Notes on statically typed parameters

Introduction

Until ROS 2 Foxy, all parameters could change type anytime, except if the user installed a parameter callback to reject a change. This could generate confusing errors, for example:

$ ros2 run demo_nodes_py listener &
$ ros2 param set /listener use_sim_time not_a_boolean
[ERROR] [1614712713.233733147] [listener]: use_sim_time parameter set to something besides a bool
Set parameter successful
$ ros2 param get /listener use_sim_time
String value is: not_a_boolean

For most use cases, having static parameter types is enough. This article documents some of the decisions that were made when implementing static parameter types enforcement in:

• https://github.com/ros2/rclcpp/pull/1522

• https://github.com/ros2/rclpy/pull/683

Allowing dynamically typed parameters

There might be some scenarios where dynamic typing is desired, so a dynamic_typing field was added to the parameter descriptor. It defaults to false.

For example:

rcl_interfaces::msg::ParameterDescriptor descriptor;
descriptor.dynamic_typing = true;

node->declare_parameter("dynamically_typed_parameter", rclcpp::ParameterValue{}, descriptor);

rcl_interfaces.msg.ParameterDescriptor descriptor;
descriptor.dynamic_typing = True;

node.declare_parameter("dynamically_typed_parameter", None, descriptor);

How is the parameter type specified?

The parameter type will be inferred from the default value provided when declaring it.

Statically typed parameters when allowing undeclared parameters

When undeclared parameters are allowed and a parameter is set without a previous declaration, the parameter will be dynamically typed. This is consistent with other allowed behaviors when undeclared parameters are allowed, e.g. trying to get an undeclared parameter returns “NOT_SET”. Parameter declarations will remain special and dynamic or static typing will be used based on the parameter descriptor (default to static).

Declaring a parameter without a default value

There might be cases were a default value does not make sense and the user must always provide an override. In those cases, the parameter type can be specified explicitly:

// method signature
template<typename T>
Node::declare_parameter<T>(std::string name, rcl_interfaces::msg::ParameterDescriptor = rcl_interfaces::msg::ParameterDescriptor{});
// or alternatively
Node::declare_parameter(std::string name, rclcpp::ParameterType type, rcl_interfaces::msg::ParameterDescriptor = rcl_interfaces::msg::ParameterDescriptor{});

// examples
node->declare_parameter<int64_t>("my_integer_parameter");  // declare an integer parameter
node->declare_parameter("another_integer_parameter", rclcpp::ParameterType::PARAMETER_INTEGER);  // another way to do the same

# method signature
Node.declare_parameter(name: str, param_type: rclpy.Parameter.Type, descriptor: rcl_interfaces.msg.ParameterDescriptor = rcl_interfaces.msg.ParameterDescriptor())

# example
node.declare_parameter('my_integer_parameter', rclpy.Parameter.Type.INTEGER);  # declare an integer parameter

If the parameter may be unused, but when used requires a parameter override, then you could conditionally declare it:

auto mode = node->declare_parameter("mode", "modeA");  // "mode" parameter is an string
if (mode == "modeB") {
    node->declare_parameter<int64_t>("param_needed_when_mode_b");  // when "modeB", the user must provide "param_needed_when_mode_b"
}

Other migration notes

Declaring a parameter with only a name is deprecated:

node->declare_parameter("my_param");  // this generates a build warning

node.declare_parameter("my_param");  # this generates a python user warning

Before, you could initialize a parameter with the “NOT SET” value (in cpp rclcpp::ParameterValue{}, in python None). Now this will throw an exception in both cases:

node->declare_parameter("my_param", rclcpp::ParameterValue{});  // not valid, will throw exception

node.declare_parameter("my_param", None);  # not valid, will raise error

Possible improvements

Easier way to declare dynamically typed parameters

Declaring a dynamically typed parameter in rclcpp could be considered to be a bit verbose:

rcl_interfaces::msg::ParameterDescriptor descriptor;
descriptor.dynamic_typing = true;

node->declare_parameter(name, rclcpp::ParameterValue{}, descriptor);

the following ways could be supported to make it simpler:

node->declare_parameter(name, rclcpp::PARAMETER_DYNAMIC);
node->declare_parameter(name, default_value, rclcpp::PARAMETER_DYNAMIC);

or alternatively:

node->declare_parameter(name, default_value, rclcpp::ParameterDescriptor{}.dynamic_typing());

For rclpy, there’s already a short way to do it:

node.declare_parameter(name, default_value, rclpy.ParameterDescriptor(dynamic_typing=true));