Improve hybrid level handling

src/earthkit/data/field/component/vertical.py

@@ -356,8 +356,6 @@ class Vertical(VerticalBase):
         metadata such as units and CF attributes.
     """
 
-    _level_parameters: Optional[LevelParameters] = None
-
     def __init__(
         self,
         level: Union[int, float] = None,

src/earthkit/data/field/grib/vertical.py

@@ -27,6 +27,7 @@ class GribHybridLevelParameters(HybridLevelParametersBase):
     def __init__(self, handle):
         self._handle = handle
 
+    # TODO: this method should be cached
     def number_of_levels(self):
         coeff_num = self._handle.get("NV", default=None)
         if coeff_num is not None:
@@ -170,11 +171,10 @@ def to_grib(self, component, handle=None):
 
         # this tries to avoid writing the coefficients back to the handle if they are already
         # present and correct, which can be expensive for large coefficient arrays. The check
-        # is not robust enough and should be improved.
-        # TODO: should be: if handle is not None and hasattr(component, "_level_parameters"): ?
-        if handle is not None and hasattr(component, "level_parameters"):
-            level_parameters = component._level_parameters  # it seems nobody sets it to other than None!
-            if isinstance(level_parameters, GribFieldComponentHandler):
+        # is not robust enough, and should be improved.
+        if handle is not None and hasattr(component, "_coefficients"):
+            level_parameters = component._coefficients
+            if isinstance(level_parameters, GribHybridLevelParameters):
                 nv = handle.get("NV", default=None)
                 if level_parameters.coefficient_size() == nv:
                     return r

tests/grib/test_grib_set_vertical.py

@@ -117,6 +117,16 @@ def test_grib_set_vertical_hybrid_1(fl_type):
     assert len(A) == 138
     assert len(B) == 138
 
+    with temp_file() as tmp:
+        f.to_target("file", tmp)
+        f_saved = from_source("file", tmp).to_fieldlist()[0]
+        assert f_saved.vertical.level() == 2
+        assert f_saved.vertical.level_type() == "hybrid"
+        assert f_saved.vertical.number_of_levels() == 137
+        A1, B1 = f_saved.vertical.coefficients()
+        assert np.allclose(A, A1)
+        assert np.allclose(B, B1)
+
 
 @pytest.mark.parametrize("fl_type", ["file"])
 @pytest.mark.parametrize("coeff_mode", ["object", "tuple"])