1 files changed, 60 insertions, 0 deletions
diff --git a/taskcluster/taskgraph/util/templates.py b/taskcluster/taskgraph/util/templates.py
new file mode 100644
index 0000000000..d0cfb251c3
--- /dev/null
+++ b/taskcluster/taskgraph/util/templates.py
@@ -0,0 +1,60 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from __future__ import absolute_import, print_function, unicode_literals
+
+import copy
+
+
+def merge_to(source, dest):
+    """
+    Merge dict and arrays (override scalar values)
+
+    Keys from source override keys from dest, and elements from lists in source
+    are appended to lists in dest.
+
+    :param dict source: to copy from
+    :param dict dest: to copy to (modified in place)
+    """
+
+    for key, value in source.items():
+        if (
+            isinstance(value, dict)
+            and len(value) == 1
+            and list(value)[0].startswith("by-")
+        ):
+            # Do not merge by-* values as this is likely to confuse someone
+            dest[key] = value
+            continue
+
+        # Override mismatching or empty types
+        if type(value) != type(dest.get(key)):  # noqa
+            dest[key] = value
+            continue
+
+        # Merge dict
+        if isinstance(value, dict):
+            merge_to(value, dest[key])
+            continue
+
+        if isinstance(value, list):
+            dest[key] = dest[key] + value
+            continue
+
+        dest[key] = value
+
+    return dest
+
+
+def merge(*objects):
+    """
+    Merge the given objects, using the semantics described for merge_to, with
+    objects later in the list taking precedence.  From an inheritance
+    perspective, "parents" should be listed before "children".
+
+    Returns the result without modifying any arguments.
+    """
+    if len(objects) == 1:
+        return copy.deepcopy(objects[0])
+    return merge_to(objects[-1], merge(*objects[:-1]))