how do I run my own node by launch file?

edit

I created one python file in my own pakage. when I execute the following code in command window, it works well.

rosrun my_package my_python.py

but when I add it in launch file and run it by following code, but it cannot work, just output one ERROR like:

roslaunch my_package my_launch.launch

ERROR cannot launch node of type [my_package/my_python.py]: can't locate node [my_python] in package [my_package]

No processes to monitor

my launch file like this(my_launch.launch):

<launch>
  <node name="my_python" type="my_python" pkg="my_package" />
</launch>