Add conditional substitution (#734)

* Add conditional substitution

Closes: #727

Signed-off-by: Nick Lamprianidis <[email protected]>

launch/doc/source/architecture.rst

@@ -131,6 +131,10 @@ There are many possible variations of a substitution, but here are some of the c
 
   - This substitution gets a launch configuration value, as a string, by name.
 
+- :class:`launch.substitutions.IfElseSubstitution`
+
+  - This substitution takes a substitution, and if it evaluates to true, then the result is the if_value, else the result is the else_value.
+
 - :class:`launch.substitutions.LaunchDescriptionArgument`
 
   - This substitution gets the value of a launch description argument, as a string, by name.

launch/launch/substitutions/__init__.py

@@ -25,6 +25,7 @@
 from .equals_substitution import EqualsSubstitution
 from .file_content import FileContent
 from .find_executable import FindExecutable
+from .if_else_substitution import IfElseSubstitution
 from .launch_configuration import LaunchConfiguration
 from .launch_log_dir import LaunchLogDir
 from .local_substitution import LocalSubstitution
@@ -46,6 +47,7 @@
     'EnvironmentVariable',
     'FileContent',
     'FindExecutable',
+    'IfElseSubstitution',
     'LaunchConfiguration',
     'LaunchLogDir',
     'LocalSubstitution',

launch/launch/substitutions/if_else_substitution.py

@@ -0,0 +1,117 @@
+# Copyright 2023 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.
+
+"""Module for the IfElseSubstitution substitution."""
+
+from typing import List
+from typing import Sequence
+from typing import Text
+
+from .substitution_failure import SubstitutionFailure
+from ..frontend import expose_substitution
+from ..launch_context import LaunchContext
+from ..some_substitutions_type import SomeSubstitutionsType
+from ..substitution import Substitution
+from ..utilities import normalize_to_list_of_substitutions
+from ..utilities import perform_substitutions
+from ..utilities.type_utils import perform_typed_substitution
+
+
+@expose_substitution('if')
+class IfElseSubstitution(Substitution):
+    """
+    Substitution that conditionally returns one of two substitutions.
+
+    Depending on whether the condition substitution evaluates to true, either it returns
+    the if_value substitution or the else_value substitution.
+
+    Example with a boolean launch configuration:
+
+        .. doctest::
+
+            >>> from launch.substitutions import LaunchConfiguration
+            >>> subst = IfElseSubstitution(
+            ...     LaunchConfiguration("arg"),
+            ...     if_value="arg_evaluated_to_true",
+            ...     else_value="arg_evaluated_to_false")
+
+    Combine with boolean substitutions to create more complex conditions.
+    Example with multiple boolean launch configurations:
+
+        .. doctest::
+
+            >>> from launch.substitutions import AllSubstitution
+            >>> from launch.substitutions import EqualsSubstitution
+            >>> from launch.substitutions import LaunchConfiguration
+            >>> from launch.substitutions import NotSubstitution
+            >>> subst = IfElseSubstitution(
+            ...     AllSubstitution(EqualsSubstitution(LaunchConfiguration("arg1"),
+            ...                                        LaunchConfiguration("arg2")),
+            ...                     NotSubstitution(LaunchConfiguration("arg3"))),
+            ...     if_value="all_args_evaluated_to_true",
+            ...     else_value="at_least_one_arg_evaluated_to_false")
+
+    """
+
+    def __init__(self, condition: SomeSubstitutionsType,
+                 if_value: SomeSubstitutionsType = '',
+                 else_value: SomeSubstitutionsType = '') -> None:
+        """Create a IfElseSubstitution substitution."""
+        super().__init__()
+        if if_value == else_value == '':
+            raise RuntimeError('One of if_value and else_value must be specified')
+        self._condition = normalize_to_list_of_substitutions(condition)
+        self._if_value = normalize_to_list_of_substitutions(if_value)
+        self._else_value = normalize_to_list_of_substitutions(else_value)
+
+    @classmethod
+    def parse(cls, data: Sequence[SomeSubstitutionsType]):
+        """Parse `IfElseSubstitution` substitution."""
+        if len(data) < 2 or len(data) > 3:
+            raise TypeError('if substitution expects from 2 or 3 arguments')
+        kwargs = {'condition': data[0], 'if_value': data[1]}
+        if len(data) == 3:
+            kwargs['else_value'] = data[2]
+        return cls, kwargs
+
+    @property
+    def condition(self) -> List[Substitution]:
+        """Getter for condition."""
+        return self._condition
+
+    @property
+    def if_value(self) -> List[Substitution]:
+        """Getter for if value."""
+        return self._if_value
+
+    @property
+    def else_value(self) -> List[Substitution]:
+        """Getter for else value."""
+        return self._else_value
+
+    def describe(self) -> Text:
+        """Return a description of this substitution as a string."""
+        return f'IfElseSubstitution({self.condition}, {self.if_value}, {self.else_value})'
+
+    def perform(self, context: LaunchContext) -> Text:
+        """Perform the substitution by evaluating the condition."""
+        try:
+            condition = perform_typed_substitution(context, self.condition, bool)
+        except (TypeError, ValueError) as e:
+            raise SubstitutionFailure(e)
+
+        if condition:
+            return perform_substitutions(context, self.if_value)
+        else:
+            return perform_substitutions(context, self.else_value)

launch/launch/substitutions/python_expression.py

@@ -92,7 +92,7 @@ def expression(self) -> List[Substitution]:
 
     @property
     def python_modules(self) -> List[Substitution]:
-        """Getter for expression."""
+        """Getter for python modules."""
         return self.__python_modules
 
     def describe(self) -> Text:
@@ -108,7 +108,7 @@ def perform(self, context: LaunchContext) -> Text:
         module_objects = [importlib.import_module(name) for name in module_names]
         expression_locals = {}
         for module in module_objects:
-            # For backwards compatility, we allow math definitions to be implicitly
+            # For backwards compatibility, we allow math definitions to be implicitly
             # referenced in expressions, without prepending the math module name
             # TODO: This may be removed in a future release.
             if module.__name__ == 'math':

launch/test/launch/substitutions/test_if_else_substitution.py

@@ -0,0 +1,72 @@
+# Copyright 2023 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.
+
+"""Tests for the IfElseSubstitution substitution class."""
+
+from launch import LaunchContext
+from launch.substitutions import IfElseSubstitution
+
+import pytest
+
+
+def test_if_else_substitution_no_values():
+    """Check that construction fails if no values are specified."""
+    # Should raise an error since neither the if value nor the else value is given
+    with pytest.raises(RuntimeError):
+        IfElseSubstitution('true')
+
+
+def test_if_else_substitution_both_values():
+    """Check that the right value is returned when both values are given."""
+    # Condition is true case
+    subst = IfElseSubstitution('true', 'ivalue', 'evalue')
+    result = subst.perform(LaunchContext())
+    assert result == 'ivalue'
+    subst = IfElseSubstitution('true', if_value='ivalue', else_value='evalue')
+    result = subst.perform(LaunchContext())
+    assert result == 'ivalue'
+
+    # Condition is false case
+    subst = IfElseSubstitution('false', 'ivalue', 'evalue')
+    result = subst.perform(LaunchContext())
+    assert result == 'evalue'
+
+
+def test_if_else_substitution_if_value():
+    """Check that the right value is returned when only the if value is given."""
+    # Condition is true case
+    subst = IfElseSubstitution('1', 'ivalue')
+    result = subst.perform(LaunchContext())
+    assert result == 'ivalue'
+    subst = IfElseSubstitution('1', if_value='ivalue')
+    result = subst.perform(LaunchContext())
+    assert result == 'ivalue'
+
+    # Condition is false case
+    subst = IfElseSubstitution('0', 'ivalue')
+    result = subst.perform(LaunchContext())
+    assert result == ''
+
+
+def test_if_else_substitution_else_value():
+    """Check that the right value is returned when only the else value is given."""
+    # Condition is true case
+    subst = IfElseSubstitution('on', else_value='evalue')
+    result = subst.perform(LaunchContext())
+    assert result == ''
+
+    # Condition is false case
+    subst = IfElseSubstitution('off', else_value='evalue')
+    result = subst.perform(LaunchContext())
+    assert result == 'evalue'