Handle empty strings when evaluating parameters (#300)

If a substitution evaluates to the empty string, then yaml.safe_load() will return None, resulting
in an exception being raised. Instead, this commit allows for the empty string by first wrapping
it in an extra set of quotes before passing to yaml.safe_load().

Signed-off-by: Jacob Perron <[email protected]>

Author: jacobperron

launch_ros/launch_ros/utilities/evaluate_parameters.py

@@ -71,6 +71,10 @@ def check_sequence_type_is_allowed(sequence):
                 # Value is a list of substitutions, so perform them to make a string
                 evaluated_value = perform_substitutions(context, list(value))
 
+                # Handle special case where yaml.safe_load will return None given an empty string
+                if len(evaluated_value) == 0:
+                    evaluated_value = "''"
+
                 try:
                     yaml_evaluated_value = yaml.safe_load(evaluated_value)
                 except yaml.YAMLError:

test_launch_ros/test/test_launch_ros/utilities/test_normalize_parameters.py

@@ -396,3 +396,11 @@ def test_unallowed_yaml_types_as_strings():
     evaluate_parameters(LaunchContext(), norm)
     expected = ({'foo': 1, 'fiz': 'Text That : Cannot Be Parsed As : Yaml'},)
     assert evaluate_parameters(LaunchContext(), norm) == expected
+
+
+def test_empty_string_evalutates_to_empty_string():
+    # Regression test for https://github.com/ros2/launch_ros/pull/289#discussion_r818070166
+    orig = [{'foo': TextSubstitution(text='')}]
+    norm = normalize_parameters(orig)
+    expected = ({'foo': ''},)
+    assert evaluate_parameters(LaunchContext(), norm) == expected