0
$\begingroup$

Rosanswers logo

I have a python script which reads data from 4 txt files and stores the values in 4 numpy arrays. I put the txt files in the same directory with my python node, but when I run the node the files cannot be located and I get the error:

IOError: [Errno 2] No such file or directory: 'xdot.txt'

That works when I run the code on a python IDE, but I don't know where is the problem here. I am new to ROS and I use ROS Kinetic, Python 2.7.12 on Ubuntu 16.04 LTS. Thank you in advance.

#!/usr/bin/python

import rospy
import serial
import time
from std_msgs.msg import String
from decimal import Decimal
import numpy as np
from scipy.integrate import odeint, solve_ivp
import matplotlib.pyplot as plt
from scipy.interpolate import interp1d

xdot_list = []
ydot_list = []
xdotdot_list = []
ydotdot_list = []

with open('xdot.txt', 'r') as filehandle:
    filecontents = filehandle.readlines()

    for line in filecontents:
        current_place = line[:-1]
        xdot_list.append(current_place)

xdot = np.array(xdot_list, dtype=np.float32) 

with open('ydot.txt', 'r') as filehandle:
    filecontents = filehandle.readlines()

    for line in filecontents:
        current_place = line[:-1]
        ydot_list.append(current_place)

ydot = np.array(ydot_list, dtype=np.float32) 

with open('xdotdot.txt', 'r') as filehandle:
    filecontents = filehandle.readlines()

    for line in filecontents:
        current_place = line[:-1]
        xdotdot_list.append(current_place)

xdotdot = np.array(xdotdot_list, dtype=np.float32) 

with open('ydotdot.txt', 'r') as filehandle:
    filecontents = filehandle.readlines()

    for line in filecontents:
        current_place = line[:-1]
        ydotdot_list.append(current_place)

ydotdot = np.array(ydotdot_list, dtype=np.float32)

Originally posted by Spyros on ROS Answers with karma: 51 on 2019-02-25

Post score: 0

Improve this question
$\endgroup$

1 Answer 1

Reset to default
0
$\begingroup$

Rosanswers logo

I put the txt files in the same directory with my python node, but when I run the node the files cannot be located and I get the error:

IOError: [Errno 2] No such file or directory: 'xdot.txt'

That works when I run the code on a python IDE, but I don't know where is the problem here.

The problem is in the difference in working directory when running the script directly versus using roslaunch or rosrun.

See #q235337 for more info on that.

In short: don't use relative paths, but absolute ones. To avoid hard-coding paths specific to your own PC, use ROS parameters with substitution args.

Edit:

I checked the links you pointed out. In my occasion, it is not a problem to use an absolute path to open the file.

Absolute paths are not the problem. The problem would be to embed paths that are only valid on your own machine in your code. That is never a good idea.

But which is the recommended way to do it in ROS applications in general?

If you want to keep things relative to package locations, you could use either something like rospkg or the substitution args I mentioned earlier. If you have the option, I'd go for the substitution args (as you avoid adding another dependency to your program).

Edit2:

I have my 4 files in the path: ~/catkin_ws/src/usb_rs232/scripts in the usb_rs232 package. So the command would be:

rosrun usb_rs232 serial_connection.py _arg_name:=$(rospack find usb_rs232)~/catkin_ws/src/usb_rs232/scripts

No.

The command would be (provided you have an actual ROS parameter in your Python script called arg_name):

rosrun usb_rs232 serial_connection.py _arg_name:=$(rospack find usb_rs232)/scripts

There's no need to repeat the path. $(rospack find usb_rs232) returns the path to $HOME/catkin_ws/src/usb_rs232, but without hard-coding it. The /scripts part would be appended to that. So in the end, arg_name gets the value $HOME/catkin_ws/src/usb_rs232/scripts assigned to it. You can then retrieve that in your script using rospy.get_param('arg_name') and use it as a base path, relative to which you could then open your 4 files.

Edit 3:

I changed my code to:

rospy.get_param('arg_name')

with open('arg_name/xdot.txt', 'r') as filehandle:

with the same command and I get the error:

KeyError: 'arg_name'

This is starting to become more of a Python question and not so much a ROS question, but: that's not how this works.

rospy.get_param(..) returns a str object, so you'll have to store that somewhere. Then, you should use that str and prefix it to the filename to get a valid path.

Something like the following should work:

base_dir = rospy.get_param('~arg_name')
with open(base_dir + '/xdot.txt', 'r') as filehandle:
    ...

Note: you only need to retrieve arg_name once. And you'll probably want to add some error checking for when the user hasn't actually set arg_name to a legal/proper value.

I would actually recommend to do something like this though:

base_dir = rospy.get_param('~arg_name')
with open(os.path.join(base_dir, 'xdot.txt'), 'r') as filehandle:
    ...

as that would be both more sane and portable (see os.path.join(..)).

Edit 4:

File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/client.py", line 465, in get_param 

  return _param_server[param_name] #MasterProxy does all the magic for us
File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/msproxy.py", line 123, in __getitem__
  raise KeyError(key)
KeyError: arg_name

This is basically rospy telling you that it cannot find the parameter.

Two things:

  1. parameters set using _name:=value syntax of rosrun are private parameters
  2. your script does not initialise a node (I don't see rospy.init_node(..))

If you don't initialise a node, you cannot access private parameters (as private parameters are always private to a node: no node -> no parameters).

To retrieve a private parameter, you need to prefix the parameter name with a ~. So arg_name becomes ~arg_name.

I've also updated the code snippets in my earlier edits to show this.

Originally posted by gvdhoorn with karma: 86574 on 2019-02-25

This answer was ACCEPTED on the original site

Post score: 2

Original comments

Comment by Spyros on 2019-02-26:
Thanks for the response. I checked the links you pointed out. In my occasion, it is not a problem to use an absolute path to open the file. But which is the recommended way to do it in ROS applications in general?

Comment by Spyros on 2019-02-26:
the substitution args can be applied with rosrun with the same way the do with roslaunch ? I prefer to use rosrun but I am a bit lost in the documentation of substitution args. How can this be applied with rosrun?

Comment by gvdhoorn on 2019-02-26:
With rosrun it's no longer a substitution arg, as those are roslaunch specific.

But rospack find can be used on the command line, so something like this should work:

rosrun your_pkg your_script _arg_name:=$(rospack find pkg_with_txt_file)/path/to/file.txt

Comment by Spyros on 2019-02-26:
how can this work for several txt files that open in the same script? (4 in my case). The files are in the same directory.

Comment by gvdhoorn on 2019-02-26:
Pass the path to a directory instead of a file?

Comment by Spyros on 2019-02-26:
it doesn't seems to work. I have already done it with absolute path, I just wanted to know my alternatives. But it is not really convenient to type all the path every time I run the node

Comment by gvdhoorn on 2019-02-26:\

it doesn't seems to work.

If you can show us what "doesn't work", perhaps we can help.

But it is not really convenient to type all the path every time I run the node

well, that is what roslaunch would be for.

Comment by Spyros on 2019-02-26:
I have my 4 files in the path: ~/catkin_ws/src/usb_rs232/scripts in the usb_rs232 package. So the command would be: rosrun usb_rs232 serial_connection.py _arg_name:=$(rospack find usb_rs232)~/catkin_ws/src/usb_rs232/scripts ?

Comment by Spyros on 2019-02-26:
I changed my code to:

rospy.get_param('arg_name')
with open('arg_name/xdot.txt', 'r') as filehandle:

with the same command and I get the error:

KeyError: 'arg_name'

Comment by Spyros on 2019-02-27:
File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/client.py", line 465, in get_param return _param_server[param_name] #MasterProxy does all the magic for us File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/msproxy.py", line 123, in getitem raise KeyError(key) KeyError: arg_name

Comment by Spyros on 2019-02-27:
thanks for the patience. I still don't know what I am doing wrong here.

Comment by Spyros on 2019-02-27:
Now it works fine!I was initializing the node after the reading of the txt files. So I changed the order of my code and the node runs without errors. I will keep both methods in mind (absolute path and path set from command line) for future work. Thanks a lot for the help!!!

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.