added test for the `to_dict()` method

pytest.ini

@@ -1,2 +1,2 @@
 [pytest]
-python_files = *.py
+python_files = *.py

split_uniques.py

@@ -0,0 +1,30 @@
+import pandas as pd
+from typing import Iterable, NamedTuple
+
+SplitUniqueKeys = dict[str:str]
+
+
+class SplitUniqueResult(NamedTuple):
+    keys: SplitUniqueKeys
+    data: pd.DataFrame
+
+
+def split(
+    data: pd.DataFrame,
+    columns: str | Iterable[str],
+    *,
+    prevkeys: SplitUniqueKeys = None
+) -> Iterable[SplitUniqueResult]:
+    if isinstance(columns, str):
+        columns = [columns]
+    if prevkeys is None:
+        prevkeys = {}
+    current, *rest = columns
+    for value in data[current].unique():
+        selection = data[current] == value
+        selected = data.loc[selection].copy()
+        keys = prevkeys | {current: value}
+        if rest:
+            yield from split(selected, rest, prevkeys=keys)
+        else:
+            yield SplitUniqueResult(keys, selected)