get_B0FieldIdentifiers errors

[bpinsard]

I am trying to apply the new spec for B0 fieldmap Intent to bypass limitations with sdcflows nipreps/sdcflows#266

However I have fmriprep crashing in pybids.BIDSLayout.get_B0FieldIdentifiers

lib/python3.7/site-packages/bids/layout/layout.py in get(self, return_type, target, scope, regex_search, absolute
_paths, invalid_filters, **filters)                                                                                                                       
    676                 ent_iter = (x.get_entities(metadata=metadata) for x in results)                                                                   
    677                 results = list({                                                                                                                  
--> 678                     ents[target] for ents in ent_iter if target in ents                                                                           
    679                 })                                                                                                                                
    680                                                                                                                                                   
                                                                                                                                                          
lib/python3.7/site-packages/bids/layout/layout.py in <setcomp>(.0)                                               
    676                 ent_iter = (x.get_entities(metadata=metadata) for x in results)                                                                   
    677                 results = list({                                                                                                                  
--> 678                     ents[target] for ents in ent_iter if target in ents                                                                           
    679                 })                                                                                                                                
    680                                                                                                                                                   
                                                                                                                                                          
TypeError: unhashable type: 'list' 

From the spec, B0FieldIdentifier can be a string or an array of string, which I stored as a json list if I am correct.
https://bids-specification.readthedocs.io/en/stable/04-modality-specific-files/01-magnetic-resonance-imaging-data.html#echo-planar-imaging-and-b0-mapping

So I am not sure why it is converted to a dict here.

pybids/bids/layout/layout.py

Lines 677 to 679 in 5ea3103

	results = list({
	ents[target] for ents in ent_iter if target in ents
	})

[effigies]

Hi @bpinsard, thanks for posting this! I think this is a definitive use case for #684. If you're doing some testing, would you mind seeing whether merging that would resolve the issue? I can get back to this issue next week.

[bpinsard]

@effigies thanks for the pointer, weirdly I had not found that PR in my search. I will test it on our data.
Yesterday I had implemented a quick and dirty fix in the pybids code to do something similar (str representation of the list for hashing/unique) and it allowed to pass that error, so that should work.

[bpinsard]

It works with the PR.