Cannot retrieve parameters from YAML file (even if the file is found)

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Hi,

I wrote a very simple program in python to read parameters from a YAML file. The YAML file is the following:

mqtt_node:
  ros__parameters:
    variable: 9494.98

and the minimal code of the node is this one:

class mqtt_class(Node):

    def __init__(self):

        super().__init__('mqtt_class')

        print()
        print(self.get_parameter('variable')._value)   # <- here is the problem
        print()
)


def main(args = None):

    rclpy.init(args = args)

    my_publisher = mqtt_class()

    rclpy.spin(my_publisher)


    my_publisher.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':

    main()

This is the error I get:

 Traceback (most recent call last):
   File "/home/kkk/workspace/install/xxx/lib/xxx/xxx", line 11, in <module>
   load_entry_point('xxx', 'console_scripts', 'xxx')()
 File "/home/kkk/workspace/build/xxx/xxx/xxx.py", line 108, in main
  my_publisher = mqtt_class()
  File "/home/kkk/workspace/build/xxx/xxx/xxx.py", line 54, in __init__
    print(self.get_parameter('variable')._value)
   File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/node.py", line 512, in get_parameter
    raise ParameterNotDeclaredException(name)
 rclpy.exceptions.ParameterNotDeclaredException: ('Invalid access to undeclared parameter(s)', 'variable')
[ERROR] [zzzz-2]: process has died [pid 19805, exit code 1, cmd '/home/kkk/workspace/install/xxx/lib/xxx/xxx --ros-args -r __xxx:=xxx --params-file /home/kkk/workspace/install/xxx/share/xxx/params.yaml'].

What is wrong with the code?