Implement union simplification.

ppp_libmodule/simplification.py

+import itertools
+
+from ppp_datamodel.nodes import Resource
+from ppp_datamodel.nodes.list_operators import *
+
+__all__ = ['simplify']
+
+def simplify_union(tree):
+    if len(tree.list) == 0:
+        return List([])
+    elif len(tree.list) == 1:
+        return tree.list[0]
+    list_ = [x for x in tree.list if x]
+    lists = (x.list for x in list_ if isinstance(x, List))
+    not_lists = [x for x in list_ if not isinstance(x, List)]
+    lists = list(set(itertools.chain(*lists)))
+    if not_lists:
+        all_ = not_lists
+        all_.append(List(lists))
+        return Union(all_)
+    else:
+        return List(lists)
+
+
+predicates = {
+        Union: simplify_union
+        }
+
+def predicate(tree):
+    for (cls, f) in predicates.items():
+        if isinstance(tree, cls):
+            return f(tree)
+    return tree
+
+def simplify(tree):
+    return tree.traverse(predicate)

setup.py

@@ -24,7 +24,7 @@ setup(
         'Topic :: Software Development :: Libraries',
     ],
     install_requires=[
-        'ppp_datamodel>=0.5.2,<0.6',
+        'ppp_datamodel>=0.5.22,<0.6',
     ],
     packages=[
         'ppp_libmodule',

tests/test_simplification.py

+import unittest
+
+from ppp_datamodel.nodes import Triple as T
+from ppp_datamodel.nodes import Missing as M
+from ppp_datamodel.nodes import Resource as R
+from ppp_datamodel.nodes.list_operators import *
+from ppp_libmodule.simplification import simplify
+
+
+class SimplificationTestCase(unittest.TestCase):
+    def testUnionTrivial(self):
+        self.assertEqual(simplify(Union([])), List([]))
+        self.assertEqual(simplify(Union([List([R('a')])])),
+                List([R('a')]))
+    def testUnionResourceLists(self):
+        t = Union([List([R('a'), R('b')]), List([R('c')])])
+        t = simplify(t)
+        self.assertIsInstance(t, List)
+        self.assertEqual(set(t.list), {R('a'), R('b'), R('c')})
+    def testUnionMixed(self):
+        t = Union([List([R('a'), R('b')]), T(M(), M(), M())])
+        t = simplify(t)
+        self.assertIsInstance(t, Union)
+        self.assertEqual(t.list[0], T(M(), M(), M()))
+        self.assertIsInstance(t.list[1], List)
+        self.assertEqual(set(t.list[1].list), {R('a'), R('b')})