Expose StringJoinSubstitution to frontend

launch/launch/substitutions/string_join_substitution.py

@@ -14,14 +14,16 @@
 
 """Module for the StringJoinSubstitution substitution."""
 
-from typing import Iterable, List, Text
+from typing import Any, Dict, Iterable, List, Sequence, Text, Tuple, Type
 
+from ..frontend.expose import expose_substitution
 from ..launch_context import LaunchContext
 from ..some_substitutions_type import SomeSubstitutionsType
 from ..substitution import Substitution
 from ..utilities import perform_substitutions
 
 
+@expose_substitution('string-join')
 class StringJoinSubstitution(Substitution):
     """
     Substitution that joins strings and/or other substitutions.
@@ -34,13 +36,31 @@ class StringJoinSubstitution(Substitution):
 
     .. code-block:: python
 
-        StringJoinSubstitution(
-            [['https', '://'], LaunchConfiguration('subdomain')], 'ros', 'org'],
+        subdomain = LaunchConfiguration(variable_name='subdomain', default='docs')
+        url = StringJoinSubstitution(
+            [['https', '://', subdomain], 'ros', 'org'],
             delimiter='.'
         )
 
-    If the ``subdomain`` launch configuration was set to ``docs``
-    and the ``delimiter`` to ``.``, this would result in a string equal to
+    .. code-block:: xml
+
+        <launch>
+            <arg name="subdomain" default="docs"/>
+            <let name="url" value="$(string-join . https://$(var subdomain) ros org)"/>
+        </launch>
+
+    .. code-block:: yaml
+
+            launch:
+                - arg:
+                    name: subdomain
+                    default: "docs"
+                - let:
+                    name: url
+                    value: "$(string-join . https://$(var subdomain) ros org)"
+
+    If the ``subdomain`` launch configuration was set to ``docs`` and the ``delimiter`` to ``.``,
+    then any of the above launch descriptions would result in a string equal to
 
     .. code-block:: python
 
@@ -76,6 +96,18 @@ def delimiter(self) -> List[Substitution]:
         """Getter for delimiter."""
         return self.__delimiter
 
+    @classmethod
+    def parse(
+        cls, data: Sequence[SomeSubstitutionsType]
+    ) -> Tuple[Type['StringJoinSubstitution'], Dict[str, Any]]:
+        """Parse `StringJoinSubstitution` substitution."""
+        if len(data) < 2:
+            raise TypeError(
+                'string-join substitution expects at least 2 arguments: '
+                '1 delimiter + at least 1 component'
+            )
+        return cls, {'delimiter': data[0], 'substitutions': data[1:]}
+
     def __repr__(self) -> Text:
         """Return a description of this substitution as a string."""
         string_components = [

launch_xml/test/launch_xml/test_string_join_substitution.py

@@ -0,0 +1,71 @@
+# Copyright 2025 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test parsing a StringJoinSubstitution in XML launch file."""
+
+import io
+import textwrap
+
+from launch.actions import DeclareLaunchArgument, SetLaunchConfiguration
+from launch.frontend import Parser
+from launch.launch_context import LaunchContext
+from launch.substitutions import StringJoinSubstitution
+
+
+def test_nested():
+    xml_file = textwrap.dedent(
+        """
+        <launch>
+            <arg name="subdomain" default="wiki"/>
+            <let name="url" value="$(string-join . https://$(var subdomain) ros org)"/>
+        </launch>
+        """
+    )
+    root_entity, parser = Parser.load(io.StringIO(xml_file))
+    ld = parser.parse_description(root_entity)
+
+    assert len(ld.entities) == 2
+    assert isinstance(ld.entities[0], DeclareLaunchArgument)
+    assert isinstance(ld.entities[1], SetLaunchConfiguration)
+
+    lc = LaunchContext()
+    ld.entities[0].visit(lc)
+
+    let = ld.entities[1]
+    assert isinstance(let.value[0], StringJoinSubstitution)
+    assert let.value[0].perform(lc) == 'https://wiki.ros.org'
+
+
+def test_delimiter():
+    yaml_file = textwrap.dedent(
+        """
+        <launch>
+            <arg name="delimiter" default="^_^"/>
+            <let name="text" value="$(string-join '($(var delimiter))' a b c)"/>
+        </launch>
+        """
+    )
+    root_entity, parser = Parser.load(io.StringIO(yaml_file))
+    ld = parser.parse_description(root_entity)
+
+    assert len(ld.entities) == 2
+    assert isinstance(ld.entities[0], DeclareLaunchArgument)
+    assert isinstance(ld.entities[1], SetLaunchConfiguration)
+
+    lc = LaunchContext()
+    ld.entities[0].visit(lc)
+
+    let = ld.entities[1]
+    assert isinstance(let.value[0], StringJoinSubstitution)
+    assert let.value[0].perform(lc) == 'a(^_^)b(^_^)c'

launch_yaml/test/launch_yaml/test_string_join_substitution.py

@@ -0,0 +1,77 @@
+# Copyright 2025 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test parsing a StringJoinSubstitution in yaml launch file."""
+
+import io
+import textwrap
+
+from launch.actions import DeclareLaunchArgument, SetLaunchConfiguration
+from launch.frontend import Parser
+from launch.launch_context import LaunchContext
+from launch.substitutions import StringJoinSubstitution
+
+
+def test_nested():
+    yaml_file = textwrap.dedent(
+        """
+        launch:
+            - arg:
+                name: subdomain
+                default: "wiki"
+            - let:
+                name: url
+                value: "$(string-join . https://$(var subdomain) ros org)"
+        """
+    )
+    root_entity, parser = Parser.load(io.StringIO(yaml_file))
+    ld = parser.parse_description(root_entity)
+
+    assert len(ld.entities) == 2
+    assert isinstance(ld.entities[0], DeclareLaunchArgument)
+    assert isinstance(ld.entities[1], SetLaunchConfiguration)
+
+    lc = LaunchContext()
+    ld.entities[0].visit(lc)
+
+    let = ld.entities[1]
+    assert isinstance(let.value[0], StringJoinSubstitution)
+    assert let.value[0].perform(lc) == 'https://wiki.ros.org'
+
+
+def test_delimiter():
+    yaml_file = textwrap.dedent(
+        """
+        launch:
+            - arg:
+                name: delimiter
+                default: "^_^"
+            - let:
+                name: text
+                value: "$(string-join '($(var delimiter))' a b c)"
+        """
+    )
+    root_entity, parser = Parser.load(io.StringIO(yaml_file))
+    ld = parser.parse_description(root_entity)
+
+    assert len(ld.entities) == 2
+    assert isinstance(ld.entities[0], DeclareLaunchArgument)
+    assert isinstance(ld.entities[1], SetLaunchConfiguration)
+
+    lc = LaunchContext()
+    ld.entities[0].visit(lc)
+
+    let = ld.entities[1]
+    assert isinstance(let.value[0], StringJoinSubstitution)
+    assert let.value[0].perform(lc) == 'a(^_^)b(^_^)c'