Initial data value sketch.

configurator/data.py

@@ -0,0 +1,7 @@
+class DataValue:
+
+    def __init__(self, value):
+        self.value = value
+
+    def get(self):
+        return self.value

configurator/node.py

@@ -1,5 +1,6 @@
 from pprint import pformat
 
+from .data import DataValue
 from .path import parse_text, NotPresent
 
 
@@ -23,6 +24,8 @@ def __init__(self, data=None, container=None, accessor=None):
         self._accessor = accessor
 
     def _wrap(self, accessor, value):
+        if isinstance(value, DataValue):
+            value = value.get()
         if isinstance(value, (dict, list)):
             value = ConfigNode(value, self.data, accessor)
         return value

tests/test_functional.py

@@ -2,6 +2,7 @@
 
 from testfixtures import compare
 from configurator import Config
+from configurator.data import DataValue
 from configurator.mapping import target, convert
 import pytest
 
@@ -87,6 +88,36 @@ def test_overlay(self, dir):
         compare(config.user, expected=2)
         compare(config.file, expected=3)
 
+    def test_lazy_load(self, dir):
+        yaml = pytest.importorskip("yaml")
+
+        class LazyVault(DataValue):
+            def __init__(self, key):
+                self.key = key
+
+            @classmethod
+            def from_yaml(cls, loader, node):
+                return cls(loader.construct_mapping(node)['key'])
+
+            def get(self):
+                assert self.key == 'someuser'
+                return {'name': 'Some User', 'password': '...'}
+
+        class Loader(yaml.Loader):
+            pass
+
+        Loader.add_constructor('!from_vault', LazyVault.from_yaml)
+
+        path = dir.write('lazy.yml', '''
+            users:
+            - !from_vault {key: someuser}
+        ''')
+        config = Config.from_path(path, parser=lambda p: yaml.load(p, Loader))
+        compare(config.users[0].data, expected={'name': 'Some User', 'password': '...'})
+
+        # make sure clone still works:
+        config.clone()
+        compare(config.users.data[0], expected=LazyVault('someuser'))
 
 def test_fake_fs(fs):
     fs.create_file('/foo/bar.yml', contents='foo: 1\n')