Implement list_concat function

This function uses python's extend function for lists in order to
concatenate lists. So, given the input: [['v1', 'v2'], ['v3', 'v4']]
the result would be ['v1', 'v2', 'v3', 'v4'].

This comes as an alternative to using yaql's concat.

Change-Id: I082833e73388540b29f71b78293fd36fe04b7b92

doc/source/template_guide/hot_spec.rst

@@ -304,6 +304,7 @@ for the ``heat_template_version`` key:
       get_resource
       list_join
       make_url
+      list_concat
       map_merge
       map_replace
       repeat
@@ -1893,3 +1894,28 @@ For example
 ``server_url`` will be evaluated to a URL in the form::
 
     http://[<server IP>]:8080/hello?recipient=world#greeting
+
+list_concat
+-----------
+
+The ``list_concat`` function concatenates lists together.
+
+The syntax of the ``list_concat`` function is
+
+.. code-block:: yaml
+
+    list_concat:
+      - <list #1>
+      - <list #2>
+      - ...
+
+
+For example
+
+.. code-block:: yaml
+
+    list_concat: [['v1', 'v2'], ['v3', 'v4']]
+
+Will resolve to the list ``['v1', 'v2', 'v3', 'v4']``.
+
+Null values will be ignored.

heat/engine/hot/functions.py

@@ -1436,3 +1436,47 @@ class MakeURL(function.Function):
 
         return urlparse.urlunsplit((scheme, ''.join(netloc()),
                                     path, query, fragment))
+
+
+class ListConcat(function.Function):
+    """A function for extending lists.
+
+    Takes the form::
+
+        list_concat:
+          - [<value 1>, <value 2>]
+          - [<value 3>, <value 4>]
+
+    And resolves to::
+
+        [<value 1>, <value 2>, <value 3>, <value 4>]
+
+    """
+
+    def __init__(self, stack, fn_name, args):
+        super(ListConcat, self).__init__(stack, fn_name, args)
+        example = (_('"%s" : [ [ <value 1>, <value 2> ], '
+                     '[ <value 3>, <value 4> ] ]')
+                   % fn_name)
+        self.fmt_data = {'fn_name': fn_name, 'example': example}
+
+    def result(self):
+        args = function.resolve(self.args)
+
+        if not isinstance(args, collections.Sequence):
+            raise TypeError(_('Incorrect arguments to "%(fn_name)s" '
+                              'should be: %(example)s') % self.fmt_data)
+
+        def ensure_list(m):
+            if m is None:
+                return []
+            elif isinstance(m, collections.Sequence):
+                return m
+            else:
+                msg = _('Incorrect arguments: Items to concat must be lists.')
+                raise TypeError(msg)
+
+        ret_list = []
+        for m in args:
+            ret_list.extend(ensure_list(m))
+        return ret_list

heat/engine/hot/template.py

@@ -590,6 +590,7 @@ class HOTemplate20170901(HOTemplate20170224):
 
         # functions added in 2017-09-01
         'make_url': hot_funcs.MakeURL,
+        'list_concat': hot_funcs.ListConcat,
 
         # functions removed from 2015-10-15
         'Fn::Select': hot_funcs.Removed,

heat/tests/test_hot.py

@@ -2060,6 +2060,26 @@ conditions:
         stack.validate()
         self.assertEqual({'one', 'three'}, set(stack.resources))
 
+    def test_list_concat(self):
+        snippet = {'list_concat': [['v1', 'v2'], ['v3', 'v4']]}
+        snippet_resolved = ['v1', 'v2', 'v3', 'v4']
+        tmpl = template.Template(hot_pike_tpl_empty)
+        resolved = self.resolve(snippet, tmpl)
+        self.assertEqual(snippet_resolved, resolved)
+
+    def test_list_concat_none(self):
+        snippet = {'list_concat': [['v1', 'v2'], ['v3', 'v4'], None]}
+        snippet_resolved = ['v1', 'v2', 'v3', 'v4']
+        tmpl = template.Template(hot_pike_tpl_empty)
+        resolved = self.resolve(snippet, tmpl)
+        self.assertEqual(snippet_resolved, resolved)
+
+    def test_list_concat_invalid(self):
+        snippet = {'list_concat': [{'k1': 'v2'}, ['v3', 'v4']]}
+        tmpl = template.Template(hot_pike_tpl_empty)
+        exc = self.assertRaises(TypeError, self.resolve, snippet, tmpl)
+        self.assertIn('Incorrect arguments', six.text_type(exc))
+
 
 class HotStackTest(common.HeatTestCase):
     """Test stack function when stack was created from HOT template."""

releasenotes/notes/add-list_concat-function-c28563ab8fb6362e.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - The list_concat function was added, which concats several lists using
+    python's extend function.