Raise TypeError when calling AtomGroup[None] (#3517)

Fixes #3092
* Raise TypeError when a group is indexed with None

package/CHANGELOG

@@ -43,6 +43,8 @@ Enhancements
   * Added MDAnalysis.lib.distances.minimize_vectors
 
 Changes
+  * Indexing a group (AtomGroup, ResidueGroup, or SegmentGroup) with None
+    raises a TypeError (Issue #3092, PR #3517)
   * TRZReader now defaults to a `dt` of 1.0 (instead of 0.0 previously) when
     it cannot be obtained from file (Issue #3257)
   * Dropped python 3.6 support and raised minimum numpy version to 1.18.0

package/MDAnalysis/core/groups.py

@@ -569,7 +569,9 @@ def __getitem__(self, item):
         # because our _ix attribute is a numpy array
         # it can be sliced by all of these already,
         # so just return ourselves sliced by the item
-        if isinstance(item, numbers.Integral):
+        if item is None:
+            raise TypeError('None cannot be used to index a group.')
+        elif isinstance(item, numbers.Integral):
             return self.level.singular(self.ix[item], self.universe)
         else:
             if isinstance(item, list) and item:  # check for empty list
@@ -2488,6 +2490,8 @@ class AtomGroup(GroupBase):
     .. versionchanged:: 2.0.0
        :class:`AtomGroup` can always be pickled with or without its universe,
        instead of failing when not finding its anchored universe.
+    .. versionchanged:: 2.1.0
+       Indexing an AtomGroup with ``None`` raises a ``TypeError``.
     """
 
     def __reduce__(self):
@@ -3582,6 +3586,8 @@ class ResidueGroup(GroupBase):
        *Instant selectors* of Segments will be removed in the 1.0 release.
     .. versionchanged:: 1.0.0
        Removed instant selectors, use select_atoms instead
+    .. versionchanged:: 2.1.0
+       Indexing an ResidueGroup with ``None`` raises a ``TypeError``.
     """
 
     @property
@@ -3773,6 +3779,8 @@ class SegmentGroup(GroupBase):
        *Instant selectors* of Segments will be removed in the 1.0 release.
     .. versionchanged:: 1.0.0
        Removed instant selectors, use select_atoms instead
+    .. versionchanged:: 2.1.0
+       Indexing an SegmentGroup with ``None`` raises a ``TypeError``.
     """
 
     @property

testsuite/MDAnalysisTests/core/test_groups.py

@@ -326,6 +326,10 @@ def test_integer_getitem(self, group, nparray, idx, singular):
 
         assert a.ix == ref
         assert isinstance(a, singular)
+
+    def test_none_getitem(self, group):
+        with pytest.raises(TypeError):
+            group[None]
 
 
 def _yield_groups(group_dict, singles, levels, groupclasses, repeat):