Add a warning whenever a _mesh is different for a field when added to a FieldSet, or during Fieldset creation (#2761)

* added a warning whenever added fields use different meshes

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Added mesh warning to add_constant_field and revised test suite to only combine/create FieldSets with the same underlying mesh

---------

Co-authored-by: Wyatt Sieminski <wyatt_sieminski@brown.edu>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

src/parcels/_core/fieldset.py

@@ -2,6 +2,7 @@
 
 import functools
 import sys
+import warnings
 from collections.abc import Iterable
 from typing import IO, TYPE_CHECKING
 
@@ -21,6 +22,7 @@
 from parcels._core.utils.string import _assert_str_and_python_varname
 from parcels._core.utils.time import get_datetime_type_calendar
 from parcels._core.utils.time import is_compatible as datetime_is_compatible
+from parcels._core.warnings import FieldSetWarning
 from parcels._python import NOTSET, NotSetType
 from parcels._reprs import fieldset_describe
 from parcels.interpolators import (
@@ -73,6 +75,7 @@ def __init__(self, models: list[ModelData]):
         self._fields: dict[str, Field | VectorField] | None = None
         self.reconstruct_fields()
         self.context: dict[str, float] = {}
+        _warn_if_fields_use_different_meshes(self.fields.values())
 
     def __setattr__(self, name, value):
         """Set field attribute by name. If context exists and name in context, raise error to prevent overwriting context variable."""
@@ -151,6 +154,7 @@ def add_field(self, field: Field, name: str | None = None):
             raise ValueError(f"FieldSet already has a Field with name '{name}'")
 
         self.fields[name] = field
+        _warn_if_fields_use_different_meshes(self.fields.values())
 
     def to_windowed_arrays(self, *, max_levels: int | None = None):
         """Wrap dask-backed field data in rolling time-window caches.
@@ -215,6 +219,7 @@ def add_constant_field(self, name: str, value, mesh: ptyping.Mesh = "spherical")
         self.reconstruct_fields()
         field = getattr(self, name)
         field.interp_method = XConstantField()
+        _warn_if_fields_use_different_meshes(self.fields.values())
 
     def add_context(self, name, value):
         """Add context variable to the FieldSet.
@@ -362,6 +367,28 @@ def assert_compatible_fieldsets(left: FieldSet, right: FieldSet) -> None:
         )
 
 
+def _warn_if_fields_use_different_meshes(fields: Iterable[Field | VectorField]):
+    """Warn if multiple fields use different meshes on the underlying grids.
+
+    Parameters
+    ----------
+    fields : Iterable[Field | VectorField]
+        The fields to check for conflicting meshes.
+
+    Warns
+    -----
+    FieldSetWarning
+        If the fields have different meshes on the underlying grids.
+    """
+    meshes = {field.grid._mesh for field in fields}
+    if len(meshes) > 1:
+        warnings.warn(
+            f"FieldSet has multiple different meshes: {meshes}. This may lead to unexpected behavior during execution.",
+            category=FieldSetWarning,
+            stacklevel=3,
+        )
+
+
 class CalendarError(Exception):  # TODO: Move to a parcels errors module
     """Exception raised when the calendar of a field is not compatible with the rest of the Fields. The user should ensure that they only add fields to a FieldSet that have compatible CFtime calendars."""
 

tests/test_fieldset.py

@@ -32,7 +32,7 @@ def fieldset_two_models():
     fset2 = FieldSet.from_sgrid_conventions(ds2, mesh="flat", vector_fields={"UV_wind": ("U_wind", "V_wind")})
     fset2.add_context("my_value", 2.0)
     fset2.add_context("my_list", [1, 2, "hello"])
-    fset2.add_constant_field("constant_field", 3.0)
+    fset2.add_constant_field("constant_field", 3.0, mesh="flat")
     return fset1 + fset2
 
 
@@ -69,7 +69,7 @@ def test_fieldset_add_context_invalid_name(fieldset, name):
 
 
 def test_fieldset_add_constant_field(fieldset):
-    fieldset.add_constant_field("test_constant_field", 1.0)
+    fieldset.add_constant_field("test_constant_field", 1.0, mesh="flat")
 
     # Get a point in the domain
     time = ds["time"].mean()
@@ -86,7 +86,7 @@ def test_fieldset_gridset(fieldset):
     assert fieldset.fields["UV"].grid in fieldset.gridset
     assert len(fieldset.gridset) == 1
 
-    fieldset.add_constant_field("constant_field", 1.0)
+    fieldset.add_constant_field("constant_field", 1.0, mesh="flat")
     assert len(fieldset.gridset) == 2
 
 
@@ -233,7 +233,7 @@ def test_fieldset_time_interval():
     field2 = Field("field2", ds2["U_A_grid"], grid2, interp_method=XLinear)
 
     fieldset = FieldSet([field1, field2])
-    fieldset.add_constant_field("constant_field", 1.0)
+    fieldset.add_constant_field("constant_field", 1.0, mesh="flat")
 
     assert fieldset.time_interval.left == np.datetime64("2000-01-02")
     assert fieldset.time_interval.right == np.datetime64("2001-01-01")