Python on ROS

• WIKI
  

Services:

ROSServices are defined bysrv files, which contains a request message and a response message. These are identical to the messages used with ROSTopics (see rospy message overview). rospy converts these srv files into Python source code and creates three classes that you need to be familiar with: service definitions, request messages, and response messages. The names of these classes come directly from thesrv filename:

my_package/srv/Foo.srv →my_package.srv.Foo

my_package/srv/Foo.srv →my_package.srv.FooRequest

my_package/srv/Foo.srv → my_package.srv.FooResponse

       srv : http://wiki.ros.org/srv

A service description file consists of a request and aresponsemsg type, separated by'---'. Any two.msg files concatenated together with a'---' are a legal service description.

Client Library Support

In Python, the generated Python service type file (e.g.foo_srvs.srv.Foo) provides nearly all the information you might want about a.srv file. You can examine the __slots__ and_slot_types and other fields to introspect information about the request and reply Messages. Foradvanced users, theroslib.srvs module in theroslib Package provides basic support for parsing.srv files in Python. This library is only recommended for those implementing service generators.

Creating a ROS msg and srv

Reference article: 如何创建使用ros的service？Python版

• Notes
  • Files we need to create
    1.  filename.srv file; This file is used to define the input(request message) and output(response message) for this service class. rospy converts thesesrv files into Python source code and creates three classes that you need to be familiar with: service definitions, request messages, and response messages. The names of these classes come directly from thesrv filename.
       
    2.  server.py file; This file is to create a service.
    3. client.py file; This file is to produce a request for the service.
       

• SRV file (e.g WordCount.srv and AddTwoInts.srv in the package of 'py_package')
  
  • This file usually locate in the 'srv' folder of your package. So when we use this srv, we can using following code in the server.py to import three classes (WordCount,WordCountRequest,WordCountResponse) (if pyLint prompt error for this line that can not find the py_package.srv, we ignore it. If using python command to run this file and an error of 'ImportError: No module named py_package.srv',then try with'source devel/setup.bash' at first in your terminal where running the python command, if error still shows again ,then try catkin_make to rebuild again not just with roboware's builder tool because it sometimes does not work well . Here the py_pacakge.srv is the path of srv folder rather than a srv file)
    

from py_package.srv import AddTwoInts,AddTwoIntsResponse,WordCount,WordCountResponse

• AddTwoInts.srv
  

int64 a

int64 b

---

int64 sum

• WordCount.srv

string words

---

uint32 count

• server.py

from py_package.srv import AddTwoInts,AddTwoIntsResponse,WordCount,WordCountResponse

import rospy

def handle_add_two_ints(req):

    print "Returning [%s + %s = %s]"%(req.a, req.b, (req.a + req.b))

    return AddTwoIntsResponse(req.a + req.b)

def handle_word_count(req):

        print "Return [%s] "% (req.words)

        return WordCountResponse(len(req.words.split()))

def add_two_ints_server():

    rospy.init_node('add_two_ints_server')

    s = rospy.Service('add_two_ints', AddTwoInts, handle_add_two_ints)

    print "Ready to add two ints."

    w = rospy.Service('word_count',WordCount,handle_word_count)

    print "Ready to count the length."

    rospy.spin()

if __name__ == "__main__":

    add_two_ints_server()

• client.py

#!/usr/bin/env python

import sys

import rospy

from py_package.srv import AddTwoInts,WordCount

def add_two_ints_client(x, y):

    rospy.wait_for_service('add_two_ints')

    try:

        add_two_ints = rospy.ServiceProxy('add_two_ints', AddTwoInts)

        resp1 = add_two_ints(x, y)

        return resp1.sum

    except rospy.ServiceException, e:

        print "Service add_two_ints call failed: %s"%e

def count_length_words(words):

    rospy.wait_for_service('word_count')

    try:

        word_counter = rospy.ServiceProxy('word_count',WordCount)

        word_count = word_counter(words)

        print words,'->',word_count.count

        return word_count.count

    except rospy.ServiceException,e:

        print "Service word_count call failed: %s"%e

def usage():

    return "%s [x y]"%sys.argv[0]

if __name__ == "__main__":

    if len(sys.argv) >= 3:

        x = int(sys.argv[1])

        y = int(sys.argv[2])

        words = ' '.join(sys.argv[3:])

    else:

        print usage()

        sys.exit(1)

    print "Requesting %s+%s"%(x, y)

    print "%s + %s = %s"%(x, y, add_two_ints_client(x, y))

    print "The count of \' %s \'is %s "%(words,count_length_words(words))

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