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In the ROS2 tutorials located at https://docs.ros.org/en/foxy/Tutorial..., the 'default_value' within the 'DeclareLaunchArgument' directive is assigned to TextSubstitution(text='0') or similar.
My question is why that is necessary. Can't you just do default_value='0'? It seems to work without the TextSubstitution directive.
Thanks
TextSubstitution is usually used to wrap literals in the launch description so they can be concatenated with other substitutions - but these substitutions must only be evaluated when the specific launch description is executed. In that context, it provides a uniform interface for all "substitutions".
https://github.com/ros2/launch/blob/b...
A partial purpose is that TextSubstitution must be constructed with a Text type, which indicates a value that contains a unicode string in a manner that is compatible with both Python 2 and Python 3 - so to work-around the differences in string handling/usage in Python2/3.
I'd say that's partly the reason - at least entirely the reason for there being the Text type.
More importantly, TextSubstitution is usually used to wrap literals in the launch description so they can be concatenated with other substitutions - but these substitutions must only be evaluated when the specific launch description is executed. In that context, it provides a uniform interface for all "substitutions".
@swiz23 I believe so, yes. Having said that, you never know how downstream launch descriptions will try to wrap around what you have - very contrived example, but still. As such, it's probably best practice to just use Substitution types for default values.
Asked: 2021-11-09 22:42:05 -0600
Seen: 1,462 times
Last updated: Nov 10 '21
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