T8488: add utility to define exclusion mask from list of partial paths

A partial path is one that may or may not include intervening tag node
values. For unspecified tag node values, the matching subtree is
returned.

For example ['interfaces', 'ethernet', 'address'] will return the
subtree of the config for which ethernet tag values have defined
address.

1 files changed, 56 insertions, 0 deletions

diff --git a/python/vyos/derivedtree.py b/python/vyos/derivedtree.py
new file mode 100644
index 000000000..0a57921b2
--- /dev/null
+++ b/python/vyos/derivedtree.py
@@ -0,0 +1,56 @@
+# Copyright (C) VyOS Inc.
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this library.  If not, see <http://www.gnu.org/licenses/>.
+
+
+from vyos.referencetree import ReferenceTree
+from vyos.configtree import ConfigTree
+from vyos.configtree import ConfigTreeError
+from vyos.configtree import subtree_from_partial
+
+
+class DerivedTreeError(Exception):
+    """Error to be raised by functions of derivedtree"""
+
+
+def subtree_from_list_of_partial_paths(
+    ctree: ConfigTree, paths: list[list[str]], accumulator: ConfigTree = None
+):
+    """Return the union of subtrees of the ConfigTree argument matching each
+    of the 'partial' paths. A partial path is one that may or may not
+    contain intervening tag node values, in which case it will match for all
+    values that apply.
+
+    An existing subtree may be passed as the initial value of accumulator.
+    """
+    if accumulator:
+        if not isinstance(accumulator, ConfigTree):
+            raise TypeError("Argument 'accumulator' must be an instance of ConfigTree")
+    else:
+        accumulator = ConfigTree('')
+
+    rtree = ReferenceTree()
+
+    errors = []
+    for path in paths:
+        try:
+            accumulator = subtree_from_partial(ctree, path, rtree, accumulator)
+        except ConfigTreeError as e:
+            errors.append(str(e))
+            continue
+
+    if errors:
+        raise DerivedTreeError(f'Nonsensical paths: {errors}')
+
+    return accumulator