improved the code

practice/etl/etl.py

@@ -1,6 +1,28 @@
-def transform(old):
-    new = {}
-    for point, letters in old.items():
+def transform(old_dict):
+    """
+    Transforms the input dictionary by swapping keys with values.
+
+    The function iterates over each key-value pair in the input dictionary.
+    Each value is expected to be a list. For each element in the list, a new
+    key-value pair is added to the output dictionary, where the key is the
+    element (converted to lowercase) and the value is the original key from
+    the input dictionary.
+
+    Args:
+        old_dict (dict): A dictionary to be transformed. Each value should be
+            a list.
+
+    Returns:
+        dict: The transformed dictionary.
+
+    Raises:
+        TypeError: If a value in the input dictionary is not a list.
+    """
+    new_dict = {}
+    for point, letters in old_dict.items():
+        if not isinstance(letters, list):
+            raise TypeError('Each value in the input dictionary should be a list.')
         for letter in letters:
-            new[letter.lower()] = point
-    return new
+            # Convert the letter to lowercase and add it to the new dictionary
+            new_dict[letter.lower()] = point
+    return new_dict