MRG: Allow removal of active projectors if channels they applied to h…

…ave meanwhile been dropped (#8003)

* Allow removal of active proj if chs were dropped

In cases where the channels that an applied projector
applies to have been removed from the data (pick
after apply_proj() call), it should besafe to allow
users to remove the projector. Currently we disallow
deletion of any "active" (applied) projectors, so this
commit adds more flexibility there.

* Typo

* Fix reference

* Test that projector got removed

doc/changes/latest.inc

@@ -125,6 +125,8 @@ 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`_
+
 Bug
 ~~~
 

mne/io/fiff/tests/test_raw_fiff.py

@@ -739,6 +739,18 @@ def test_proj(tmpdir):
     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'])

mne/io/proj.py

@@ -222,7 +222,8 @@ 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).
+                  (has not been applied to the data), unless the channels it
+                  was applied to no longer exist in the data.
 
         Parameters
         ----------
@@ -238,9 +239,17 @@ def del_proj(self, idx='all'):
         if isinstance(idx, str) and idx == 'all':
             idx = list(range(len(self.info['projs'])))
         idx = np.atleast_1d(np.array(idx, int)).ravel()
-        if any(self.info['projs'][ii]['active'] for ii in idx):
-            raise ValueError('Cannot remove projectors that have already '
-                             'been applied')
+
+        for ii in idx:
+            proj = self.info['projs'][ii]
+            if (proj['active'] and
+                    set(self.info['ch_names']) &
+                    set(proj['data']['col_names'])):
+                msg = (f'Cannot remove projector that has already been '
+                       f'applied, unless you first remove all channels it '
+                       f'applies to. The problematic projector is: {proj}')
+                raise ValueError(msg)
+
         keep = np.ones(len(self.info['projs']))
         keep[idx] = False  # works with negative indexing and does checks
         self.info['projs'] = [p for p, k in zip(self.info['projs'], keep) if k]