Initial validation of functions

All template functions contain only 'result' function for resolving
snippet of template. Current patch presents initial implementation of
validate method and support function 'validate' similar to 'resolve'.

Related-Bug: #1273490
Change-Id: Id81493bd72e75da746101c4c741eaf846abd8360

heat/engine/function.py

@@ -36,6 +36,15 @@ class Function(object):
         self.fn_name = fn_name
         self.args = args
 
+    def validate(self):
+        """
+        Validate arguments without resolving the function.
+
+        Function subclasses must override this method to validate their
+        args.
+        """
+        validate(self.args)
+
     @abc.abstractmethod
     def result(self):
         """
@@ -110,3 +119,15 @@ def resolve(snippet):
         return [resolve(v) for v in snippet]
 
     return snippet
+
+
+def validate(snippet):
+    if isinstance(snippet, Function):
+        snippet.validate()
+    elif isinstance(snippet, collections.Mapping):
+        for v in snippet.values():
+            validate(v)
+    elif (not isinstance(snippet, basestring) and
+          isinstance(snippet, collections.Iterable)):
+        for v in snippet:
+            validate(v)

heat/tests/test_function.py

@@ -19,6 +19,10 @@ from heat.engine import function
 
 
 class TestFunction(function.Function):
+    def validate(self):
+        if len(self.args) < 2:
+            raise Exception(_('Need more arguments'))
+
     def result(self):
         return 'wibble'
 
@@ -80,3 +84,42 @@ class ResolveTest(HeatTestCase):
         self.assertEqual(['foo', {'bar': ['baz', {'blarg': 'wibble'}]}],
                          result)
         self.assertIsNot(result, snippet)
+
+
+class ValidateTest(HeatTestCase):
+    def setUp(self):
+        super(ValidateTest, self).setUp()
+        self.func = TestFunction(None, 'foo', ['bar', 'baz'])
+
+    def test_validate_func(self):
+        self.assertIsNone(function.validate(self.func))
+        self.func = TestFunction(None, 'foo', ['bar'])
+        ex = self.assertRaises(Exception, function.validate, self.func)
+        self.assertEqual('Need more arguments', str(ex))
+
+    def test_validate_dict(self):
+        snippet = {'foo': 'bar', 'blarg': self.func}
+        function.validate(snippet)
+
+        self.func = TestFunction(None, 'foo', ['bar'])
+        snippet = {'foo': 'bar', 'blarg': self.func}
+        ex = self.assertRaises(Exception, function.validate, snippet)
+        self.assertEqual('Need more arguments', str(ex))
+
+    def test_validate_list(self):
+        snippet = ['foo', 'bar', 'baz', 'blarg', self.func]
+        function.validate(snippet)
+
+        self.func = TestFunction(None, 'foo', ['bar'])
+        snippet = {'foo': 'bar', 'blarg': self.func}
+        ex = self.assertRaises(Exception, function.validate, snippet)
+        self.assertEqual('Need more arguments', str(ex))
+
+    def test_validate_all(self):
+        snippet = ['foo', {'bar': ['baz', {'blarg': self.func}]}]
+        function.validate(snippet)
+
+        self.func = TestFunction(None, 'foo', ['bar'])
+        snippet = {'foo': 'bar', 'blarg': self.func}
+        ex = self.assertRaises(Exception, function.validate, snippet)
+        self.assertEqual('Need more arguments', str(ex))