Make Slot class better

[WolfgangFahl]

Description

The class Slot needs:

• Better documentation
• More robust code

 self.get_stack().pop()

should not lead to

IndexError: pop from empty list

see #1786

[falkoschindler]

Thanks for bringing this up, @WolfgangFahl.

The Slot class is primarily for internal use, which is why there is no extensive documentation about it (yet). But here and there the user might get into contact with slots, and we should make sure there is no confusion or unexpected behavior.

Regarding your suggestions:

1. What exactly would you like to find in the documentation? And where should we put it?
2. Why do you think the code isn't robust?
3. When exiting a slot, a non-empty slot stack is a valid assumption, because we must have entered the slot which puts it on the stack. If there is an exception, there is a bug somewhere (in NiceGUI or user code), but not necessarily in the Slot class.
4. In Testing via FastAPI Testclient #1786 you seem to use multiple threads. This can cause problems, since NiceGUI assumes only one thread interacting with the UI. As soon as multiple threads enter and exit slots, a corrupt slot stack is no surprise.
5. We could detect an empty slot stack. But I'm not sure what to tell the user in this case.

[kewartoscar]

I couldn't agree with @WolfgangFahl any better. Though slot is from Vue, there must be some transparency about its usage.
My suggestion is that for each widget for which slot can be applied, you give some notes on common usage. For instance, how to use slot to add a search button to a search field. Or using slot to add an update and delete button to ui.table rows etc.

[WolfgangFahl]

@falkoschindler thank you for looking into this.

1. nicegui/nicegui/slot.py

   Line 16 in bddc263

   """Maps asyncio task IDs to slot stacks, which keep track of the current slot in each task."""

   currently has
   Maps asyncio task IDs to slot stacks, which keep track of the current slot in each task.

and then mostly just the code with enter, exit and stack functionalities. It seems that This means you can use a Slot in a with statement, and when you exit the with block, the exit method is called automatically to clean up or finalize the slot.

I already asked: Is it related to https://vuejs.org/guide/components/slots.html#slot-content-and-outlet and https://vuejs.org/guide/components/slots.html#named-slots as the properties name and template would suggest?

2: First of all the code should be debuggable so if the pop is not possible the code for this should be on a separate line to allow to set
a breakpoint to find the conditions.
3. the Reason for the situation is hard to find if the context is not given.
4. the assumption
NiceGUI assumes only one thread interacting with the UI. should be clearly stated. There is a need to have background task interact with the UI - so how should this be done properly?
5. Furthermore in debug and or verbose mode it should be explained what's going on e.g.

[falkoschindler]

1. Yes, I answered that we borrow the slot concept from Vue.
2. While being debuggable, the code should be readable. I don't think that splitting expressions into many lines with lots of local variables helps in this regard. Sure, there might be different opinions about optimal line lengths etc. But I still would call this line robust and debuggable. If you need to see the stack in a breakpoint, you can easily edit the installed slot.py as you wish.
3. Sure.
4. Sure. I'm just not sure where to put such information. It can easily get lost within the existing documentation.
5. A debug mode would be an interesting new feature, with its own challenges.

I think we can agree that

• a note about single-threading would be helpful (just where?) and
• catching the index error and raising some informative exception instead would be nice (just what message?).

[rodja]

I just created a short pull request as a suggestion on how to mention the fact that UI updates need to happen on the main thread: #2306

[WolfgangFahl]

if only the one liner self.get_stack().pop() would be replaced by a multiline version with the option to set a breakpoint to handle the case when self.get_stack() is empty ...
I'd then be able to exactly show the circumstances under which this happens.

[falkoschindler]

As I said, you can simply edit the slot.py wherever it is installed on your system.

[WolfgangFahl]

@falkoschindler The reason for this issue is to change the nicegui slot.py and improve it. I already changed the slot.py multiple times and will have to do it again and again on every release.

[WolfgangFahl]

The problem is still happening.

here is a recent example:

Traceback (most recent call last):
  File "/Users/wf/Documents/pyworkspace/py-yprinciple-gen/yprinciple/ypgenapp.py", line 171, in show_GenerateGrid
    self.generatorGrid.add_topic_rows(context)
  File "/Users/wf/Documents/pyworkspace/py-yprinciple-gen/yprinciple/gengrid.py", line 435, in add_topic_rows
    checkbox=self.add_yp_cell(ypCell)
  File "/Users/wf/Documents/pyworkspace/py-yprinciple-gen/yprinciple/gengrid.py", line 396, in add_yp_cell
    checkbox = self.create_check_box_for_cell(ypCell, parent=self.grid)
  File "/Users/wf/Documents/pyworkspace/py-yprinciple-gen/yprinciple/gengrid.py", line 327, in create_check_box_for_cell
    with parent:
  File "/Users/wf/Library/Python/3.10/lib/python/site-packages/nicegui/element.py", line 176, in __exit__
    self.default_slot.__exit__(*_)
  File "/Users/wf/Library/Python/3.10/lib/python/site-packages/nicegui/slot.py", line 29, in __exit__
    self.get_stack().pop()
IndexError: pop from empty list

why would a

with parent

be able to cause this?

What does

Keep in mind that all UI updates must happen on the main thread with its event loop.

mean? What is the necessary style/pattern to be applied here?

[WolfgangFahl]

I am back at

def __exit__(self, *_) -> None:
        stack=self.get_stack()
        if not stack:
            pass
        stack.pop()
        self.prune_stack()

which at least allows debugging ... it would be good to have a setting that allows to have some kind of debug/warning mode in which it is pointed out how typical scenarios look like in which the problem above may happen. In my case it was simply a call:

with parent:
  yp_cell_card=ui.card() 
  label_text = yp_cell.getLabelText()
  checkbox = self.create_simple_checkbox(
       parent=yp_cell_card, 
       label_text=label_text,
       title=label_text
   ) 

versus

with parent:
   yp_cell_card=ui.card() 
label_text = yp_cell.getLabelText()
checkbox = self.create_simple_checkbox(
    parent=yp_cell_card, 
    label_text=label_text,
    title=label_text
) 

for some reason or another to nested

with parent

won't work ... and the with parent in create_simple_checkbox is not obvious from the code above. Such small typos/glitches easily lead to the Slot class throwing an error that looks strange.

[rodja]

@WolfgangFahl What kind of debugging would help you? A better error message? How can __exit__ be improved?

[WolfgangFahl]

@rodja
the one liner:

 self.get_stack().pop()

may fail since get_stack() might deliver an empty stack. To make the software more robust a check is IMHO needed. The software will then at least not immediately crash. My solution is currently to offer a debug break point for developers which does nothing but allows to set catch the situation when debugging. Having a general option for such debugging or security/robustness checks might be an improvement.

[rodja]

Do you have an example code where the current error message is not good enough to see where the code breaks? I can not think of any scenario where it would be good to keep running after self.get_stack() fails.

[WolfgangFahl]

@rodja
The scenario is a failing background task. I think Tim prepared a positive example for the #2729 discussion but did not post it yet. I am going to remind him and hope we can include a negative example while at it.

[MaidScientistIzutsumiMarin]

This is something we are facing a lot in io_bound functions. We get the error when it successfully completes. We can try to figure out a MRE.

[WolfgangFahl]

@MaidScientistIzutsumiMarin Thank you for reminding. The hints are much better these days https://ngdemo.bitplan.com/taskrunner and #2729 are updated