Inserting objects with reference to object into list

Question

I've encountered a behavior in a Python program that I've been able to condense into a smaller program. I'm pretty sure I've encountered it and solved it and the past, but can't remember how.

Consider the following Python program:

import random

class B:
    def __init__(self):
        self.attr=random.randrange(0,10)

class A:
    def __init__(self):
        self.b=B()

list=[]
print "First pass"
for i in range(0,10):
    a=A()
    print "Random number in b: ",a.b.attr
    list.insert(0,a)

print "Second pass"
for l in list:
    print "Random number in b: ",a.b.attr

It typically outputs something such as:

First pass
Random number in b:  9
Random number in b:  7
Random number in b:  1
Random number in b:  5
Random number in b:  9
Random number in b:  7
Random number in b:  0
Random number in b:  6
Random number in b:  2
Random number in b:  7

Second pass
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7
Random number in b:  7

How do we solve this? I know it has to do with how Python copies object etc., but having:

import copy
list.insert(0,copy.deepcopy(a))

Doesn't solve it as one could expect.

Uh, in your second pass, you are displaying a.b.attr instead of l.b.attr. a is left over from the first pass loop, so you are seeing the constant output. — MJZ
– MJZ, Commented Dec 13, 2012 at 6:13

warvariuc · Accepted Answer · 2012-12-13 06:11:42Z

1

Maybe instead of

print "Second pass"
for l in list:
    print "Random number in b: ",a.b.attr

Use

print "Second pass"
for l in list:
    print "Random number in b: ", l.b.attr

answered Dec 13, 2012 at 6:11

warvariuc

60.1k45 gold badges183 silver badges234 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

bitgarden Over a year ago

I am an idiot. This solves my question, although I still have a similar issue; now to write a proper test program that reproduces the bug I'm experiencing.

bitgarden Over a year ago

Here's a basic test program that expresses the gist of the problem I was having: pastebin.com/MQMKpNhh - I've figured it though (for those stumbling on this: it's because values is initiated with an empty dict, which is mutable; you want your construction vars to be initiated with immutable values)

warvariuc Over a year ago

Glad you figured it out! Actually not "because values is initiated with an empty dict", but because you made values a class attribute, instead of creating it per instance inside __init__

Collectives™ on Stack Overflow

Inserting objects with reference to object into list

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related