refs #2 Support for referencing child node as attributes on NonTerminal

This works for rules that are referenced from the rule which created
NonTerminal.

arpeggio/__init__.py

@@ -615,6 +615,9 @@ class NonTerminal(ParseTreeNode):
         super(NonTerminal, self).__init__(type, position, error)
         self.nodes = flatten([nodes])
 
+        # Child nodes cache. Used for lookup by rule name.
+        self._child_cache = {}
+
     @property
     def desc(self):
         return self.name
@@ -628,6 +631,26 @@ class NonTerminal(ParseTreeNode):
     def __repr__(self):
         return "[ %s ]" % ", ".join([repr(x) for x in self.nodes])
 
+    def __getattr__(self, item):
+        """
+        Find a child (non)terminal by the rule name.
+
+        Args:
+            item(str): The name of the child node.
+        """
+        # First check the cache
+        if item in self._child_cache:
+            return self._child_cache[item]
+
+        # If not found in the cache find it and store it in the
+        # cache for later.
+        for n in self.nodes:
+            if n.type == item:
+                self._child_cache[item] = n
+                return n
+
+        raise AttributeError
+
 
 # ----------------------------------------------------
 # Semantic Actions

tests/test_rulename_lookup.py

+# -*- coding: utf-8 -*-
+#######################################################################
+# Name: test_peg_parser
+# Purpose: Test for parser constructed using PEG textual grammars.
+# Author: Igor R. Dejanović <igor DOT dejanovic AT gmail DOT com>
+# Copyright: (c) 2014 Igor R. Dejanović <igor DOT dejanovic AT gmail DOT com>
+# License: MIT License
+#######################################################################
+
+from unittest import TestCase
+
+# Grammar
+from arpeggio import ParserPython, ZeroOrMore
+
+
+def foo(): return "a", bar, "b", baz
+def bar(): return "c"
+def baz(): return "d"
+
+
+
+class TestPEGParser(TestCase):
+
+    def test_lookup_single(self):
+
+        parser = ParserPython(foo)
+
+        result = parser.parse("a c b d")
+
+        self.assertTrue(hasattr(result, "bar"))
+        self.assertTrue(hasattr(result, "baz"))
+        self.assertTrue(not hasattr(result, "unexisting"))
+