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[rospy] suscriber: run a callback once using rospy.spin

asked 2012-03-26 06:56:52 -0500

cduguet gravatar image


I am sorry if I am not using the proper words when I refer to "run a callback function" from a subscriber while using rospy. I'm not sure I got the concept of keeping the nodes sleeping. However, this is not my question.

I want to make a logger node node which takes data from the first message received in a topic, and writes it as a header in a file. Then it will be storing the raw data as long as rospy.spin() says so. The problem is, I have not found an example of how to do this, or if there is such a function. All I can think about are pretty cluttered solutions, which take the time stamp and compares them in every message.

my question is, is there a way to call

rospy.Subscriber("Flow", OpticalFlow2D.msg.FrameFlow, callbackOF)

many times and

rospy.Subscriber("Flow", OpticalFlow2D.msg.FrameFlow, callbackOFonce)

just on the first time, to write the header?

Many thanks,


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5 Answers

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answered 2012-03-26 08:27:59 -0500

updated 2012-03-26 08:43:59 -0500

If you want a particular callback for a topic only to be run for the first message received, you can unsubscribe from within the callback after you've finished your processing. For example:

def cb_once(msg, subscriber):
    #do processing here

sub_once = None
sub_once = rospy.Subscriber('my_topic,', MyType, cb_once, sub_once)

In your case, however, you could have a single callback, and just keep a flag around like wrote_header, which you can set to true after the first message was received so you only write the header once.

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The above may not be entirely correct. The callback args may have to be an iterable, so the line would be sub_once = rospy.Subscriber('my_topic,', MyType, cb_once, [sub_once])

Dan Lazewatsky gravatar imageDan Lazewatsky ( 2012-03-27 06:54:38 -0500 )edit

Based on the upvotes, I'm guessing this works for others. For me though, cb_once is receiving a None, and I get the exception "AttributeError: 'NoneType' object has no attribute 'unregister'".

Benjamin Blumer gravatar imageBenjamin Blumer ( 2014-06-10 18:11:16 -0500 )edit

answered 2014-06-10 18:18:52 -0500

Benjamin Blumer gravatar image

This: solution works, at the cost of having to make the subscriber global.

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answered 2012-03-27 06:24:47 -0500

cduguet gravatar image

Thank you!

I did it using a flag, because (I don't know why it doesn't work, because it usually does to me) the callback function does not get the argument sub_once, therefore throwing the error "cb_once needs exactly two arguments", and it is being given one (I don't know why it is not passing the argument, and yes, sub_once is given after the callback function name just as you wrote it).

Anyway, with the flags the problem is solved.

Thank you!

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Would you mind giving an example of how you accomplished this in code?

Seanny123 gravatar imageSeanny123 ( 2013-11-20 19:18:24 -0500 )edit

answered 2014-05-21 23:49:53 -0500

Captcha gravatar image

Hi, i tried the given code but my program was stuck on the rospy.spin() procedure after calling the callback for the forst time ! is there a way i can exit from that blocking statement to a next statement ? Thanks

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answered 2016-09-07 01:25:12 -0500

siddian93 gravatar image

2 Steps : Flag set to 0 in the class __init__ method. Step 1 : I believe a better alternate is to define the call back method in a class along with the subscriber. And then setting a flag which will be set to one once the callback method has been called for the first time.

Step 2 : Once the Flag has been set 1, in the subscriber declaration block, return to the called block.

An Example is given below : (Tested Works)

class frontCam : def __init__(self): self.flag = 0 rospy.init_node("DFC") self.getVideo()

def getVideo(self) :
    self.image_sub = rospy.Subscriber('Subscribed TOPIC', Message_Type, self.callback )
    print 'Getting Video Feed . . . ' 
    if (self.flag == 1)
    try :
    except KeyboardInterrupt :
        print "Shutting Down"

def callback(self, data) :
    self.flag = 1

However if you guys want, can close the callback method after a certain period using the ropy.sleep() and the return statement as well.

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Asked: 2012-03-26 06:56:52 -0500

Seen: 6,454 times

Last updated: May 21 '14