rospy init_node() inside imported file

Question

I'd like to perform a rospy.init_node() inside a file imported by another file. However, this doesn't seem to work - the program hangs whenever I try this.

Example (broken, both files in same folder):

main.py:

import test

test.py:

   import rospy
   rospy.init_node("node_name")

Running python main.py results in the program hanging. However, running python test.py works.

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Originally posted by zplizzi on ROS Answers with karma: 1 on 2017-07-17

Post score: 0

Answer 1

I have a similar project, it works like that:

1. Inside the project folder, I have a folder called packages, with a init.py inside. It's empty, just to make the module available

2. In the packages folder yet, a file called initialize.py, where I have:

   import rospy as _rospy from std_msgs.msg import String as _String, Empty as _Empty

   def init():

       _rospy.init_node('controller', anonymous=True)
       global _sub
       _sub = _rospy.Subscriber("/model_name", _String, _nameCallback)
       global _pub
       _pub = _rospy.Publisher(_controllerName+'/encoder_updated',_Empty)
   

3. In the root folder, my script that imports the initialize library:

   import packages.initialize

   initialize.init()

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Originally posted by marcoarruda with karma: 541 on 2017-07-18

This answer was ACCEPTED on the original site

Post score: 0

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Original comments

Comment by zplizzi on 2017-07-18:
Yep, this works - since the init_node() call is in a method called by the main file, it seems to work. What I can't figure out is why it doesn't work in the example I described - where init_node() is called implicitly during the import. This method works, but still curious why the other doesn't.

Comment by marcoarruda on 2017-07-18:
I think the answers for this question https://stackoverflow.com/questions/6523791/why-is-python-running-my-module-when-i-import-it-and-how-do-i-stop-it can explain better. The question is the opposite of yours, but there are some explanation there.

Comment by zplizzi on 2017-07-18:
Yep, I understand how that works. Python will run all the code in an imported file upon import. What my question is asking is why the init_node() function doesn't work when run in that way (versus being run directly from the main script).