What is the reason, sometimes the parameters in class does not defined in __init__ but added as instance variables?

Question

Why does these attributes (commented) does not included in the init ?

class RRT(object):
  def __init__(self, start, stop, obstacle_list, rand_area):
    self.start = Node(start[0], start[1])
    self.stop = Node(stop[0], stop[1])
    self.min_rand = rand_area[0]        
    self.max_rand = rand_area[1]
    self.exapandDis = 1.0            # just like this one
    self.goalSampleRate = 0.05       # just like this one
    self.maxIter = 500               # just like this one
    self.obstacle_list = obstacle_list

Answer 1

Have you ever heard of constructor? The __init__ is a constructor in Python.

A constructor is used to initialized member variables of a class. It is not necessary for a constructor to initialize each and every variable.

Sometimes, we want a constructor to initialize variables, which aren't passed in as parameters, with some default values.

It is not necessary for a construtor to only intialize the variables which are passed to it!

Thus, your code will initialize: exapandDis, goalSampleRate, maxIter with default values 1.0, 0.05, 500

Answer 2

You can give parameters default values. These are called "keyword arguments".

class RRT(object):
  def __init__(self, start, stop, obstacle_list, rand_area,
        exapandDis=1.0, goalSampleRate=.05, maxIter=500):
    self.start = Node(start[0], start[1])
    self.stop = Node(stop[0], stop[1])
    self.min_rand = rand_area[0]        
    self.max_rand = rand_area[1]
    self.exapandDis = expandDis                # just like this one
    self.goalSampleRate = goalSampleRate       # just like this one
    self.maxIter = maxIter                     # just like this one
    self.obstacle_list = obstacle_list

Now, when someone creates an instance of the class, they must supply the positional arguments start, stop, obstacle_list, rand_area but the others are optional. For instance, if you want to keep defaults expandDis and goalSapleRate but want a different maxIter, you would do

RRT(1, 100, [], 66, maxIter=1000)