adding radd dunder method to Volumetric data + test_outputs (#4417)

* adding radd dunder method to Volumetric data + test_outputs

* more robust with warnings

* swapped dos and volumetric, my bad

* fixing indentation

Author: wladerer

src/pymatgen/io/common.py

@@ -129,6 +129,15 @@ def get_axis_grid(self, ind):
     def __add__(self, other):
         return self.linear_add(other, 1.0)
 
+    def __radd__(self, other):
+        if other == 0 or other is None:
+            # sum() calls 0 + self first; we treat 0 as the identity element
+            return self
+        if isinstance(other, self.__class__):
+            return self.__add__(other)
+
+        raise TypeError(f"Unsupported operand type(s) for +: '{type(other).__name__}' and '{type(self).__name__}'")
+
     def __sub__(self, other):
         return self.linear_add(other, -1.0)
 

tests/io/vasp/test_outputs.py

@@ -1585,6 +1585,9 @@ def test_init(self):
         chgcar = self.chgcar_spin + self.chgcar_spin
         assert chgcar.get_integrated_diff(0, 1)[0, 1] == approx(-0.0043896932237534022 * 2)
 
+        chgcar = sum([self.chgcar_spin, self.chgcar_spin])
+        assert chgcar.get_integrated_diff(0, 1)[0, 1] == approx(-0.0043896932237534022 * 2)
+
         chgcar = self.chgcar_spin - self.chgcar_spin
         assert chgcar.get_integrated_diff(0, 1)[0, 1] == approx(0)