Add option to preserve order of inserts. Only allow OrderedDicts for

this. Make command line always preserve order too.

json_merge_patch/cli.py

@@ -5,15 +5,15 @@
 from collections import OrderedDict
 
 
-def merge(files, output):
+def merge(files, output, position):
     result = None
     for file in files:
         with open(file) as input_file:
             input_json = json.load(input_file, object_pairs_hook=OrderedDict)
         if result is None:
             result = input_json
         else:
-            result = json_merge_patch.merge(result, input_json)
+            result = json_merge_patch.merge(result, input_json, position=position)
 
     merged = json.dumps(result, indent=4)
 
@@ -46,6 +46,7 @@ def main():
       'merge', help='Merge json documents together using JSON merge patch')
 
     parser_merge.add_argument('-o', '--output', help='path of output file, if none specified will print to stdout')
+    parser_merge.add_argument('-f', '--first', action='store_true', help='when merging new properties of object put them first instead of last')
     parser_merge.add_argument('files', help='JSON files to merge in order', nargs='+')
 
     parser_create_patch = subparsers.add_parser(
@@ -57,7 +58,7 @@ def main():
 
     args = parser.parse_args()   
     if args.subparser_name == 'merge':
-        merge(args.files, args.output)
+        merge(args.files, args.output, 'first' if args.first else 'last')
     elif args.subparser_name == 'create-patch':
         create_patch(args.original, args.target, args.output)
     else:

json_merge_patch/lib.py

@@ -1,31 +1,57 @@
+from collections import OrderedDict
+import sys
 
-def merge(*objs):
+def merge(*objs, **kw):
     result = objs[0]
     for obj in objs[1:]:
-        result = merge_obj(result, obj)
+        result = merge_obj(result, obj, kw.get('position'))
     return result
 
-def merge_obj(result, obj):
+def move_to_start(result, key):
+    result_copy = result.copy()
+    result.clear()
+    result[key] = result_copy.pop(key)
+    result.update(result_copy)
+
+def merge_obj(result, obj, position=None):
     if not isinstance(result, dict):
-        result = {}
+        result = OrderedDict() if position else {}
 
     if not isinstance(obj, dict):
         return obj
 
+    if position:
+        if position not in ('first', 'last'):
+            raise ValueError("position can either be first or last")
+        if not isinstance(result, OrderedDict) or not isinstance(obj, OrderedDict):
+            raise ValueError("If using position all dicts need to be OrderedDicts")
+
     for key, value in obj.items():
         if isinstance(value, dict):
             target = result.get(key)
             if isinstance(target, dict):
-                merge_obj(target, value)
+                merge_obj(target, value, position)
                 continue
-            result[key] = {}
-            merge_obj(result[key], value)
+            result[key] = OrderedDict() if position else {}
+            if position and position == 'first':
+                if sys.version_info >= (3, 2):
+                    result.move_to_end(key, False)
+                else:
+                    move_to_start(result, key)
+            merge_obj(result[key], value, position)
             continue
-
         if value is None:
             result.pop(key, None)
             continue
-        result[key] = value
+        if key not in result and position == 'first':
+            result[key] = value
+            if sys.version_info >= (3, 2):
+                result.move_to_end(key, False)
+            else:
+                move_to_start(result, key)
+        else:
+            result[key] = value
+
     return result
 
 def create_patch(source, target):

json_merge_patch/tests.py

@@ -1,5 +1,6 @@
 import unittest
 import lib as merge
+from collections import OrderedDict
 
 fixtures = [
 [{
@@ -58,6 +59,55 @@ def test_create_patch(self):
                 self.assertEqual(merge.create_patch(fixture[0], fixture[2]), fixture[1])
 
 
+ordered_fixtures = [[
+    OrderedDict([
+        ("title", "Goodbye!"),
+        ("content", "This will be unchanged")
+    ]),
+    OrderedDict([
+        ("title", "Goodbye!"),
+        ("new", "Where will this go?"),
+        ("content", "content")
+    ]),
+    OrderedDict([
+        ("title", "Goodbye!"),
+        ("content", "content"),
+        ("new", "Where will this go?")
+    ]),
+    OrderedDict([
+        ("new", "Where will this go?"),
+        ("title", "Goodbye!"),
+        ("content", "content")
+    ])
+],[
+    OrderedDict([
+        ("title", "Goodbye!"),
+        ("content", "This will be unchanged")
+    ]),
+    OrderedDict([
+        ("title", "Goodbye!"),
+        ("new", OrderedDict([("where", "will I go")])),
+        ("content", "content"),
+    ]),
+    OrderedDict([
+        ("title", "Goodbye!"),
+        ("content", "content"),
+        ("new", OrderedDict([("where", "will I go")])),
+    ]),
+    OrderedDict([
+        ("new", OrderedDict([("where", "will I go")])),
+        ("title", "Goodbye!"),
+        ("content", "content"),
+    ])
+]]
+
+class TestOrdered(unittest.TestCase):
+
+    def test_merge(self):
+        for fixture in ordered_fixtures:
+            self.assertEqual(merge.merge(fixture[0].copy(), fixture[1], position='last'), fixture[2])
+            self.assertEqual(merge.merge(fixture[0].copy(), fixture[1], position='first'), fixture[3])
+
 if __name__ == '__main__':
     unittest.main()