Having a hard time reading *.urdf and *.rob file in Klampt - ros

Question

Hopefully, somebody used Klampt before here, but I am not able to read. So this question is merely about what am I missing in the urdf file, it seems correct, and this is straight from an example in a e-book that used Klampt for modeling and simulation of robots
from __future__ import print_function,division
from klampt import *
import math
import os
world = WorldModel()
fileurdf = r"planar2R.urdf"
path = os.path.join(r"data",fileurdf)
res = world.readFile(path)
assert res, "There was an error loading data/planar2R.urdf"
the urdf file is well formated
<robot name="test_robot">
<link name="base_link">
</link>
<link name="link1">
</link>
<link name="link2">
</link>
<link name="link3">
</link>
<link name="end_effector">
</link>
<joint name="joint1" type="revolute">
<parent link="base_link"/>
<child link="link1"/>
</joint>
<joint name="joint2" type="revolute">
<origin xyz="1 0 0" rpy="0 0 0"/>
<axis xyz="0 1 0"/>
<parent link="link1"/>
<child link="link2"/>
</joint>
<joint name="joint3" type="revolute">
<parent link="link2"/>
<child link="link3"/>
</joint>
<joint name="joint4" type="fixed">
<parent link="link3"/>
<child link="end_effector"/>
</joint>
</robot>
I get a failure of parsing the file

Answer 1

There could be many reasons for which you'll get "unable to load urdf" error.
Reason 1: File address specified is not correct/readable.
Reason 2: urdf file itself has an error. keep in mind that it is possible to load urdf file in one urdf simulator without any error, but same file may cause an error in klampt.
I faced this problem for quite some time and had to debug it by trying and writing my whole urdf file, learning from example urdf.
Try 1: First make sure the file address is not making any issue.
Try 2: Start with an given example. Make small changes in the urdf file and debug simultaneously.
Try 3: about urdf specifically for klampt, it need last <klampt> </klampt> data. unless it is properly given, compiling will give "unable to load urdf" error.
<klampt> </klampt> data could be given something like this:
<klampt world_frame="world" defaultAccMax="4" use_vis_geom="1">
<noselfcollision group1="" group2="" />
<link name="world" />
<link name="base_link" />
<link name="link_1" servoP="1400" servoI="0" servoD="6" dryFriction="0.5" viscousFriction="8" />
<link name="link_2" servoP="1400" servoI="0" servoD="6" dryFriction="0.5" viscousFriction="8" />
<link name="fcp" />
</klampt>
</robot>
One thing that I don't like about this klampt package is that they don't specify the reason for failure to load urdf file and thus make it difficult to understand for beginner.