Python Spin Once Equivalent

asked 2013-12-13 12:56:33 -0500

atp
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updated 2014-01-28 17:06:37 -0500

ngrennan
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Is there an equivalent to spin_once (C++) in ROS for Python?

In C++ I have a loop with spin_once and during each iteration of the loop the code checks a couple of global variables for changes and then does some action based on the value of those global variables. The global variables are based on messages from other nodes' topics to which my C++ ROS node is subscribed. How would this code structure translate into Python?

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answered 2013-12-16 00:07:25 -0500

demmeln
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No, there is no spin_once in rospy. In rospy the subscriber's callbacks are executed in separate threads. Google quickly produces these old questions including answers:

To translate your loop from C++ you could either check periodically in a loop in the main thread, or check in each of the callbacks. Either way you likely need some thread synchronization.

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Nabble doesn't work any longer, but I was able to find this one (it's nice the nabble url contained some searchable text, though this once has a date encoded) http://lists.ros.org/lurker/message/2...

You don't need spinOnce in rospy as the I/O is multithreaded. You can use the Rate object to contrain the rate of processing, though it does not have the same effect as spinOnce in roscpp. - Ken

lucasw ( 2022-01-12 10:48:03 -0500 )edit

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answered 2019-06-12 09:00:07 -0500

byque

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The rospy function wait_for_message() allowed me to receive one message from topic in Python.

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Exactly what I wanted, thanks.

I have used below code:

msg = rospy.wait_for_message("/camera/depth/color/points", PointCloud2, timeout=None)

askkvn ( 2020-05-19 03:24:48 -0500 )edit

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Please note that wait_for_message does the following internally: it subscribes to the topic, waits for a message, then unsubscribes. This is only useful if you want to get an odd message once in a while. Don't do this in a loop - it will stress the ros master and all nodes publishing on that topic, and you'll lose all the messages between unsubscribing and subscribing again.

Martin Günther ( 2020-10-02 03:37:38 -0500 )edit

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