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Writing a Simple Publisher and Subscriber (Python) (plain cmake)
Description: This tutorial covers how to write a publisher and subscriber node in Python using plain cmake (i.e., not catkin). Actually, in this tutorial, we won't even use cmake, but future tutorials in this series do.Tutorial Level: BEGINNER
Contents
Note: Be sure that you have already installed ROS and remember that you must source the appropriate setup file in every shell you create.
Making a directory in which to work
To help keep yourself organized, make a directory in which to work. You can use any name; we'll just call it pubsub:
mkdir -p pubsub cd pubsub
Writing the Publisher Node
"Node" is the ROS term for an executable that is connected to the ROS network. Here we'll create the publisher ("talker") node which will continually broadcast a message.
The Code
Create a file named talker.py within the pubsub directory and paste the following inside it:
1 #!/usr/bin/env python
2 # license removed for brevity
3 import rospy
4 from std_msgs.msg import String
5
6 def talker():
7 pub = rospy.Publisher('chatter', String, queue_size=10)
8 rospy.init_node('talker', anonymous=True)
9 rate = rospy.Rate(10) # 10hz
10 while not rospy.is_shutdown():
11 hello_str = "hello world %s" % rospy.get_time()
12 rospy.loginfo(hello_str)
13 pub.publish(hello_str)
14 rate.sleep()
15
16 if __name__ == '__main__':
17 try:
18 talker()
19 except rospy.ROSInterruptException:
20 pass
Add the following to your CMakeLists.txt. This makes sure the python script gets installed properly, and uses the right python interpreter.
catkin_install_python(PROGRAMS scripts/talker.py
DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)
The Code Explained
Now, let's break the code down.
1 #!/usr/bin/env python
Every Python ROS Node will have this declaration at the top. The first line makes sure your script is executed as a Python script.
You need to import rospy if you are writing a ROS Node. The std_msgs.msg import is so that we can reuse the std_msgs/String message type (a simple string container) for publishing.
This section of code defines the talker's interface to the rest of ROS. pub = rospy.Publisher("chatter", String, queue_size=10) declares that your node is publishing to the chatter topic using the message type String. String here is actually the class std_msgs.msg.String. The queue_size argument is New in ROS hydro and limits the amount of queued messages if any subscriber is not receiving them fast enough. In older ROS distributions just omit the argument.
The next line, rospy.init_node(NAME, ...), is very important as it tells rospy the name of your node -- until rospy has this information, it cannot start communicating with the ROS Master. In this case, your node will take on the name talker. NOTE: the name must be a base name, i.e. it cannot contain any slashes "/".
anonymous = True ensures that your node has a unique name by adding random numbers to the end of NAME. Refer to Initialization and Shutdown - Initializing your ROS Node in the rospy documentation for more information about node initialization options.
9 rate = rospy.Rate(10) # 10hz
This line creates a Rate object rate. With the help of its method sleep(), it offers a convenient way for looping at the desired rate. With its argument of 10, we should expect to go through the loop 10 times per second (as long as our processing time does not exceed 1/10th of a second!)
This loop is a fairly standard rospy construct: checking the rospy.is_shutdown() flag and then doing work. You have to check is_shutdown() to check if your program should exit (e.g. if there is a Ctrl-C or otherwise). In this case, the "work" is a call to pub.publish(hello_str) that publishes a string to our chatter topic. The loop calls rate.sleep(), which sleeps just long enough to maintain the desired rate through the loop.
(You may also run across rospy.sleep() which is similar to time.sleep() except that it works with simulated time as well (see Clock).)
This loop also calls rospy.loginfo(str), which performs triple-duty: the messages get printed to screen, it gets written to the Node's log file, and it gets written to rosout. rosout is a handy tool for debugging: you can pull up messages using rqt_console instead of having to find the console window with your Node's output.
std_msgs.msg.String is a very simple message type, so you may be wondering what it looks like to publish more complicated types. The general rule of thumb is that constructor args are in the same order as in the .msg file. You can also pass in no arguments and initialize the fields directly, e.g.
msg = String() msg.data = str
or you can initialize some of the fields and leave the rest with default values:
String(data=str)
You may be wondering about the last little bit:
In addition to the standard Python __main__ check, this catches a rospy.ROSInterruptException exception, which can be thrown by rospy.sleep() and rospy.Rate.sleep() methods when Ctrl-C is pressed or your Node is otherwise shutdown. The reason this exception is raised is so that you don't accidentally continue executing code after the sleep().
Now we need to write a node to receive the messages.
Writing the Subscriber Node
The Code
Create a file named listener.py within the pubsub directory and paste the following inside it:
1 #!/usr/bin/env python
2 import rospy
3 from std_msgs.msg import String
4
5 def callback(data):
6 rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)
7
8 def listener():
9
10 # In ROS, nodes are uniquely named. If two nodes with the same
11 # name are launched, the previous one is kicked off. The
12 # anonymous=True flag means that rospy will choose a unique
13 # name for our 'listener' node so that multiple listeners can
14 # run simultaneously.
15 rospy.init_node('listener', anonymous=True)
16
17 rospy.Subscriber("chatter", String, callback)
18
19 # spin() simply keeps python from exiting until this node is stopped
20 rospy.spin()
21
22 if __name__ == '__main__':
23 listener()
Then, edit the catkin_install_python() call in your CMakeLists.txt so it looks like the following:
catkin_install_python(PROGRAMS scripts/talker.py scripts/listener.py
DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)
The Code Explained
The code for listener.py is similar to talker.py, except we've introduced a new callback-based mechanism for subscribing to messages.
This declares that your node subscribes to the chatter topic which is of type std_msgs.msgs.String. When new messages are received, callback is invoked with the message as the first argument.
We also changed up the call to rospy.init_node() somewhat. We've added the anonymous=True keyword argument. ROS requires that each node have a unique name. If a node with the same name comes up, it bumps the previous one. This is so that malfunctioning nodes can easily be kicked off the network. The anonymous=True flag tells rospy to generate a unique name for the node so that you can have multiple listener.py nodes run easily.
The final addition, rospy.spin() simply keeps your node from exiting until the node has been shutdown. Unlike roscpp, rospy.spin() does not affect the subscriber callback functions, as those have their own threads.
Building your nodes
Because all we did was write some Python code (and not, for example, define new ROS messages or services that would need to be processed by code generators), there's nothing to build!
Running the nodes
Running nodes requires you have a ROS core started. A ROS core is a collection of nodes and programs that are pre-requisites of a ROS-based system. You must have a roscore running in order for ROS nodes to communicate. Open a new shell, and type:
$ roscore
roscore will output something similar to this:
... logging to /u/nkoenig/ros-jaunty/ros/log/d92b213a-90d4-11de-9344-00301b8246bf/roslaunch-ncq-11315.log ... loading XML file [/u/nkoenig/ros-jaunty/ros/tools/roslaunch/roscore.xml] Added core node of type [rosout/rosout] in namespace [/] started roslaunch server http://ncq:60287/ SUMMARY ====== NODES starting new master (master configured for auto start) process[master]: started with pid [11338] ROS_MASTER_URI=http://ncq:11311/ setting /run_id to d92b213a-90d4-11de-9344-00301b8246bf +PARAM [/run_id] by /roslaunch +PARAM [/roslaunch/uris/ncq:60287] by /roslaunch process[rosout-1]: started with pid [11353] started core service [/rosout] +SERVICE [/rosout/get_loggers] /rosout http://ncq:36277/ +SERVICE [/rosout/set_logger_level] /rosout http://ncq:36277/ +SUB [/time] /rosout http://ncq:36277/ +PUB [/rosout_agg] /rosout http://ncq:36277/ +SUB [/rosout] /rosout http://ncq:36277/
Now everything is set to run talker/listener. Open a new shell, go to your pubsub directory and type:
python talker.py
Now in the original shell type:
python listener.py
Talker should begin outputting text similar to:
[INFO] [WallTime: 1394915011.927728] hello world 1394915011.93 [INFO] [WallTime: 1394915012.027887] hello world 1394915012.03 [INFO] [WallTime: 1394915012.127884] hello world 1394915012.13 [INFO] [WallTime: 1394915012.227858] hello world 1394915012.23 ...
And listener should begin outputting text similar to:
[INFO] [WallTime: 1394915043.555022] /listener_9056_1394915043253I heard hello world 1394915043.55 [INFO] [WallTime: 1394915043.654982] /listener_9056_1394915043253I heard hello world 1394915043.65 [INFO] [WallTime: 1394915043.754936] /listener_9056_1394915043253I heard hello world 1394915043.75 [INFO] [WallTime: 1394915043.854918] /listener_9056_1394915043253I heard hello world 1394915043.85 ...
Now that you've written your first listener node, you should also know that ROS comes with its own generic listener for any topic called rostopic. If you run rostopic echo topic_name, you get output similar to what you wrote with listener.py:
$ rostopic echo chatter
data: hello world 1394915083.35 --- data: hello world 1394915083.45 --- data: hello world 1394915083.55 --- data: hello world 1394915083.65 --- data: hello world 1394915083.75 --- ...
Congratulations! You've just run your first ROS python nodes. For more example code, see the rospy_tutorials package, or move on to the next tutorial rospy_tutorials/Tutorials/WritingServiceClient.
