How to troubleshoot nicegui issues?

[steweg]

First Check

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Example Code

I don't have exact example of code. I can reproduce the issue on my PC, but I am not able to create simplified version of code to demonstrate the issue and cannot really share whole application code here.

Description

Hi,

I am getting weird errors from nicegui, which I am not sure what can cause this nor how to properly troubleshoot it. Here is and output which I am getting:

dictionary changed size during iteration
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/nicegui/outbox.py", line 115, in loop
    raise e
  File "/usr/local/lib/python3.10/dist-packages/nicegui/outbox.py", line 106, in loop
    data = {
  File "/usr/local/lib/python3.10/dist-packages/nicegui/outbox.py", line 106, in <dictcomp>
    data = {
  File "/usr/lib/python3.10/weakref.py", line 214, in items
    for k, wr in self.data.items():
RuntimeError: dictionary changed size during iteration

I understood what is the issue itself, but this is just a consequence and I wanted to find out the root cause. I have only very limited idea on how it can actually happen, so I would really appreciate some guidance or if you have experience with such a issue, feel free to provide some hints. My best guess was that it is something related multi-threading, where io_bound thread accidentally triggers something on UI element itself, but I am not yet able to confirm nor deny it.

Any ideas? Thanks

NiceGUI Version

3.1.0

Python Version

3.10.12

Browser

Firefox

Operating System

Linux

Additional Context

No response

[evnchn]

Hello @steweg. Thanks for the report.

I think you got a big one. This appears to be a race-condition where when the Outbox is just about to enqueue the updates into sending, the update is gone because self.updates is a weakref.WeakValueDictionary meaning if there are no more references to the value (Element or Deleted, and only the former can lost reference) that key-value pair is gone immediately.

You may use AI tools to help you "bisect" your way into just the offending code. Process goes:

1. Take some code out
2. Did the issue still occur?
   a. If so, go to step 1 further
   b. If not, undo, and go to step 1 but take out some other code

Thank you!

CC: @falkoschindler

[steweg]

Thanks for the hints... I was able to pin point it by putting some additional logs and stack trace outputs in nicegui. It turned out that my best guess was right. It was issue with threads, one of the io_bound threads was accidentally triggering UI changes and therefore causing update of nicegui outbox, while the main thread was in the process of sending data..... I needed to completely rework that part of the code to strictly segregate background processing and UI elements processing