Better find_events message if no stim channel is found (mne-tools#6347)

* Better find_events message if no stim channel is found

* Actually check if annotations are present and show custom error message

mne/event.py

@@ -678,7 +678,16 @@ def find_events(raw, stim_channel=None, output='onset',
     min_samples = min_duration * raw.info['sfreq']
 
     # pull stim channel from config if necessary
-    stim_channel = _get_stim_channel(stim_channel, raw.info)
+    try:
+        stim_channel = _get_stim_channel(stim_channel, raw.info)
+    except ValueError:
+        if len(raw.annotations) > 0:
+            raise ValueError("No stim channels found, but the raw object has "
+                             "annotations. Consider using "
+                             "mne.events_from_annotations to convert these to "
+                             "events.")
+        else:
+            raise
 
     picks = pick_channels(raw.info['ch_names'], include=stim_channel)
     if len(picks) == 0:

mne/tests/test_event.py

@@ -8,7 +8,8 @@
 
 from mne import (read_events, write_events, make_fixed_length_events,
                  find_events, pick_events, find_stim_steps, pick_channels,
-                 read_evokeds, Epochs, create_info, compute_raw_covariance)
+                 read_evokeds, Epochs, create_info, compute_raw_covariance,
+                 Annotations)
 from mne.io import read_raw_fif, RawArray
 from mne.utils import _TempDir, run_tests_if_main
 from mne.event import (define_target_events, merge_events, AcqParserFIF,
@@ -362,6 +363,17 @@ def test_find_events():
     assert_array_equal(find_events(raw, 'MYSTI', initial_event=True),
                        [[0, 0, 100], [30, 0, 200]])
 
+    # test error message for raw without stim channels
+    raw = read_raw_fif(raw_fname, preload=True)
+    raw.pick_types(meg=True, stim=False)
+    # raw does not have annotations
+    with pytest.raises(ValueError, match="'stim_channel'"):
+        find_events(raw)
+    # if raw has annotations, we show a different error message
+    raw.set_annotations(Annotations(0, 2, "test"))
+    with pytest.raises(ValueError, match="mne.events_from_annotations"):
+        find_events(raw)
+
 
 def test_pick_events():
     """Test pick events in a events ndarray."""