BUG: Fix bug with BTi loc (mne-tools#10662)

* BUG: Fix bug with BTi loc

* FIX: Counting

doc/changes/latest.inc

@@ -94,6 +94,8 @@ Bugs
 
 - Fix bug in :meth:`raw.crop(start, stop) <mne.io.Raw.crop>` that would cause annotations to be erroneously shifted when ``start != 0`` and no measurement date was set. (:gh:`10491` by `Eric Larson`_)
 
+- Fix bug in :func:`mne.io.read_raw_bti` where unknown electrode locations were not handled properly (:gh:`10662` by `Eric Larson`_)
+
 - Rendering issues with recent MESA releases can be avoided by setting the new environment variable``MNE_3D_OPTION_MULTI_SAMPLES=1`` or using :func:`mne.viz.set_3d_options` (:gh:`10513` by `Eric Larson`_)
 
 - Fix behavior for the ``pyvista`` 3D renderer's ``quiver3D`` function so that default arguments plot a glyph in ``arrow`` mode (:gh:`10493` by `Alex Rockhill`_)

mne/io/bti/bti.py

@@ -855,7 +855,7 @@ def _read_bti_header(pdf_fname, config_fname, sort_by_ch_name=True):
         if ch_cfg.get('dev', dict()).get('transform', None) is not None:
             ch['loc'] = _coil_trans_to_loc(ch_cfg['dev']['transform'])
         else:
-            ch['loc'] = None
+            ch['loc'] = np.full(12, np.nan)
         if pdf_fname is not None:
             if info['data_format'] <= 2:  # see DTYPES, implies integer
                 ch['cal'] = ch['scale'] * ch['upb'] / float(ch['gain'])

mne/io/bti/tests/test_bti.py

@@ -12,15 +12,15 @@
                            assert_allclose, assert_equal)
 import pytest
 
+import mne
 from mne.datasets import testing
 from mne.io import read_raw_fif, read_raw_bti
 from mne.io._digitization import _make_bti_dig_points
-from mne.io.bti.bti import (_read_config,
+from mne.io.bti.bti import (_read_config, _read_head_shape,
                             _read_bti_header, _get_bti_dev_t,
                             _correct_trans, _get_bti_info,
                             _loc_to_coil_trans, _convert_coil_trans,
                             _check_nan_dev_head_t, _rename_channels)
-from mne.io.bti.bti import _read_head_shape
 from mne.io.tests.test_raw import _test_raw_reader
 from mne.io.pick import pick_info
 from mne.io.constants import FIFF
@@ -55,6 +55,25 @@ def test_read_2500():
     _test_raw_reader(read_raw_bti, pdf_fname=fname_2500, head_shape_fname=None)
 
 
+def test_no_loc_none(monkeypatch):
+    """Test that we don't set loc to None when no trans is found."""
+    ch_name = 'MLzA'
+
+    def _read_config_bad(*args, **kwargs):
+        cfg = _read_config(*args, **kwargs)
+        idx = [ch['name'] for ch in cfg['chs']].index(ch_name)
+        del cfg['chs'][idx]['dev']['transform']
+        return cfg
+
+    monkeypatch.setattr(mne.io.bti.bti, '_read_config', _read_config_bad)
+    kwargs = dict(pdf_fname=pdf_fnames[0], config_fname=config_fnames[0],
+                  head_shape_fname=hs_fnames[0], rename_channels=False,
+                  sort_by_ch_name=False)
+    raw = read_raw_bti(**kwargs)
+    idx = raw.ch_names.index(ch_name)
+    assert_allclose(raw.info['chs'][idx]['loc'], np.full(12, np.nan))
+
+
 def test_read_config():
     """Test read bti config file."""
     # for config in config_fname, config_solaris_fname: