Implement first set of review comments.

src/_gettsim/functions/loader.py

@@ -8,14 +8,13 @@
 from typing import Any, Literal, TypeAlias
 
 from _gettsim.config import PATHS_TO_INTERNAL_FUNCTIONS, RESOURCE_DIR
+from _gettsim.functions.policy_function import PolicyFunction
 from _gettsim.gettsim_typing import NestedFunctionDict
 from _gettsim.shared import (
     get_path_from_qualified_name,
     tree_update,
 )
 
-from .policy_function import PolicyFunction
-
 
 def load_functions_tree_for_date(date: datetime.date) -> NestedFunctionDict:
     """
@@ -219,6 +218,8 @@ def _create_policy_function_from_decorated_callable(
     """
 
     # Only needed until the directory structure is cleaned up
+    # TODO(@MImmesberger): Remove the removeprefix calls once the directory
+    # structure is cleaned up
     clean_module_name = (
         module_name.removeprefix("_gettsim__")
         .removeprefix("taxes__")
@@ -269,20 +270,25 @@ def _load_aggregation_dict(
             .removeprefix("transfers__")
         )
         tree_keys = get_path_from_qualified_name(clean_module_name)
-        dicts_in_module = _load_dicts_in_module(path, package_root, f"{variant}_")
-        _fail_if_more_than_one_dict_loaded(dicts_in_module)
-        tree = tree_update(tree, tree_keys, *dicts_in_module)
+        derived_function_specs = load_functions_to_derive(
+            path, package_root, f"{variant}_"
+        )
+        tree = tree_update(tree, tree_keys, *derived_function_specs)
 
     return tree
 
 
-def _load_dicts_in_module(
+def load_functions_to_derive(
     path: Path,
     package_root: Path,
     prefix_filter: str,
 ) -> list[dict]:
     """
-    Load dictionaries defined in a module.
+    Load the dictionary that specifies which functions to derive from the module.
+
+    Returns one aggregation dictionary where keys are the names of the functions to
+    derive and values are dictionaries with the aggregation specifications ('aggr',
+    'source_col', 'p_id_to_aggregate_by').
 
     Parameters
     ----------
@@ -297,17 +303,19 @@ def _load_dicts_in_module(
         Loaded dictionaries.
     """
     module = _load_module(path, package_root)
-
-    return [
+    module_name = _convert_path_to_module_name(path, package_root)
+    dicts_in_module = [
         member
         for name, member in inspect.getmembers(module)
         if isinstance(member, dict) and name.startswith(prefix_filter)
     ]
 
+    _fail_if_more_than_one_dict_loaded(dicts_in_module, module_name)
+    return dicts_in_module
+
 
-def _fail_if_more_than_one_dict_loaded(dicts: list[dict]) -> None:
+def _fail_if_more_than_one_dict_loaded(dicts: list[dict], module_name: str) -> None:
     if len(dicts) > 1:
         raise ValueError(
-            "More than one dictionary found in the module. "
-            "Only one dictionary is allowed."
+            "More than one dictionary found in the module:\n\n" f"{module_name}\n\n"
         )