Reading in bvct digitizer file without LPA and RPA throws key error

Describe the new feature or enhancement

Hi,

I was trying to read a .bvct file and make a DigitMontage from it, but I noticed that _parse_brainvision_dig_montage in _dig_montage_utils.py assumes that an LPA and RPA coordinate is in the CapTrackElectrodeList. I don’t think this should be a rigid assumption since mne.channels.make_dig_montage can take in an LPA and RPA that are none. I was wondering if it was possible to change the implementation in _parse_brainvision_dig_montage from:

return dict( # BVCT stuff nasion=fids["nasion"], lpa=fids["lpa"], rpa=fids["rpa"], ch_pos=dig_ch_pos, coord_frame="unknown", )

to

return dict( # BVCT stuff nasion=fids.get("nasion", None), lpa=fids.get("lpa", None), rpa=fids.get("rpa", None), ch_pos=dig_ch_pos, coord_frame="unknown", )

as this would prevent the function from throwing a key error if no LPA or RPA exist within the bvct file.

Describe your proposed implementation

return dict( # BVCT stuff nasion=fids["nasion"], lpa=fids["lpa"], rpa=fids["rpa"], ch_pos=dig_ch_pos, coord_frame="unknown", )

to

return dict( # BVCT stuff nasion=fids.get("nasion", None), lpa=fids.get("lpa", None), rpa=fids.get("rpa", None), ch_pos=dig_ch_pos, coord_frame="unknown", )

Describe possible alternatives

Please let me know if there are alternatives for defaulting dictionary key access values in Python as I typically use this to resolve to access a key that doesn't exist

Additional context

No response

Inspect _parse_brainvision_dig_montage in _dig_montage_utils.py and reproduce the failure with a .bvct file that lacks LPA or RPA coordinates. Update the handling so missing fiducials do not cause a key error, then verify that make_dig_montage accepts the resulting missing coordinates without failing.