Add an inverse function to flatten_mapping

`deepen_mapping` recostruct a deep mapping from a flat one given a
separator character and reference mapping to reconstruct paths.

Part of the ground work for
freeipa#26

ipadocker/config.py

@@ -223,6 +223,44 @@ def flatten_mapping(mapping, separator='_', path=()):
     return flat_mapping
 
 
+def deepen_mapping(mapping, separator='_',
+                   reference=None, path=()):
+    """
+    re-create a nested mapping from a flat representation created by
+    @flatten_mapping. A reference dictionary is needed to unambiguously
+    construct all paths
+
+    :param mapping: flat mapping to transform
+    :param separator: separator that was used to concatenate keys in the
+        original nested mapping
+    :param path: stack holding keys of previous nesting levels
+    """
+    if reference is None:
+        reference = constants.DEFAULT_CONFIG
+
+    deep_mapping = {}
+
+    key_prefix = ''
+
+    if path:
+        key_prefix = separator.join(path)
+
+    for key, value in reference.items():
+        path_to_set = path + (key,)
+        if isinstance(value, dict):
+            deep_mapping[key] = deepen_mapping(
+                mapping,
+                separator=separator,
+                reference=value,
+                path=path_to_set)
+        else:
+            flat_key = separator.join([key_prefix, key]) if key_prefix else key
+            if flat_key in mapping:
+                deep_mapping[key] = mapping[flat_key]
+
+    return deep_mapping
+
+
 class IPADockerConfig(object):
     """
     An object which encapsulates the merged default options and options

tests/test_config.py

@@ -219,3 +219,14 @@ def test_flattened_mapping():
     """
     flat_mapping = config.flatten_mapping(NESTED_MAPPING)
     assert flat_mapping == FLAT_MAPPING
+
+
+def test_deepened_mapping():
+    """
+    Tests that the `deepen_mapping` function does its job
+    """
+
+    nested_mapping = config.deepen_mapping(FLAT_MAPPING,
+                                           reference=NESTED_MAPPING)
+
+    assert nested_mapping == NESTED_MAPPING