Do not lose track of dtypes in get_array_ragged

pyiron_base/generic/flattenedstorage.py

@@ -364,8 +364,12 @@ def get_array_ragged(self, name: str) -> np.ndarray:
         """
         if name in self._per_chunk_arrays:
             return self.get_array(name)
-        return np.array([self.get_array(name, i) for i in range(len(self))],
-                        dtype=object)
+        # pre-allocated as dtype=object, then setting individual elements makes sure that element arrays retain their
+        # dtype
+        result = np.empty(len(self), dtype=object)
+        for i in range(len(self)):
+            result[i] = self.get_array(name, i)
+        return result
 
     def get_array_filled(self, name: str) -> np.ndarray:
         """

tests/generic/test_flattenedstorage.py

@@ -238,6 +238,20 @@ def test_get_array_ragged(self):
                            store.get_array_ragged("chunk"),
                         ), "get_array_ragged does not give same result as get_array for per chunk array")
 
+    def test_get_array_ragged_dtype_stability(self):
+        """get_array_ragged should (only!) convert top-most dimension to dtype=object and be of shape (n,) """
+        # regression test
+        store = FlattenedStorage(elem=[ [1, 2], [3, 4], [5, 6] ])
+        ragged = store.get_array_ragged("elem")
+        self.assertEqual(ragged.dtype, np.dtype("O"),
+                         "Top most dtype not object!")
+        self.assertEqual(len(ragged.shape), 1,
+                         "Shape not (n,)!")
+        for array in store._per_element_arrays:
+            for a in ragged:
+                self.assertEqual(a.dtype, store._per_element_arrays[array].dtype,
+                                 "Nested array returned from get_array_ragged has wrong dtype!")
+
     def test_has_array(self):
         """hasarray should return correct information for added array; None otherwise."""