Add diff_from(self, other) method to CfgNode

yacs/config.py

@@ -270,6 +270,29 @@ def _immutable(self, is_immutable):
             if isinstance(v, CfgNode):
                 v._immutable(is_immutable)
 
+    def diff_from(self, other):
+        out = CfgNode()
+        out["add"] = CfgNode()
+        out["minus"] = CfgNode()
+        out["change"] = CfgNode()
+
+        return self._diff_from(other, out)
+
+    def _diff_from(self, other, out):
+        for key in self.keys():
+            if key not in other.keys():
+                out.minus[key] = self[key]
+        for key in other.keys():
+            if key not in self.keys():
+                out.add[key] = other[key]
+            elif self[key] != other[key]:
+                if isinstance(other[key], CfgNode):
+                    out.change[key] = CfgNode()
+                    out.change[key] = self[key]._diff_from(other[key], out.change[key])
+                else:
+                    out[key] = other[key]
+        return out
+
     def clone(self):
         """Recursively copy this CfgNode."""
         return copy.deepcopy(self)

yacs/tests.py

@@ -253,6 +253,24 @@ def test_load_cfg_from_file(self):
             with open(f.name, "rt") as f_read:
                 yacs.config.load_cfg(f_read)
 
+    def test_diff_cfg(self):
+        # Test a change in config gives correct diff config
+        cfg = get_cfg()
+        cfg2 = cfg.clone()
+        cfg2.pop("NUM_GPUS")
+        cfg2.MODEL.TYPE = "a_bar_model"
+        cfg2.TEST = "new_key"
+        expected = CN()
+        expected["add"] = CN()
+        expected.add.TEST = "new_key"
+        expected["minus"] = CN()
+        expected.minus.NUM_GPUS = 8
+        expected["change"] = CN()
+        expected.change.MODEL = CN()
+        expected.change.MODEL.TYPE = "a_bar_model"
+        print(cfg.diff_from(cfg2))
+        assert cfg.diff_from(cfg2) == expected
+
     def test_load_from_python_file(self):
         # Case 1: exports CfgNode
         cfg = get_cfg()