2 files changed, 93 insertions, 1 deletions
diff --git a/python/vyos/compose_config.py b/python/vyos/compose_config.py
new file mode 100644
index 000000000..efa28babe
--- /dev/null
+++ b/python/vyos/compose_config.py
@@ -0,0 +1,84 @@
+# Copyright 2024 VyOS maintainers and contributors <maintainers@vyos.io>
+#
+# 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/>.
+
+"""This module allows iterating over function calls to modify an existing
+config.
+"""
+
+from pathlib import Path
+from typing import TypeAlias, Union, Callable
+
+from vyos.configtree import ConfigTree
+from vyos.configtree import deep_copy as ct_deep_copy
+from vyos.utils.system import load_as_module
+
+ConfigObj: TypeAlias = Union[str, ConfigTree]
+
+class ComposeConfigError(Exception):
+    """Raised when an error occurs modifying a config object.
+    """
+
+class ComposeConfig:
+    """Apply function to config tree: for iteration over functions or files.
+    """
+    def __init__(self, config_obj: ConfigObj, checkpoint_file=None):
+        if isinstance(config_obj, ConfigTree):
+            self.config_tree = config_obj
+        else:
+            self.config_tree = ConfigTree(config_obj)
+
+        self.checkpoint = self.config_tree
+        self.checkpoint_file = checkpoint_file
+
+    def apply_func(self, func: Callable):
+        """Apply the function to the config tree.
+        """
+        if not callable(func):
+            raise ComposeConfigError(f'{func.__name__} is not callable')
+
+        if self.checkpoint_file is not None:
+            self.checkpoint = ct_deep_copy(self.config_tree)
+
+        try:
+            func(self.config_tree)
+        except Exception as e:
+            self.config_tree = self.checkpoint
+            raise ComposeConfigError(e) from e
+
+    def apply_file(self, func_file: str, func_name: str):
+        """Apply named function from file.
+        """
+        try:
+            mod_name = Path(func_file).stem.replace('-', '_')
+            mod = load_as_module(mod_name, func_file)
+            func = getattr(mod, func_name)
+        except Exception as e:
+            raise ComposeConfigError(f'Error with {func_file}: {e}') from e
+
+        try:
+            self.apply_func(func)
+        except ComposeConfigError as e:
+            raise ComposeConfigError(f'Error in {func_file}: {e}') from e
+
+    def to_string(self, with_version=False) -> str:
+        """Return the rendered config tree.
+        """
+        return self.config_tree.to_string(no_version=not with_version)
+
+    def write(self, config_file: str, with_version=False):
+        """Write the config tree to a file.
+        """
+        config_str = self.to_string(with_version=with_version)
+        Path(config_file).write_text(config_str)
diff --git a/python/vyos/configtree.py b/python/vyos/configtree.py
index e4b282d72..afd6e030b 100644
--- a/python/vyos/configtree.py
+++ b/python/vyos/configtree.py
@@ -175,9 +175,11 @@ class ConfigTree(object):
     def get_version_string(self):
         return self.__version
 
-    def to_string(self, ordered_values=False):
+    def to_string(self, ordered_values=False, no_version=False):
         config_string = self.__to_string(self.__config, ordered_values).decode()
         config_string = unescape_backslash(config_string)
+        if no_version:
+            return config_string
         config_string = "{0}\n{1}".format(config_string, self.__version)
         return config_string
 
@@ -482,3 +484,9 @@ class DiffTree:
         add = self.add.to_commands()
         delete = self.delete.to_commands(op="delete")
         return delete + "\n" + add
+
+def deep_copy(config_tree: ConfigTree) -> ConfigTree:
+    """An inelegant, but reasonably fast, copy; replace with backend copy
+    """
+    D = DiffTree(None, config_tree)
+    return D.add