Exception deep within kivy in certain focus changes

[John Perry]

Hi

I've observed a problem that I think is a bug, but I want to run it by the folks here to see if there's an issue with my usage. I've worked hard to get an MWE; see below. To reproduce the problem, click first on the text input with hint text "one", then click on the text input with hint text "three". The result is:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
...[snip -- you can recreate by running the MWE]...

  File "/home/jperry/.local/lib/python3.11/site-packages/kivy/uix/behaviors/focus.py", line 412, in _on_focus
    self._unbind_keyboard()
  File "/home/jperry/.local/lib/python3.11/site-packages/kivy/uix/textinput.py", line 3057, in _unbind_keyboard
    super()._unbind_keyboard()
  File "/home/jperry/.local/lib/python3.11/site-packages/kivy/uix/behaviors/focus.py", line 454, in _unbind_keyboard
    del FocusBehavior._keyboards[keyboard]
        ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: <kivy.core.window.Keyboard object at 0x7f362d22e190>

You don't have to type anything for the failure to occur, and in any case typing doesn't help.

As far as I can tell, the problem goes away only if I remove either the validation or the focus monitoring, but I'm pretty sure I need both, since (a) I need to validate / modify the input for both Tab and Enter, (b) when the user pressed Enter, I need the indicated text input to acquire focus, and (c) I also need the grouping of text inputs, which seems to be related to the problem -- once I take that grouping away, the problem goes away.

Any advice would be appreciated, especially if there's a smarter way to do what I'm trying to do. If it is indeed a bug, I'll report it.

thanks in advance

john perry

# interface.kv

#:set CHAR_HEIGHT 30

#:set CHAR_WIDTH 10

<SingleLineInput@TextInput>:

text: ''

multiline: False

write_tab: False

Root:

BoxLayout:

size_hint_y: 0.1

SingleLineInput:

id: one

hint_text: 'one'

focus_next: two.__self__

on_focus: root.focused(self)

on_text_validate: root.validate(self, self.text)

SingleLineInput:

id: two

hint_text: 'two'

focus_next: one.__self__

on_focus: root.focused(self)

on_text_validate: root.validate(self, self.text)

SingleLineInput:

id: three

hint_text: 'three'

on_focus: root.focused(self)

on_text_validate: root.validate(self, self.text)

#main.py

from kivy.app import App

from kivy.factory import Factory

from kivy.uix.floatlayout import FloatLayout

class Root(FloatLayout):

def __init__(self, **kwargs) -> None:

super(Root, self).__init__(**kwargs)

def focus_first(self, _dt) -> None:

self.ids.one.focus = True

def validate(self, widget, value) -> None:

if widget == self.ids.one:

self.ids.one.text = value.capitalize()

self.ids.two.focus = True

if widget == self.ids.two:

self.ids.two.text = value.capitalize()

self.ids.one.focus = True

if widget == self.ids.three:

self.ids.three.text = value.capitalize()

def focused(self, widget) -> bool:

if not widget.focus:

self.validate(widget, widget.text)

return True

class Interface(App):

pass

Factory.register('Root', cls=Root)

if __name__ == '__main__':

Interface().run()

[ElliotG]

The focus behavior maintains a list, rather than setting focus dynamically, you can set the focus_next property.

Also there is no need to Register Root.  It is registered automatically..

If the goal is to capitalize the input, you can use the on_text event of the TextInput.  This will let you remove the on_focus and on_text_validate calls.  This create a better user experience.

<SingleLineInput@TextInput>:
    text: ''
    multiline: False
    write_tab: False

    on_text: self.text = self.text.capitalize()

Below are the update files.

# interface.kv

#:set CHAR_HEIGHT 30
#:set CHAR_WIDTH 10

<SingleLineInput@TextInput>:
    text: ''
    multiline: False
    write_tab: False

Root:
    BoxLayout:
        size_hint_y: 0.1

        SingleLineInput:
            id: one
            hint_text: 'one'
            focus_next: two.__self__
            on_focus: root.focused(self)
            on_text_validate: root.validate(self, self.text)

            focus_next: two

        SingleLineInput:
            id: two
            hint_text: 'two'
            focus_next: one.__self__
            on_focus: root.focused(self)
            on_text_validate: root.validate(self, self.text)

            focus_next: one

        SingleLineInput:
            id: three
            hint_text: 'three'
            on_focus: root.focused(self)
            on_text_validate: root.validate(self, self.text)

from kivy.app import App
# from kivy.factory import Factory

from kivy.uix.floatlayout import FloatLayout


class Root(FloatLayout):

    def __init__(self, **kwargs) -> None:
        super(Root, self).__init__(**kwargs)

    def focus_first(self, _dt) -> None:
        self.ids.one.focus = True


    def validate(self, widget, value) -> None:

        # if widget == self.ids.one:
        #     self.ids.one.text = value.capitalize()
        #     # self.ids.two.focus = True
        # if widget == self.ids.two:
        #     self.ids.two.text = value.capitalize()
        #     # self.ids.one.focus = True
        # if widget == self.ids.three:
        #     self.ids.three.text = value.capitalize()
        p = self.ids
        if widget in {p.one, p.two, p.three}:
            widget.text = value.capitalize()

    def focused(self, widget) -> bool:
        if not widget.focus:
            self.validate(widget, widget.text)
        return True


class Interface(App):
    pass

# Factory.register('Root', cls=Root)

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

[John Perry]

Hi ElliotG

Thank you, but your proposal doesn't satisfy requirement (b): when the user presses Enter, I need the indicated text input to acquire focus. As far as I can tell, no element has focus after the user presses Enter.

[ElliotG]

I've added setting the focus on enter:

from kivy.app import App

from kivy.uix.floatlayout import FloatLayout


class Root(FloatLayout):

    def __init__(self, **kwargs) -> None:
        super(Root, self).__init__(**kwargs)

    def focus_first(self, _dt) -> None:
        self.ids.one.focus = True

    def validate(self, widget) -> None:
        if widget == self.ids.one:

            self.ids.two.focus = True
        if widget == self.ids.two:

            self.ids.one.focus = True

    def focused(self, widget) -> bool:
        if not widget.focus:
            self.validate(widget, widget.text)
        return True


class Interface(App):
    pass

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

============================

# interface.kv

#:set CHAR_HEIGHT 30
#:set CHAR_WIDTH 10

<SingleLineInput@TextInput>:
    text: ''
    multiline: False
    write_tab: False

    on_text: self.text = self.text.capitalize()

Root:
    BoxLayout:
        size_hint_y: 0.1

        SingleLineInput:
            id: one
            hint_text: 'one'

            on_text_validate: root.validate(self)

            focus_next: two

        SingleLineInput:
            id: two
            hint_text: 'two'

            on_text_validate: root.validate(self)

            focus_next: one

        SingleLineInput:
            id: three
            hint_text: 'three'

[John Perry]

Thank you again! This seems to work, and it should satisfy my purposes. However, it validates whenever the user presses a key. In principle, it should work only when the user presses Tab or Enter. If validation requires a time-consuming computation, it would be a terrible idea to validate on every keypress. Is there a way to make it work without that?

[elli...@cox.net]

It is not validating on evey key, it is running capitalize() on every key.    I think that creates a better user experience.  You could move the capialitze out of on_text, and into the validate method if you prefer.

---

From: kivy-...@googlegroups.com <kivy-...@googlegroups.com> on behalf of John Perry <cantani...@gmail.com>
Sent: Tuesday, October 28, 2025 2:28 PM
To: Kivy users support <kivy-...@googlegroups.com>
Subject: [kivy-users] Re: Exception deep within kivy in certain focus changes

 

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[John Perry]

Sure, but if I move it out of on_text and into validate, it no longer capitalizes (i.e., validates) when I press Tab, which is something I need. I could uncomment the on_focus for the TextInputs, but then we're back where we started with the crashing.

[ElliotG]

The code below works... I changed the name validate() to focus_next() to match it's role.

Is this what your looking for? 

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout


class Root(BoxLayout):
    def focus_next(self, widget) -> None:

        if widget == self.ids.one:
            self.ids.two.focus = True
        if widget == self.ids.two:
            self.ids.one.focus = True

class Interface(App):
    pass


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

======================================
# interface.kv
<SingleLineInput@TextInput>:
    multiline: False
    write_tab: False
    on_focus: if not self.focus: self.text = self.text.capitalize()
    on_text_validate: app.root.focus_next(self)

BoxLayout:
    size_hint_y: None
    height: dp(48)

    SingleLineInput:
        id: one
        hint_text: 'one'

        focus_next: two

    SingleLineInput:
        id: two
        hint_text: 'two'

        focus_next: one

    SingleLineInput:
        id: three
        hint_text: 'three'

[ElliotG]

I was thinking that another way to fix the problem was to delay calling validate to it is called after the focus change from on_touch_validate had ended.

This also works.

from kivy.app import App
from kivy.uix.floatlayout import FloatLayout

from kivy.clock import Clock


class Root(FloatLayout):

    def validate(self, widget, value) -> None:

        print('validate called')

        if widget == self.ids.one:
            self.ids.one.text = value.capitalize()
            self.ids.two.focus = True
        if widget == self.ids.two:
            self.ids.two.text = value.capitalize()
            self.ids.one.focus = True
        if widget == self.ids.three:
            self.ids.three.text = value.capitalize()

    def focused(self, widget) -> bool:
        if not widget.focus:

            # delay validate by one clock... 
            Clock.schedule_once(lambda dt: self.validate(widget, widget.text))

class Interface(App):
    pass


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

==========================================

# interface.kv

#:set CHAR_HEIGHT 30
#:set CHAR_WIDTH 10

<SingleLineInput@TextInput>:

    multiline: False
    write_tab: False

Root:
    BoxLayout:
        size_hint_y: 0.1

        SingleLineInput:
            id: one
            hint_text: 'one'
            focus_next: two.__self__
            on_focus: root.focused(self)
            on_text_validate: root.validate(self, self.text)

        SingleLineInput:
            id: two
            hint_text: 'two'
            focus_next: one.__self__
            on_focus: root.focused(self)
            on_text_validate: root.validate(self, self.text)

        SingleLineInput:
            id: three
            hint_text: 'three'
            on_focus: root.focused(self)
            on_text_validate: root.validate(self, self.text)

[John Perry]

This code crashes when I press "Enter" in a box:

AttributeError: 'BoxLayout' object has no attribute 'focus_next'

[John Perry]

...and this code, whenever I do something in box A or B, then click in box C, immediately focuses on one of A or B.

[ElliotG]

 I must not have been running the correct code... embarrassing.

Here is a fix for the first one:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout


class Root(BoxLayout):
    def focus_next(self, widget) -> None:
        if widget == self.ids.one:
            self.ids.two.focus = True
        if widget == self.ids.two:
            self.ids.one.focus = True


class Interface(App):
    pass


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

==============================

# interface.kv
<SingleLineInput@TextInput>:
    multiline: False
    write_tab: False
    on_focus: if not self.focus: self.text = self.text.capitalize()
    on_text_validate: app.root.focus_next(self)

Root:

    size_hint_y: None
    height: dp(48)

    SingleLineInput:
        id: one
        hint_text: 'one'
        focus_next: two

    SingleLineInput:
        id: two
        hint_text: 'two'
        focus_next: one

    SingleLineInput:
        id: three
        hint_text: 'three'

[ElliotG]

and here is a fix for the second version using the delay...

from kivy.app import App
from kivy.uix.floatlayout import FloatLayout
from kivy.clock import Clock


class Root(FloatLayout):

    def validate(self, widget) -> None:
        if not widget.focus:
            widget.text = widget.text.capitalize()
            return

        if widget == self.ids.one:
            self.ids.two.focus = True
        if widget == self.ids.two:
            self.ids.one.focus = True

    def focused(self, widget):
        if not widget.focus:
            Clock.schedule_once(lambda dt: self.validate(widget))

class Interface(App):
    pass


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

=========================================

# interface.kv

<SingleLineInput@TextInput>:
    multiline: False
    write_tab: False

    on_focus: app.root.focused(self)
    on_text_validate: app.root.validate(self)

Root:
    BoxLayout:
        size_hint_y: 0.1

        SingleLineInput:
            id: one
            hint_text: 'one'

            focus_next: two

        SingleLineInput:
            id: two
            hint_text: 'two'
            focus_next: one

        SingleLineInput:
            id: three
            hint_text: 'three'

[John Perry]

Thank you, that first version seems to work very well, even when I extend it. It's a clever solution, as well.

That aside, was there anything specifically wrong about the approach I used in the OP? That is, should I report it as a bug? If nothing else, it seems that kivy shouldn't crash gracelessly that way; it should log an error, or something.

[elli...@cox.net]

Feel free to report an issue on the kivy github.  FWIW I'm not sure what the fix would be - if nothing else a more useful error message would be helpful.

It seems to me that the way the code was setting focus was causing a conflict.

When the focus changes the widget that loses focus fires, and the next clock the widget that get focus fires.  There was a problem in there somewhere.

---

From: kivy-...@googlegroups.com <kivy-...@googlegroups.com> on behalf of John Perry <cantani...@gmail.com>

Sent: Thursday, October 30, 2025 8:01 AM

To: Kivy users support <kivy-...@googlegroups.com>

Subject: Re: [kivy-users] Re: Exception deep within kivy in certain focus changes

 

To view this discussion visit https://groups.google.com/d/msgid/kivy-users/a6374420-4fc8-4aad-aea8-d74074220086n%40googlegroups.com.

[John Perry]

> When the focus changes the widget that loses focus fires, and the next clock the widget that get focus fires.  There was a problem in there somewhere.

That was my impression, too. Ultimately I think on_text_validate should automatically be called when an item loses focus, since information may have changed, and it is a common practice to trigger action on focus changes. But I could see how it might be too late to change that now.

I'll report the issue, if only in the hope that it handles things better than crashing. Thanks again.