Add doctest to topological_sort

sorts/topological_sort.py

@@ -1,4 +1,11 @@
-"""Topological Sort."""
+"""Topological Sort.
+
+https://en.wikipedia.org/wiki/Topological_sorting
+https://en.wikipedia.org/wiki/Directed_acyclic_graph
+
+Note: topological_sort() sorts a directed acyclic graph so topological_sort(2, 1, 3)
+    should fail.
+"""
 
 #     a
 #    / \
@@ -16,7 +23,26 @@
 
 
 def topological_sort(start: str, visited: list[str], sort: list[str]) -> list[str]:
-    """Perform topological sort on a directed acyclic graph."""
+    """
+    Perform topological sort on a directed acyclic graph.
+
+    >>> topological_sort('a', [], [])
+    ['c', 'd', 'e', 'b', 'a']
+
+    >>> topological_sort("a", "b", "c")
+    Traceback (most recent call last):
+        ...
+    ValueError: visited must be a list
+
+    >>> topological_sort("a", [], "c")
+    Traceback (most recent call last):
+        ...
+    ValueError: sort must be a list
+    """
+    if not isinstance(visited, list):
+        raise ValueError("visited must be a list")
+    if not isinstance(sort, list):
+        raise ValueError("sort must be a list")
     current = start
     # add current to visited
     visited.append(current)