MRG: Return empty list of SSP projectors if no ECG, EOG events were found

doc/changes/latest.inc

@@ -287,6 +287,8 @@ Bugs
 
 - :func:`mne.preprocessing.find_ecg_events` now correctly handles situation where no ECG activity could be detected, and correctly returns an empty array of ECG events (:gh:`9236` by `Richard Höchenberger`_)
 
+- `mne.preprocessing.compute_proj_eog` and `mne.preprocessing.compute_proj_ecg` now return empty lists if no EOG or ECG events, respectively, could be found. Previously, we'd return ``None`` in these situations, which does not match the documented behavior of returning a list of projectors (:gh:`9277` by `Richard Höchenberger`_)
+
 API changes
 ~~~~~~~~~~~
 - Introduced new ``'auto'`` settings for ``ICA.max_iter``. The old default ``max_iter=200`` will be removed in MNE-Python 0.24 (:gh:`9099` **by new contributor** |Cora Kim|_)

mne/preprocessing/ssp.py

@@ -69,8 +69,8 @@ def _compute_exg_proj(mode, raw, raw_event, tmin, tmax,
 
     # Check to make sure we actually got at least one usable event
     if events.shape[0] < 1:
-        warn('No %s events found, returning None for projs' % mode)
-        return (None, events) + (([],) if return_drop_log else ())
+        warn(f'No {mode} events found')
+        return ([], events) + (([],) if return_drop_log else ())
 
     logger.info('Computing projector')
     my_info = cp.deepcopy(raw.info)
@@ -120,8 +120,8 @@ def _compute_exg_proj(mode, raw, raw_event, tmin, tmax,
 
     drop_log = epochs.drop_log
     if epochs.events.shape[0] < 1:
-        warn('No good epochs found, returning None for projs')
-        return (None, events) + ((drop_log,) if return_drop_log else ())
+        warn('No good epochs found')
+        return ([], events) + ((drop_log,) if return_drop_log else ())
 
     if average:
         evoked = epochs.average()

mne/preprocessing/tests/test_ssp.py

@@ -63,8 +63,12 @@ def test_compute_proj_ecg(short_raw, average):
         projs, events, drop_log = compute_proj_ecg(
             raw, n_mag=2, n_grad=2, n_eeg=2, ch_name='MEG 1531', bads=[],
             average=average, avg_ref=True, no_proj=True, l_freq=None,
-            h_freq=None, tmax=dur_use, return_drop_log=True)
-    assert projs is None
+            h_freq=None, tmax=dur_use, return_drop_log=True,
+            # XXX can be removed once
+            # XXX https://github.com/mne-tools/mne-python/issues/9273
+            # XXX has been resolved:
+            qrs_threshold=1e-15)
+    assert projs == []
     assert len(events) == len(drop_log)
 
 
@@ -100,7 +104,12 @@ def test_compute_proj_eog(average, short_raw):
             raw, n_mag=2, n_grad=2, n_eeg=2, average=average, bads=[],
             avg_ref=True, no_proj=False, l_freq=None, h_freq=None,
             tmax=dur_use)
-    assert projs is None
+    assert projs == []
+
+    raw._data[raw.ch_names.index('EOG 061'), :] = 1.
+    with pytest.warns(RuntimeWarning, match='filter.*longer than the signal'):
+        projs, events = compute_proj_eog(raw=raw, tmax=dur_use,
+                                         ch_name='EOG 061')
 
 
 @pytest.mark.slowtest  # can be slow on OSX