Parameter value persistance causes error with napari layers as parameter types

Describe the bug Using @magicgui(persist=True) works fine in a plugin, except the case when I have something like image: napari.layers.Image as an input parameter, e.g. here.

If I change the parameters, then re-start napari, the plugin doesn’t start and I get an error.

To Reproduce Steps to reproduce the behavior:

napari Load plugin, change a parameter & close napari NAPARI_CATCH_ERRORS=0 napari Try to load plugin

(napari-cellfinder) adam@garfield:~/projects/cellfinder-napari$ NAPARI_CATCH_ERRORS=0 napari
09:58:45 WARNING Could not import pyopencl. Falling back to scipy for CPU affine transforms
09:58:47 ERROR Unhandled exception:
Traceback (most recent call last):
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/napari/_qt/qt_main_window.py", line 450, in _add_widget
    self._add_plugin_dock_widget(key)
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/napari/_qt/qt_main_window.py", line 568, in _add_plugin_dock_widget
    wdg = Widget(**kwargs)
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/magicgui/_magicgui.py", line 200, in __call__
    widget = self.func(param_options=prm_options, **{**self.keywords, **kwargs})
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/magicgui/widgets/_function_gui.py", line 199, in __init__
    self._load(quiet=True)
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/magicgui/widgets/_function_gui.py", line 379, in _load
    super()._load(path or self._dump_path, quiet=quiet)
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/magicgui/widgets/_bases/container_widget.py", line 300, in _load
    getattr(self, key).value = val
  File "/home/adam/miniconda3/envs/napari-cellfinder/lib/python3.8/site-packages/magicgui/widgets/_bases/categorical_widget.py", line 38, in value
    raise ValueError(
ValueError: None is not a valid choice. must be in ()

This error doesn’t occur if I:

• delete the cache file before re-starting napari
• Don’t have a napari layer as parameter type in my decorated function

Expected behavior All the parameters to be reloaded whether or not one of the inputs is a napari layer

Environment (please complete the following information):

• OS: Ubuntu 18.04
• backend: PySide2
• magicgui version: 0.2.7

P.S. the parameter persistence is really helpful for my plugin, thanks!

[tlambert03]

There is a debounce applied to the save-to-disk function so it only runs no more than 4 times a second. So if changes are applied in rapid succession, it’s possible that the last changes will be missed. This was to prevent something like a slider causing a ton of disk events… I’ll try to add logic to make sure that the last event is always registered.

[uschmidt83]

I ran into the same issue, but it ended with:

AttributeError: Can't pickle local object 'configure_dask.<locals>.dask_optimized_slicing'