Quick fix for atommethods to return empty residue group (#3089)

Returns empty residue group for _get_prev_residues_by_resid and _get_next_residues_by_resid

package/AUTHORS

@@ -151,7 +151,9 @@ Chronological list of authors
   - Nicholas Craven
   - Mieczyslaw Torchala
   - Haochuan Chen
-
+2021
+  - Aditya Kamath
+
 External code
 -------------
 

package/CHANGELOG

@@ -16,11 +16,12 @@ The rules for this file:
 ??/??/?? tylerjereddy, richardjgowers, IAlibay, hmacdope, orbeckst, cbouy,
          lilyminium, daveminh, jbarnoud, yuxuanzhuang, VOD555, ianmkenney,
          calcraven，xiki-tempula, mieczyslaw, manuel.nuno.melo, PicoCentauri,
-         hanatok, rmeli
+         hanatok, rmeli, aditya-kamath
 
   * 2.0.0
 
 Fixes
+  * atommethods(_get_prev/next_residues_by_resid) returns empty residue group when given empty residue group (Issue #3089)
   * MOL2 files without bonds can now be read and written (Issue #3057)
   * Write `CONECT` record only once in multi-model PDB files (Issue #3018)
   * Add '.nc' as a recognised extension for the NCDFWriter (Issue #3030)

package/MDAnalysis/core/topologyattrs.py

@@ -874,7 +874,10 @@ def _get_next_residues_by_resid(residues):
 
         .. versionadded:: 1.0.0
         """
-        u = residues[0].universe
+        try:
+            u = residues[0].universe
+        except IndexError:
+            return residues
         nxres = np.array([None]*len(residues), dtype=object)
         ix = np.arange(len(residues))
         # no guarantee residues is ordered or unique
@@ -914,7 +917,10 @@ def _get_prev_residues_by_resid(residues):
 
         .. versionadded:: 1.0.0
         """
-        u = residues[0].universe
+        try:
+            u = residues[0].universe
+        except IndexError:
+            return residues
         pvres = np.array([None]*len(residues))
         pvres[:] = prev = u.residues[residues.ix-1]
         rsid = residues.segids

testsuite/MDAnalysisTests/core/test_topologyattrs.py

@@ -180,6 +180,18 @@ class TestAtomnames(TestAtomAttr):
     single_value = 'Ca2'
     attrclass = tpattrs.Atomnames
 
+    @pytest.fixture()
+    def u(self):
+        return mda.Universe(PSF, DCD)
+
+    def test_prev_emptyresidue(self, u):
+        assert_equal(u.residues[[]]._get_prev_residues_by_resid(),
+                     u.residues[[]])
+
+    def test_next_emptyresidue(self, u):
+        assert_equal(u.residues[[]]._get_next_residues_by_resid(),
+                     u.residues[[]])
+
 
 class AggregationMixin(TestAtomAttr):
     def test_get_residues(self, attr):