabel>:

orientation: 'horizontal'

size_hint_y: None

height: '40dp'

spacing: 1

canvas.before:

Color:

rgba: 255/255, 255/255, 255/255, 1

Rectangle:

pos: self.pos

size: self.size

Image:

size_hint_x: 0.3

keep_ratio: True

source: root.source  # root - ImageLabel

Label:

size_hint_x: 0.7

markup: True

halign: 'center'

valign: 'middle'

text: root.text  # root - ImageLabel

color: 0, 0, 0, 1

<ImageLabelButton>:

on_release:

# Tuple returned in dropdown.select() method to pass two values bundled as one.

app.root.get_screen('status_screen').ids.dropdown.select((self.text, self.source))

# Invoked inside ImageLabelButton rule

# app.root - Screen, root - ImageLabelButton object, self - ImageLabelButton object

Can you please suggest as to what I am doing wrong? Is there a problem in the way I am accessing the drop down fields using their ids?

Thanks in advance.

[Elliot Garbus]

It looks like this should solve your issue:

Read:  https://kivy.org/doc/stable/guide/lang.html#referencing-widgets

 

An id is a weakref to the widget and not the widget itself. As a consequence, storing the id is not sufficient to keep the widget from being garbage collected. To demonstrate:

<MyWidget>:

    label_widget: label_widget

    Button:

        text: 'Add Button'

        on_press: root.add_widget(label_widget)

    Button:

        text: 'Remove Button'

        on_press: root.remove_widget(label_widget)

    Label:

        id: label_widget

        text: 'widget'

Although a reference to label_widget is stored in MyWidget, it is not sufficient to keep the object alive once other references have been removed because it’s only a weakref. Therefore, after the remove button is clicked (which removes any direct reference to the widget) and the window is resized (which calls the garbage collector resulting in the deletion of label_widget), when the add button is clicked to add the widget back, a ReferenceError: weakly-referenced object no longer exists will be thrown.

To keep the widget alive, a direct reference to the label_widget widget must be kept. This is achieved using id.__self__ or label_widget.__self__ in this case. The correct way to do this would be:

<MyWidget>:

    label_widget: label_widget.__self__

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[Shoumik Das]

That was magic. Fixed my issue! I was a bit confused about weakrefs since I started learning Kivy a few months ago. This has cleared my doubts. Thank you very much.

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