Make validation easier to read.

src/lcm/grids.py

@@ -46,19 +46,10 @@ def __init__(self, category_class: type) -> None:
                 values.
 
         """
-        if not is_dataclass(category_class):
-            raise GridInitializationError(
-                "category_class must be a dataclass with scalar int or float fields, "
-                f"but is {category_class}."
-            )
+        _validate_discrete_grid(category_class)
 
         names_and_values = _get_field_names_and_values(category_class)
 
-        errors = _validate_discrete_grid(names_and_values)
-        if errors:
-            msg = format_messages(errors)
-            raise GridInitializationError(msg)
-
         self.__categories = list(names_and_values.keys())
         self.__codes = list(names_and_values.values())
 
@@ -159,17 +150,21 @@ def get_coordinate(self, value: Scalar) -> Scalar:
 # ======================================================================================
 
 
-def _validate_discrete_grid(names_and_values: dict[str, Any]) -> list[str]:
+def _validate_discrete_grid(category_class: type) -> None:
     """Validate the field names and values of the category_class passed to DiscreteGrid.
 
     Args:
-        names_and_values: A dictionary with the field names as keys and the field
-            values as values.
-
-    Returns:
-        list[str]: A list of error messages.
+        category_class: The class with mappings of names to codes.
 
     """
+    if not is_dataclass(category_class):
+        raise GridInitializationError(
+            "category_class must be a dataclass with scalar int or float fields, "
+            f"but is {category_class}."
+        )
+
+    names_and_values = _get_field_names_and_values(category_class)
+
     error_messages = []
 
     if not len(names_and_values) > 0:
@@ -198,7 +193,9 @@ def _validate_discrete_grid(names_and_values: dict[str, Any]) -> list[str]:
             f"{set(duplicated_values)}"
         )
 
-    return error_messages
+    if error_messages:
+        msg = format_messages(error_messages)
+        raise GridInitializationError(msg)
 
 
 def _get_field_names_and_values(dc: type) -> dict[str, Any]: