EventedModel is incompatible with dask delayed objects

🐛 Bug

napari.utils.events.EventedModel instantiation fails if one of the attributes is a dask.Delayed object. I’m not aware of this particular usage anywhere inside napari but it could feasibly come up so I thought it best to make an issue.

I know this is appearing as a pydantic issue here (not napari or EventedModel specific) but in my use case the error appeared when equality checking two EventedModel objects which had delayed attributes, not on instantiation.

I’m looking at a few ways to solve this now, for my use case simply removing the __eq__ override in EventedModel works but this isn’t a good solution

https://github.com/napari/napari/blob/255822fb17f25c79133ec40dba0e626b2bf66b20/napari/utils/events/evented_model.py#L202-L218

To Reproduce

from dask import delayed
from dask.delayed import DelayedLeaf
from napari.utils.events import EventedModel


@delayed
def my_function():
    pass


class MyObject(EventedModel):
    attribute: DelayedLeaf = my_function

Traceback (most recent call last):
  File "/Users/aburt/ccpem-projects/pp2d-relion/bug_report_eventedmodel_equality.py", line 10, in <module>
    class MyObject(EventedModel):
  File "/opt/homebrew/Caskroom/miniforge/base/envs/pp2d-relion/lib/python3.9/site-packages/napari/utils/events/evented_model.py", line 79, in __new__
    cls = super().__new__(mcs, name, bases, namespace, **kwargs)
  File "pydantic/main.py", line 299, in pydantic.main.ModelMetaclass.__new__
  File "pydantic/fields.py", line 411, in pydantic.fields.ModelField.infer
  File "pydantic/fields.py", line 342, in pydantic.fields.ModelField.__init__
  File "pydantic/fields.py", line 445, in pydantic.fields.ModelField.prepare
  File "pydantic/fields.py", line 473, in pydantic.fields.ModelField._set_default_and_type
  File "pydantic/fields.py", line 345, in pydantic.fields.ModelField.get_default
  File "pydantic/utils.py", line 627, in pydantic.utils.smart_deepcopy
  File "/opt/homebrew/Caskroom/miniforge/base/envs/pp2d-relion/lib/python3.9/site-packages/dask/delayed.py", line 580, in __bool__
    raise TypeError("Truth of Delayed objects is not supported")
TypeError: Truth of Delayed objects is not supported

[tlambert03]

I think the key here is the pick_equality_operator function

That function takes an object (which in this case would be a dask.Delayed object) and returns an appropriate function for checking equality. In “hard” cases like this, we simply return operator.is_, which just checks for object identity (probably the best we can do). The evented model then populates the equality checker for each type in the model on this line here:

https://github.com/napari/napari/blob/609fd9f4cc54832c752d3691d7f16e15fbf86517/napari/utils/events/evented_model.py#L81-L82

So, to fix this, maybe just try adding 'dask.delayed.Delayed': operator.is_ to the _known_arrays dict in pick_equality_operator?

[alisterburt]

@tlambert03 thanks a bunch for the detailed explanation, really appreciate it! 🙂