Invalid first argument to register(). ForwardRef('PredictedInstance') is not a class.

[talmo]

In Python 3.9+, it looks like cattrs breaks when structuring forward references. At the moment this only affects M1 Macs which will use Python 3.9 environments by default, but will be an issue across all platforms in the future.

This crops up when loading a project that has an instance created from predictions such that the Instance.from_predicted field is not None (i.e., set to a PredictedInstance). Here's where this is handled:

sleap/sleap/instance.py

Lines 1228 to 1249 in be89412

	# Function to determine object type for objects being structured.
	def structure_instances_list(x, type):
	inst_list = []
	for inst_data in x:
	if "score" in inst_data.keys():
	inst = converter.structure(inst_data, PredictedInstance)
	else:
	inst = converter.structure(inst_data, Instance)
	inst_list.append(inst)
	return inst_list
	converter.register_structure_hook(
	Union[List[Instance], List[PredictedInstance]], structure_instances_list
	)
	# Structure forward reference for PredictedInstance for the Instance.from_predicted
	# attribute.
	converter.register_structure_hook(
	ForwardRef("PredictedInstance"),
	lambda x, _: converter.structure(x, PredictedInstance),
	)

This can be worked around using the approach described in python-attrs/cattrs#206.

See below for original user post that reported this.

Discussed in #966

^{Originally posted by gaojundong September 20, 2022}
Hi Team,

When I try to save the skeleton instance as a new project, a window suddenly popped up to remind me:

An error occured when attempting to save:
Invalid first argument to register(). ForwardRef('PredictedInstance') is not a class.
Try saving your project with a different filename or in a different format.

I try to modify the file name as label.slp and label.v001.slp, but it still doesn't work. this is my software version:
Software versions:
SLEAP: 1.2.8
TensorFlow: 2.9.2
Numpy: 1.22.3
Python: 3.9.13
OS: macOS-12.5.1-arm64-arm-64bit

In this software, I can save skeleton node and edges and export video, but I don't know where is the problem when I save the whole project. Can you give me some hints about this. Thank you very much.

All the best,
Gaojun