_yaml: Introduce 'get_sequence()' and 'sequence_at()'/'mapping_at()'

- Adding 'get_sequence' on MappingNode to access sequences in a mapping
- Adding 'sequence_at' on SequenceNode to access sequences in a sequence
- Adding 'mapping_at' on SequenceNode to access mappings in a sequence

Using "*_at" in sequences allows us to quickly understand if we are
dealing with a sequence or a mapping.

1 files changed, 5 insertions, 8 deletions

diff --git a/src/buildstream/buildelement.py b/src/buildstream/buildelement.py
index 158f5fc11..48be1b83a 100644
--- a/src/buildstream/buildelement.py
+++ b/src/buildstream/buildelement.py
@@ -281,14 +281,11 @@ class BuildElement(Element):
     #                   Private Local Methods                   #
     #############################################################
     def __get_commands(self, node, name):
-        list_node = self.node_get_member(node, list, name, [])
-        commands = []
-
-        for i in range(len(list_node)):
-            command = self.node_subst_list_element(node, name, [i])
-            commands.append(command)
-
-        return commands
+        raw_commands = node.get_sequence(name, []).as_str_list()
+        return [
+            self.substitute_variables(command)
+            for command in raw_commands
+        ]
 
     def __run_command(self, sandbox, cmd):
         # Note the -e switch to 'sh' means to exit with an error