Fixes sphzone, sphlayer, cyzone, cylayer empty selections

package/AUTHORS

@@ -157,6 +157,7 @@ Chronological list of authors
   - Leonardo Barneschi
   - Henrik Jäger
   - Jan Stevens
+  - Orion Cohen
 
 External code
 -------------

package/CHANGELOG

@@ -17,11 +17,13 @@ The rules for this file:
          lilyminium, daveminh, jbarnoud, yuxuanzhuang, VOD555, ianmkenney,
          calcraven，xiki-tempula, mieczyslaw, manuel.nuno.melo, PicoCentauri,
          hanatok, rmeli, aditya-kamath, tirkarthi, LeonardoBarneschi, hejamu,
-         biogen98
+         biogen98, orioncohen
 
   * 2.0.0
 
 Fixes
+  * Fixed 'sphzone', 'sphlayer', 'cyzone', and 'cylayer' to return empty if the
+   zone/layer is empty, consistent with 'around' (Issue #2915)
   * A Universe created from an ROMol with no atoms returns now a Universe 
     with 0 atoms (Issue #3142)
   * ValueError raised when empty atomgroup is given to DensityAnalysis

package/MDAnalysis/core/selection.py

@@ -217,6 +217,7 @@ def apply(self, group):
         return func(self, group)
     return apply
 
+
 class _Selectionmeta(type):
     def __init__(cls, name, bases, classdict):
         type.__init__(type, name, bases, classdict)
@@ -352,6 +353,8 @@ def __init__(self, parser, tokens):
     def apply(self, group):
         indices = []
         sel = self.sel.apply(group)
+        if len(sel) == 0:
+            return group[[]]
         box = self.validate_dimensions(group.dimensions)
         periodic = box is not None
         ref = sel.center_of_geometry().reshape(1, 3).astype(np.float32)
@@ -378,6 +381,8 @@ def __init__(self, parser, tokens):
     def apply(self, group):
         indices = []
         sel = self.sel.apply(group)
+        if len(sel) == 0:
+            return group[[]]
         box = self.validate_dimensions(group.dimensions)
         periodic = box is not None
         ref = sel.center_of_geometry().reshape(1, 3).astype(np.float32)
@@ -394,7 +399,8 @@ class CylindricalSelection(Selection):
     @return_empty_on_apply
     def apply(self, group):
         sel = self.sel.apply(group)
-
+        if len(sel) == 0:
+            return group[[]]
         # Calculate vectors between point of interest and our group
         vecs = group.positions - sel.center_of_geometry()
 

testsuite/MDAnalysisTests/core/test_atomselections.py

@@ -210,6 +210,10 @@ def test_cylayer(self, universe, selstr):
         sel = universe.select_atoms(selstr)
         assert_equal(len(sel), 88)
 
+    def test_empty_cylayer(self, universe):
+        empty = universe.select_atoms('cylayer 4.0 6.0 10 -10 name NOT_A_NAME')
+        assert_equal(len(empty), 0)
+
     @pytest.mark.parametrize('selstr', [
         'cyzone 6.0 10 -10 bynum 1281',
         'cyzone 6.0 10 -10 index 1280'
@@ -218,6 +222,10 @@ def test_cyzone(self, universe, selstr):
         sel = universe.select_atoms(selstr)
         assert_equal(len(sel), 166)
 
+    def test_empty_cyzone(self, universe):
+        empty = universe.select_atoms('cyzone 6.0 10 -10 name NOT_A_NAME')
+        assert_equal(len(empty), 0)
+
     def test_point(self, universe):
         ag = universe.select_atoms('point 5.0 5.0 5.0 3.5')
 
@@ -770,6 +778,10 @@ def test_sphlayer(self, u):
 
         assert idx == set(ag.indices)
 
+    def test_empty_sphlayer(self, u):
+        empty = u.select_atoms('sphlayer 2.4 6.0 name NOT_A_NAME')
+        assert len(empty) == 0
+
     def test_sphzone(self, u):
         r1 = u.select_atoms('resid 1')
         cog = r1.center_of_geometry().reshape(1, 3)
@@ -781,6 +793,10 @@ def test_sphzone(self, u):
 
         assert idx == set(ag.indices)
 
+    def test_empty_sphzone(self, u):
+        empty = u.select_atoms('sphzone 5.0 name NOT_A_NAME')
+        assert len(empty) == 0
+
     def test_point_1(self, u):
         # The example selection
         ag = u.select_atoms('point 5.0 5.0 5.0 3.5')