Fibonacci action server

Course Support ROS Basics In 5 Days (Python)
1

Hi,

Why aren’t feedback and results variables defined within class initialization?
Thanks

2

Hi @gianmauro.fontana,

Could you please provide the code snippet you are referring to?

4

#! /usr/bin/env python
import rospy

import actionlib

from actionlib_tutorials.msg import FibonacciFeedback, FibonacciResult, FibonacciAction

class FibonacciClass(object):

_feedback = FibonacciFeedback()
_result = FibonacciResult()

def init(self):
# creates the action server
self._as = actionlib.SimpleActionServer(“fibonacci_as”, FibonacciAction, self.goal_callback, False)
self._as.start()
…

The feedback and results variables are defined outside the class initialization. Why?

5

Oh…that’s Python magic :wink:.

Those are class variables (versus instance variables ). Initialization applies only to instance variables, which can vary from one instance of the class to another. Class variables, on the other hand, are variables that don’t need to change accross instances (usually they are constants) and can be accessed directly from the class or from any instance of the class. So you can do the following:

# Directly from anywhere where FibonacciClass is available
feedback = FibonacciClass._feedback
result = FibonacciClass._result

# From an instance method of the class
def instance_method(self):
   result = self._result
   feedback = self._feedback

# From a class method of the class
@classmethod
def class_method(cls):
   result = cls._result
   feedback = cls._feedback

Is it clear?