From 87e71bb2fccfbf8ff3127130d1716c86f6682ccb Mon Sep 17 00:00:00 2001
From: Aditya Kamath <48089312+aditya-kamath@users.noreply.github.com>
Date: Sun, 3 Jan 2021 23:38:37 -0500
Subject: [PATCH] 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                                      |  4 +++-
 package/CHANGELOG                                    |  3 ++-
 package/MDAnalysis/core/topologyattrs.py             | 10 ++++++++--
 testsuite/MDAnalysisTests/core/test_topologyattrs.py | 12 ++++++++++++
 4 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/package/AUTHORS b/package/AUTHORS
index 52b6003b9cf..960e078a627 100644
--- a/package/AUTHORS
+++ b/package/AUTHORS
@@ -151,7 +151,9 @@ Chronological list of authors
   - Nicholas Craven
   - Mieczyslaw Torchala
   - Haochuan Chen
-
+2021
+  - Aditya Kamath
+  
 External code
 -------------
 
diff --git a/package/CHANGELOG b/package/CHANGELOG
index eb6baa1bc40..590a7fb5fbc 100644
--- a/package/CHANGELOG
+++ b/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)
diff --git a/package/MDAnalysis/core/topologyattrs.py b/package/MDAnalysis/core/topologyattrs.py
index 7515a519d80..2282b706f01 100644
--- a/package/MDAnalysis/core/topologyattrs.py
+++ b/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
diff --git a/testsuite/MDAnalysisTests/core/test_topologyattrs.py b/testsuite/MDAnalysisTests/core/test_topologyattrs.py
index 2a80d99899e..bf82974550c 100644
--- a/testsuite/MDAnalysisTests/core/test_topologyattrs.py
+++ b/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):