Polished and added tests

README.md

@@ -101,11 +101,11 @@ Keep contributing!!
 Thanks to these wonderful people ✨✨:
 
 <table>
-	<tr>
-		<td>
-			<a href="https://github.com/codezonediitj/pydatastructs/graphs/contributors">
-  				<img src="https://contrib.rocks/image?repo=codezonediitj/pydatastructs" />
-			</a>
-		</td>
-	</tr>
+    <tr>
+        <td>
+            <a href="https://github.com/codezonediitj/pydatastructs/graphs/contributors">
+                  <img src="https://contrib.rocks/image?repo=codezonediitj/pydatastructs" />
+            </a>
+        </td>
+    </tr>
 </table>

pydatastructs/graphs/adjacency_list.py

@@ -52,17 +52,18 @@ def remove_vertex(self, name):
                 node_obj.adjacent.remove(name)
 
     def add_edge(self, source, target, cost=None):
-        error_msg = "Vertex %s is not present in the graph. \
-                    Call Graph.add_vertex() to add a new \
-                    vertex. Graph.add_edge is only responsible \
-                    for adding edges and it will not add new \
-                    vertices on its own. This is done to maintain \
-                    clear separation between the functionality of \
-                    the two methods."
+        source, target = str(source), str(target)
+        error_msg = ("Vertex %s is not present in the graph."
+                     "Call Graph.add_vertex to add a new"
+                     "vertex. Graph.add_edge is only responsible"
+                     "for adding edges and it will not add new"
+                     "vertices on its own. This is done to maintain"
+                     "clear separation between the functionality of"
+                     "these two methods.")
         if not hasattr(self, source):
-            raise ValueError(error_msg %(source))
+            raise ValueError(error_msg % (source))
         if not hasattr(self, target):
-            raise ValueError(error_msg %(target))
+            raise ValueError(error_msg % (target))
 
         source, target = self.__getattribute__(source), \
             self.__getattribute__(target)

pydatastructs/graphs/adjacency_matrix.py

@@ -57,6 +57,18 @@ def remove_vertex(self, node):
 
     def add_edge(self, source, target, cost=None):
         source, target = str(source), str(target)
+        error_msg = ("Vertex %s is not present in the graph."
+                     "Call Graph.add_vertex to add a new"
+                     "vertex. Graph.add_edge is only responsible"
+                     "for adding edges and it will not add new"
+                     "vertices on its own. This is done to maintain"
+                     "clear separation between the functionality of"
+                     "these two methods.")
+        if source not in self.matrix:
+            raise ValueError(error_msg % (source))
+        if target not in self.matrix:
+            raise ValueError(error_msg % (target))
+
         self.matrix[source][target] = True
         if cost is not None:
             self.edge_weights[source + "_" + target] = \

pydatastructs/graphs/graph.py

@@ -53,13 +53,11 @@ class Graph(object):
     Note
     ====
 
-    Steps to create a graph:
-
-    1. Create nodes (AdjacencyListGraphNode or AdjacencyListMatrixNode)
-
-    2. Add nodes to the graph
-
-    3. Add edges b/w these nodes
+    Make sure to create nodes (AdjacencyListGraphNode or AdjacencyMatrixGraphNode)
+    and them in your graph using Graph.add_vertex before adding edges whose
+    end points require either of the nodes that you added. In other words,
+    Graph.add_edge doesn't add new nodes on its own if the input
+    nodes are not already present in the Graph.
 
     """
 

pydatastructs/graphs/tests/test_adjacency_list.py

@@ -1,5 +1,6 @@
 from pydatastructs.graphs import Graph
 from pydatastructs.utils import AdjacencyListGraphNode
+from pydatastructs.utils.raises_util import raises
 
 def test_adjacency_list():
     v_1 = AdjacencyListGraphNode('v_1', 1)
@@ -38,3 +39,6 @@ def test_adjacency_list():
     g.remove_vertex('v')
     assert g.is_adjacent('v_2', 'v') is False
     assert g.is_adjacent('v_3', 'v') is False
+
+    assert raises(ValueError, lambda: g.add_edge('u', 'v'))
+    assert raises(ValueError, lambda: g.add_edge('v', 'x'))

pydatastructs/graphs/tests/test_adjacency_matrix.py

@@ -1,5 +1,6 @@
 from pydatastructs.graphs import Graph
 from pydatastructs.utils import AdjacencyMatrixGraphNode
+from pydatastructs.utils.raises_util import raises
 
 def test_AdjacencyMatrix():
     v_0 = AdjacencyMatrixGraphNode(0, 0)
@@ -25,3 +26,6 @@ def test_AdjacencyMatrix():
     assert neighbors == [v_1]
     g.remove_edge(0, 1)
     assert g.is_adjacent(0, 1) is False
+
+    assert raises(ValueError, lambda: g.add_edge('u', 'v'))
+    assert raises(ValueError, lambda: g.add_edge('v', 'x'))