MRG: When picking, remove inapplicable projectors

doc/changes/latest.inc

@@ -125,7 +125,7 @@ Changelog
 
 - BrainVision data format files are now parsed for EEG impedance values in :func:`mne.io.read_raw_brainvision` and provided as a ``.impedances`` attribute of ``raw`` by `Stefan Appelhoff`_ and `Jan Sedivy`_
 
-- Deletion of applied (active) projectors via `~mne.io.Raw.del_proj`, `~mne.Epochs.del_proj`, and `~mne.Evoked.del_proj` is now possible if the channels the to-be-removed projector applies to are not present in the data anymore by `Richard Höchenberger`_
+- When picking a subset of channels, or when dropping channels from `~mne.io.Raw`, `~mne.Epochs`, or `~mne.Evoked`, projectors that can only be applied to the removed channels will now be dropped automatically by `Richard Höchenberger`_
 
 Bug
 ~~~

mne/beamformer/tests/test_lcmv.py

@@ -293,7 +293,7 @@ def test_make_lcmv(tmpdir, reg, proj):
 
     # __repr__
     assert len(evoked.ch_names) == 22
-    assert len(evoked.info['projs']) == (4 if proj else 0)
+    assert len(evoked.info['projs']) == (3 if proj else 0)
     assert len(evoked.info['bads']) == 2
     rank = 17 if proj else 20
     assert 'LCMV' in repr(filters)

mne/channels/channels.py

@@ -756,8 +756,8 @@ def pick_channels(self, ch_names, ordered=False):
 
         .. versionadded:: 0.9.0
         """
-        return self._pick_drop_channels(
-            pick_channels(self.info['ch_names'], ch_names, ordered=ordered))
+        picks = pick_channels(self.info['ch_names'], ch_names, ordered=ordered)
+        return self._pick_drop_channels(picks)
 
     @fill_doc
     def pick(self, picks, exclude=()):
@@ -888,6 +888,23 @@ def _pick_drop_channels(self, idx):
             self._data = self._data.take(idx, axis=axis)
         else:
             assert isinstance(self, BaseRaw) and not self.preload
+
+        self._pick_projs()
+        return self
+
+    def _pick_projs(self):
+        """Keep only projectors which apply to at least 1 data channel."""
+        drop_idx = []
+        for idx, proj in enumerate(self.info['projs']):
+            if not set(self.info['ch_names']) & set(proj['data']['col_names']):
+                drop_idx.append(idx)
+
+        for idx in drop_idx:
+            logger.info(f"Removing projector {self.info['projs'][idx]}")
+
+        if drop_idx and hasattr(self, 'del_proj'):
+            self.del_proj(drop_idx)
+
         return self
 
     def add_channels(self, add_list, force_update_info=False):

mne/io/fiff/tests/test_raw_fiff.py

@@ -728,29 +728,32 @@ def test_proj(tmpdir):
         assert_allclose(data_proj_1, data_proj_2)
         assert_allclose(data_proj_2, np.dot(raw._projector, data_proj_2))
 
+    # Test that picking removes projectors ...
+    raw = read_raw_fif(fif_fname).apply_proj()
+    n_projs = len(raw.info['projs'])
+    raw.pick_types(meg=False, eeg=True)
+    assert len(raw.info['projs']) == n_projs - 3
+
+    # ... but only if it doesn't apply to any channels in the dataset anymore.
+    raw = read_raw_fif(fif_fname).apply_proj()
+    n_projs = len(raw.info['projs'])
+    raw.pick_types(meg='mag', eeg=True)
+    assert len(raw.info['projs']) == n_projs
+
+    # I/O roundtrip of an MEG projector with a Raw that only contains EEG
+    # data.
     out_fname = tmpdir.join('test_raw.fif')
     raw = read_raw_fif(test_fif_fname, preload=True).crop(0, 0.002)
+    proj = raw.info['projs'][-1]
     raw.pick_types(meg=False, eeg=True)
-    raw.info['projs'] = [raw.info['projs'][-1]]
+    raw.info['projs'] = [proj]  # Restore, because picking removed it!
     raw._data.fill(0)
     raw._data[-1] = 1.
     raw.save(out_fname)
     raw = read_raw_fif(out_fname, preload=False)
     raw.apply_proj()
     assert_allclose(raw[:, :][0][:1], raw[0, :][0])
 
-    # Read file again, apply proj, pick all channels one proj did NOT apply to;
-    # then try to delete this proj, which now exclusively refers to channels
-    # which are not present in the data anymore.
-    raw = read_raw_fif(fif_fname).apply_proj()
-    del_proj_idx = 0
-    picks = list(set(raw.ch_names) -
-                 set(raw.info['projs'][del_proj_idx]['data']['col_names']))
-    raw.pick(picks)
-    n_projs = len(raw.info['projs'])
-    raw.del_proj(del_proj_idx)
-    assert len(raw.info['projs']) == n_projs - 1
-
 
 @testing.requires_testing_data
 @pytest.mark.parametrize('preload', [False, True, 'memmap.dat'])
@@ -1475,6 +1478,13 @@ def test_drop_channels_mixin():
     assert len(ch_names) == len(raw._cals)
     assert len(ch_names) == raw._data.shape[0]
 
+    # Test that dropping all channels a projector applies to will lead to the
+    # removal of said projector.
+    raw = read_raw_fif(fif_fname).apply_proj()
+    n_projs = len(raw.info['projs'])
+    raw.drop_channels(raw.info['projs'][-1]['data']['col_names'])  # EEG proj
+    assert len(raw.info['projs']) == n_projs - 1
+
 
 @testing.requires_testing_data
 @pytest.mark.parametrize('preload', (True, False))

mne/io/proj.py

@@ -222,8 +222,7 @@ def del_proj(self, idx='all'):
         """Remove SSP projection vector.
 
         .. note:: The projection vector can only be removed if it is inactive
-                  (has not been applied to the data), unless the channels it
-                  was applied to no longer exist in the data.
+                  (has not been applied to the data).
 
         Parameters
         ----------