# rclpy: Parameters

## Declaring and Accessing Parameters

```python
# node is rclpy.node.Node instance
# 42 is a great default for a parameter
node.declare_parameter('my_param_name', 42)

# To get the value:
param = node.get_parameter('my_param_name').value
```

## Declaring Multiple Parameters at Once

There seems to be a fairly undocumentated part of the rclpy API:

```python
node.declare_parameters(
    namespace='',
    parameters=[
        ('my_param_name', 'default value'),
        ('my_other_param', 42)
    ]
)
```

## Dynamic Parameters

In ROS 2, all parameters can be dynamically updated through a ROS 2 service
(there is no need to define duplicate stuff as with dynamic reconfigure).

```python
from rcl_interfaces.msg import SetParametersResult

import rclpy
from rclpy.node import Node

class MyNode(Node):

    def __init__(self):
        super().__init__('my_node_name')

        # Declare a parameter
        self.declare_parameter('my_param_name', 42)

        # Then create callback
        self.add_on_set_parameters_callback(self.callback)
    
    def callback(self, parameters):
        result = SetParametersResult(successful=True)

        for p in parameters:
            if p.name == 'my_param_name':
                if p.type_ != p.Type.INTEGER:
                    result.successful = False
                    result.reason = 'my_param_name must be an Integer'
                    return result
                if p.value < 20:
                    result.successful = False
                    result.reason = 'my_param_name must be >= 20;
                    return result

        # Return success, so updates are seen via get_parameter()
        return result
```

For an example of calling the set_parameters service, see
[ROS Answers](https://answers.ros.org/question/308541/ros2-rclpy-set-parameter-example/)

## Getting parameters from other nodes

In ros2, this is done through a service call. For example, to get the `robot_description` parameter published by `robot_state_publisher`:

```python
from rcl_interfaces.srv import GetParameters

import rclpy
from rclpy.node import Node

class GetParam(Node):
    def __init__(self):
        super().__init__('joint_command_publisher')
        self.client = self.create_client(GetParameters, '/robot_state_publisher/get_parameters')
        robot_description = self.get_robot_description()

    def get_robot_description(self):
        request = GetParameters.Request()
        request.names = ["robot_description"]
        future = self.client.call_async(request)
        rclpy.spin_until_future_complete(self, future)
        if future.result() is not None:
            self.get_logger().info(future.result().values[0].string_value )
            return future.result().values[0].string_value  # Assuming the parameter is a string, adjust as necessary
        else:
            self.get_logger().error('Failed to call service')
            return None

def main(args=None):
    rclpy.init(args=args)
    node = GetParam()
    
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass
    
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()
```