Cleanup combine_vars

* Dedupe combine_vars() code (removed from VariableManager)
* Fix merge_hash algorithm to preserve the type
* unittest combine_vars and merge_hash

lib/ansible/utils/vars.py

@@ -20,39 +20,64 @@ from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
 import ast
+from collections import MutableMapping
 
-from six import string_types
+from six import iteritems, string_types
 
 from ansible import constants as C
+from ansible.errors import AnsibleError
 from ansible.parsing.splitter import parse_kv
 from ansible.utils.unicode import to_unicode
 
+def _validate_mutable_mappings(a, b):
+    """
+    Internal convenience function to ensure arguments are MutableMappings
+
+    This checks that all arguments are MutableMappings or raises an error
+
+    :raises AnsibleError: if one of the arguments is not a MutableMapping
+    """
+
+    # If this becomes generally needed, change the signature to operate on
+    # a variable number of arguments instead.
+
+    if not (isinstance(a, MutableMapping) and isinstance(b, MutableMapping)):
+        raise AnsibleError("failed to combine variables, expected dicts but"
+                " got a '{0}' and a '{1}'".format(
+                    a.__class__.__name__, b.__class__.__name__))
+
 def combine_vars(a, b):
+    """
+    Return a copy of dictionaries of variables based on configured hash behavior
+    """
 
     if C.DEFAULT_HASH_BEHAVIOUR == "merge":
         return merge_hash(a, b)
     else:
+        # HASH_BEHAVIOUR == 'replace'
+        _validate_mutable_mappings(a, b)
         result = a.copy()
         result.update(b)
         return result
 
 def merge_hash(a, b):
-    ''' recursively merges hash b into a
-    keys from b take precedence over keys from a '''
+    """
+    Recursively merges hash b into a so that keys from b take precedence over keys from a
+    """
 
-    result = {}
+    _validate_mutable_mappings(a, b)
+    result = a.copy()
 
-    for dicts in a, b:
-        # next, iterate over b keys and values
-        for k, v in dicts.iteritems():
-            # if there's already such key in a
-            # and that key contains dict
-            if k in result and isinstance(result[k], dict):
-                # merge those dicts recursively
-                result[k] = merge_hash(a[k], v)
-            else:
-                # otherwise, just copy a value from b to a
-                result[k] = v
+    # next, iterate over b keys and values
+    for k, v in iteritems(b):
+        # if there's already such key in a
+        # and that key contains a MutableMapping
+        if k in result and isinstance(result[k], MutableMapping):
+            # merge those dicts recursively
+            result[k] = merge_hash(result[k], v)
+        else:
+            # otherwise, just copy the value from b to a
+            result[k] = v
 
     return result