resolve comments

Author: ChiahsinChu

package/MDAnalysis/core/groups.py

@@ -3254,6 +3254,16 @@ def select_atoms(self, sel, *othersel, periodic=True, rtol=1e-05,
                 For example, ``prop z >= 5.0`` selects all atoms with z
                 coordinate greater than 5.0; ``prop abs z <= 5.0`` selects all
                 atoms within -5.0 <= z <= 5.0.
+            relprop [abs] *property*  *operator*  *value*  *selection*
+                selects atoms based on position relative to the center of 
+                geometry (COG) of a given selection, using *property*  
+                **x**, **y**, or **z** coordinate. Supports the **abs** 
+                keyword (for absolute value) and the following 
+                *operators*: **<, >, <=, >=, ==, !=**.
+                For example, ``relprop z >= 5.0 protein`` selects all atoms 
+                with z coordinate greater than 5.0 relative to the COG 
+                of protein; ``relprop abs z <= 5.0 protein`` selects all 
+                atoms within -5.0 <= z <= 5.0 relative to the COG of protein.
             sphzone *radius* *selection*
                 Selects all atoms that are within *radius* of the center of
                 geometry of *selection*

package/MDAnalysis/core/selection.py

@@ -1366,7 +1366,7 @@ class RelPropertySelection(PropertySelection):
     def __init__(self, parser, tokens):
         super().__init__(parser, tokens)
         self.sel = parser.parse_expression(self.precedence)
-        # self.ori_value = self.value
+        self.periodic = parser.periodic
 
     def _apply(self, group):
         try:
@@ -1382,14 +1382,18 @@ def _apply(self, group):
         try:
             col = {"x": 0, "y": 1, "z": 2}[self.prop]
         except KeyError:
-            pass
+            errmsg = f"Expected one of x y z for property, got {self.prop}"
+            raise SelectionError(errmsg) from None
         else:
-            values = values[:, col]
             sel = self.sel.apply(group)
-            rel_value = (
-                sel.center_of_geometry().reshape(3).astype(np.float32)[col]
-            )
-            values -= rel_value
+            ref_value = sel.center_of_geometry().reshape(3).astype(np.float32)
+            box = group.dimensions if self.periodic else None
+            if box is not None:
+                values = distances.minimize_vectors(
+                    values - ref_value[None, :], box=box
+                )[:, col]
+            else:
+                values = values[:, col] - ref_value[col]
 
         if self.absolute:
             values = np.abs(values)

testsuite/MDAnalysisTests/core/test_atomselections.py

@@ -1092,6 +1092,15 @@ def test_invalid_relprop_selection(self, universe):
             universe.select_atoms("relprop parsnip < 2 index 0")
         with pytest.raises(SelectionError, match="Unknown selection token"):
             universe.select_atoms("relprop z < 2")
+        with pytest.raises(SelectionError, match="Expected one of x y z for property"):
+            universe.select_atoms("relprop id < 2 index 0")
+        empty_universe = mda.Universe.empty(
+            6, 2, atom_resindex=[0, 0, 0, 1, 1, 1]
+        )
+        with pytest.raises(
+            SelectionError, match="This Universe does not contain"
+        ):
+            empty_universe.select_atoms("relprop z <= 1 index 0")
 
 
 def test_segid_and_resid():