[tune] Support Configuration Merging (#3584)

* merge configs

* deep merge

* lint

* add resolve

* test

python/ray/tune/util.py

@@ -3,6 +3,7 @@ from __future__ import division
 from __future__ import print_function
 
 import base64
+import copy
 import numpy as np
 
 import ray
@@ -35,6 +36,41 @@ def get_pinned_object(pinned_id):
             ObjectID(base64.b64decode(pinned_id[len(PINNED_OBJECT_PREFIX):]))))
 
 
+def merge_dicts(d1, d2):
+    """Returns a new dict that is d1 and d2 deep merged."""
+    merged = copy.deepcopy(d1)
+    deep_update(merged, d2, True, [])
+    return merged
+
+
+def deep_update(original, new_dict, new_keys_allowed, whitelist):
+    """Updates original dict with values from new_dict recursively.
+    If new key is introduced in new_dict, then if new_keys_allowed is not
+    True, an error will be thrown. Further, for sub-dicts, if the key is
+    in the whitelist, then new subkeys can be introduced.
+
+    Args:
+        original (dict): Dictionary with default values.
+        new_dict (dict): Dictionary with values to be updated
+        new_keys_allowed (bool): Whether new keys are allowed.
+        whitelist (list): List of keys that correspond to dict values
+            where new subkeys can be introduced. This is only at
+            the top level.
+    """
+    for k, value in new_dict.items():
+        if k not in original:
+            if not new_keys_allowed:
+                raise Exception("Unknown config parameter `{}` ".format(k))
+        if type(original.get(k)) is dict:
+            if k in whitelist:
+                deep_update(original[k], value, True, [])
+            else:
+                deep_update(original[k], value, new_keys_allowed, [])
+        else:
+            original[k] = value
+    return original
+
+
 def _to_pinnable(obj):
     """Converts obj to a form that can be pinned in object store memory.