Fix mypy errors at bfs_shortest_path algo

graphs/breadth_first_search_shortest_path.py

@@ -3,6 +3,8 @@
 """
 from __future__ import annotations
 
+from typing import Optional
+
 graph = {
     "A": ["B", "C", "E"],
     "B": ["A", "D", "E"],
@@ -15,17 +17,19 @@
 
 
 class Graph:
-    def __init__(self, graph: dict[str, str], source_vertex: str) -> None:
-        """Graph is implemented as dictionary of adjacency lists. Also,
+    def __init__(self, graph: dict[str, list[str]], source_vertex: str) -> None:
+        """
+        Graph is implemented as dictionary of adjacency lists. Also,
         Source vertex have to be defined upon initialization.
         """
         self.graph = graph
         # mapping node to its parent in resulting breadth first tree
-        self.parent = {}
+        self.parent: dict[str, Optional[str]] = {}
         self.source_vertex = source_vertex
 
     def breath_first_search(self) -> None:
-        """This function is a helper for running breath first search on this graph.
+        """
+        This function is a helper for running breath first search on this graph.
         >>> g = Graph(graph, "G")
         >>> g.breath_first_search()
         >>> g.parent
@@ -44,7 +48,8 @@ def breath_first_search(self) -> None:
                     queue.append(adjacent_vertex)
 
     def shortest_path(self, target_vertex: str) -> str:
-        """This shortest path function returns a string, describing the result:
+        """
+        This shortest path function returns a string, describing the result:
         1.) No path is found. The string is a human readable message to indicate this.
         2.) The shortest path is found. The string is in the form
             `v1(->v2->v3->...->vn)`, where v1 is the source vertex and vn is the target
@@ -64,17 +69,16 @@ def shortest_path(self, target_vertex: str) -> str:
         'G'
         """
         if target_vertex == self.source_vertex:
-            return f"{self.source_vertex}"
-        elif not self.parent.get(target_vertex):
+            return self.source_vertex
+
+        target_vertex_parent = self.parent.get(target_vertex)
+        if target_vertex_parent is None:
             return f"No path from vertex:{self.source_vertex} to vertex:{target_vertex}"
-        else:
-            return self.shortest_path(self.parent[target_vertex]) + f"->{target_vertex}"
 
+        return self.shortest_path(target_vertex_parent) + f"->{target_vertex}"
 
-if __name__ == "__main__":
-    import doctest
 
-    doctest.testmod()
+if __name__ == "__main__":
     g = Graph(graph, "G")
     g.breath_first_search()
     print(g.shortest_path("D"))