xacro problem: Invalid <param> tag: Cannot load command parameter [robot_description]

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Just like the topic I posed, when I try to use roslaunch command to run in the terminal, I met this problem (as followed):

Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/groovy/stacks/xacro/xacro.py '/home/viki/ros_workspace/smartcar_description/urdf/smartcar.urdf.xacro'] returned with code [1]. 

Param xml is <param command="$(arg urdf_file)" name="robot_description"/>

I have no idea how to solve this problem, I sincerely you could give the instructions about how to solve this problem.

Thanks so much for your time.

when I run the code rosrun xacro xacro.py /home/viki/ros_workspace/smartcar_description/urdf/smartcar.urdf.xacro to check the xacro, there is the information showed as followed:

Expat parsing error.  Check that:
 - Your XML is correctly formed
 - You have the xacro xmlns declaration: xmlns:xacro="http://www.ros.org/wiki/xacro"

Traceback (most recent call last):
  File "/opt/ros/groovy/stacks/xacro/xacro.py", line 35, in <module>
    xacro.main()
  File "/opt/ros/groovy/stacks/xacro/src/xacro.py", line 542, in main
    doc = parse(f)
  File "/usr/lib/python2.7/xml/dom/minidom.py", line 1920, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python2.7/xml/dom/expatbuilder.py", line 928, in parse
    result = builder.parseFile(file)
  File "/usr/lib/python2.7/xml/dom/expatbuilder.py", line 207, in parseFile
    parser.Parse(buffer, 0)
xml.parsers.expat.ExpatError: junk after document element: line 197, column 0

Can you give me some information about how to solve the problem?

Thanks so much!