Fixing docs for nx.info(), along with necessary tests (networkx#3893)

* documenting the behaviour correctly

* adding tests to lock the behaviour

* PEP

* one-line docstring

* pep

networkx/classes/function.py

@@ -525,14 +525,28 @@ def create_empty_copy(G, with_data=True):
 
 
 def info(G, n=None):
-    """Print short summary of information for the graph G or the node n.
+    """Return a summary of information for the graph G or a single node n.
+ 
+    The summary includes the number of nodes and edges (or neighbours for a single
+    node), and their average degree.
 
     Parameters
     ----------
     G : Networkx graph
        A graph
     n : node (any hashable)
        A node in the graph G
+
+    Returns
+    -------
+    info : str
+        A string containing the short summary
+
+    Raises
+    ------
+    NetworkXError
+        If n is not in the graph G
+
     """
     info = ''  # append this all to a string
     if n is None:

networkx/classes/tests/test_function.py

@@ -243,12 +243,16 @@ def test_info(self):
         assert info == expected_graph_info
 
         info = nx.info(G, n=1)
+        assert type(info) == str
         expected_node_info = '\n'.join(
             ['Node 1 has the following properties:',
              'Degree: 2',
              'Neighbors: 0 2'])
         assert info == expected_node_info
 
+        # must raise an error for a non-existent node
+        pytest.raises(nx.NetworkXError, nx.info, G, 1248)
+
     def test_info_digraph(self):
         G = nx.DiGraph(name='path_graph(5)')
         nx.add_path(G, [0, 1, 2, 3, 4])