Appending list in Python

Question

I am trying to create a method that allows each instance of the class to be "neighbors" of each other. If Entity A adds B as a neighbor, then B is in A's neighbor_list. However, as the output below shows, B is automatically added to B's neighbor list and that's not the desired behavior - B's neighbor list should be empty. Any thoughts?

Output:

 a's neighbor list element: b
 b's neighbor list element: b

Code:

 class Entities:
     neighbor_list = []
     name = ''

     def __init__(self,name):
         self.name = name

     def add (self, neighbor):
         self.neighbor_list.append(neighbor)  

 a = Entities ('a')
 b = Entities ('b')
 a.add(b)
 print "a's neighbor list element: %s" % a.neighbor_list[0].name
 print "b's neighbor list element: %s" % b.neighbor_list[0].name

Why does it surprise you that mutating a class attribute means that all instances can see the mutation? — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Jan 11, 2014 at 0:51
@IgnacioVazquez-Abrams: Well, presumably what surprises him is that neighbor_list is a class member rather than an instance member, because he assumed that Python did members exactly like, say, Java. — abarnert
– abarnert, Commented Jan 11, 2014 at 0:52

jonrsharpe · Accepted Answer · 2014-01-11 00:51:10Z

6

Make neighbor_list an instance, not class, attribute:

class Entities(object):
    # not here
    def __init__(self, name):
        self.name = name
        self.neighbor_list = [] # here

Class attributes, defined outside the instance methods, are shared by all instances of the class.

answered Jan 11, 2014 at 0:51

jonrsharpe

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