FIX: Restore bads when interpolate_bridged_electrodes raises for bad_limit

doc/changes/dev/14320.bugfix.rst

@@ -0,0 +1 @@
+Fix bug with :func:`mne.preprocessing.interpolate_bridged_electrodes` where ``inst.info["bads"]`` was cleared and not restored when too many electrodes were bridged together and a :class:`RuntimeError` was raised, by `Carina Forster`_.

mne/preprocessing/interpolate.py

@@ -124,9 +124,6 @@ def interpolate_bridged_electrodes(inst, bridged_idx, bad_limit=4):
         raise RuntimeError(
             f"Montage channel positions must be in ``head`` got {pos['coord_frame']}"
         )
-    # store bads orig to put back at the end
-    bads_orig = inst.info["bads"]
-    inst.info["bads"] = list()
 
     # look for group of bad channels
     nodes = sorted(set(chain(*bridged_idx)))
@@ -153,6 +150,10 @@ def interpolate_bridged_electrodes(inst, bridged_idx, bad_limit=4):
                 "might be inaccurate."
             )
 
+    # store bads orig to put back at the end
+    bads_orig = inst.info["bads"]
+    inst.info["bads"] = list()
+
     # make virtual channels
     virtual_chs = dict()
     bads = set()

mne/preprocessing/tests/test_interpolate.py

@@ -129,13 +129,16 @@ def test_interpolate_bridged_electrodes():
     data[:5, :] = np.ones((5, 1024))
     raw = io.RawArray(data, info)
     raw.set_montage("spherical_1005")
+    raw.info["bads"] = [ch_names[10]]  # a bad channel outside the bridged group
     bridged_idx = list(itertools.combinations(range(5), 2))
     with pytest.raises(
         RuntimeError,
         match="The channels AF1, AF10, AF10h, AF1h, AF2 are bridged "
         "together and form a large area of bridged electrodes.",
     ):
         interpolate_bridged_electrodes(raw, bridged_idx, bad_limit=4)
+    # bads must be preserved when the error is raised
+    assert raw.info["bads"] == [ch_names[10]]
     # increase the limit to prevent raising
     interpolate_bridged_electrodes(raw, bridged_idx, bad_limit=5)
     # invalid argument