confision about ros2 rclpy parameter

edit

I implemented a subclass of node

class MyNode(Node):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

         self.declare_parameter(name='foo',
                                             value='foo_val_init',
                                             descriptor=ParameterDescriptor(name='foo',
                                                                                              type=ParameterType.PARAMETER_STRING))
            # just for debuggin
            print(self.get_parameter('foo').value)

I have tried to override this value from the command line at the startup of the node but nothing worked. I tried the following

ros2 run mynode run --ros-args -p "foo:=foo_val_cmdline"      #(prints foo_val_init)
ros2 run mynode run --ros-args -p foo "foo_val_cmdline"      #(prints foo_val_init)
ros2 run mynode run  -p foo "foo_val_cmdline"      #(prints foo_val_init)
ros2 run mynode run foo "foo_val_cmdline"      #(prints foo_val_init)

any idea what I am doing wrong? I tried looking at demos on the main rclpy repo and documentation and I think I misunderstanding these inside-node declared params purpose.