Dodgy implementation of get item.

sasdata/trend.py

@@ -3,22 +3,41 @@
 from dataclasses import dataclass
 from typing import Self
 from sasdata.data import SasData
+from sasdata.data_backing import Dataset, Group
 
 # Axis strs refer to the name of their associated NamedQuantity.
 
+# TODO: This probably shouldn't be here but will keep it here for now.
+# TODO: Not sure how to type hint the return.
+def get_metadatum_from_path(data: SasData, metadata_path: list[str]):
+    current_group = data._raw_metadata
+    for path_item in metadata_path:
+        current_item = current_group.children.get(path_item, None)
+        if current_item is None or (isinstance(current_item, Dataset) and path_item != metadata_path[-1]):
+            raise ValueError('Path does not lead to valid a metadatum.')
+        elif isinstance(current_item, Group):
+            current_group = current_item
+        else:
+            return current_item.data
+    raise ValueError('End of path without finding a dataset.')
+
+
 @dataclass
 class Trend:
     data: list[SasData]
     # This is going to be a path to a specific metadatum.
     #
     # TODO: But what if the trend axis will be a particular NamedQuantity? Will probably need to think on this.
-    trend_axis: str
+    trend_axis: list[str]
 
     # Designed to take in a particular value of the trend axis, and return the SasData object that matches it.
     # TODO: Not exaclty sure what item's type will be. It could depend on where it is pointing to.
     def __getitem__(self, item) -> SasData:
-        raise NotImplementedError()
-
+        for datum in self.data:
+            metadatum = get_metadatum_from_path(datum, self.trend_axis)
+            if metadatum == item:
+                return datum
+        raise KeyError()
     @property
     def trend_axes(self) -> list[float]:
         raise NotImplementedError()