Fixed 'is this node a leaf?' issues.

graph.py

 # vim: set fileencoding=utf-8 :
 # XXX Used in doctests (we should use them in the __main__ section below too).
-from node import Leaf, Node
-
-
 def generate_graph(root, separator=' ', verbose=False):
     """
     Return a text-based, utf-8 graph of a tree-like structure. Each tree node
@@ -10,6 +7,7 @@ def generate_graph(root, separator=' ', verbose=False):
     ``title``, that attribute will be called. That way, the node can return a
     specific title, otherwise ``+`` is used.
 
+    >>> from node import Leaf, Node
     >>> l0, l1 = Leaf(0), Leaf(1)
     >>> n0 = Node('+', l0, l1)
     >>> l2 = Leaf(2)
@@ -42,7 +40,7 @@ def generate_graph(root, separator=' ', verbose=False):
 
         # Leaves do not have children and therefore the length of its title is
         # the width of the leaf.
-        if isinstance(node, Leaf):
+        if not node.nodes:
             node_width[node] = title_len
             node_middle[node] = int((title_len - 1) / 2)
             return title_len
@@ -80,7 +78,7 @@ def generate_graph(root, separator=' ', verbose=False):
         return width
 
     def format_lines(node):
-        if isinstance(node, Leaf):
+        if not node.nodes:
             # Leaf titles do not need to be centered, since the parent will
             # center those lines. And if there are no parents, the entire graph
             # consists of a single leaf, so in that case there still is no

line.py

-from node import Leaf
-
 def generate_line(root):
     """
     Print an expression tree in a single text line. Where needed, add
@@ -50,7 +48,7 @@ def generate_line(root):
         """
         Get the associativity of an operator node.
         """
-        if not isinstance(node, Leaf) and len(node) > 1:
+        if not node.nodes and len(node) > 1:
             op = node.title()
 
             for i, group in enumerate(operators):
@@ -65,9 +63,12 @@ def generate_line(root):
         1. Visit the root
         2. Traverse the subtrees in left-to-right order
         """
+        if not node:
+            return '<empty expression>'
+
         s = node.title()
 
-        if isinstance(node, Leaf):
+        if not node.nodes:
             return s
 
         arity = len(node)

tests/test_graph.py

@@ -170,8 +170,8 @@ class TestGraph(unittest.TestCase):
         3 5 3 7
         """)
 
-    def strip(self, str):
-        return str.replace('\n        ', '\n')[1:-1]
+    def strip(self, s):
+        return s.replace('\n        ', '\n')[1:-1]
 
     def assertEqualGraphs(self, g, expect):
         expect = self.strip(expect)