Recursion and lists in Python

Question

Here is a simplified version of what I'm trying to do:

class a():
    Requirement = 0
    def func(self, oldlist, x):
        newlist = [None]*3
        newlist = oldlist
        newlist[x]  = b()
        print "Class a"
        g(newlist)


class b():
    Requirement = 1


def g(list):
    for i in range(3):
        if list[i].Requirement==0:
            list[i].func(list,i)

Initiallist=[None]*3
Initiallist[0]=a()
Initiallist[1]=b()
Initiallist[2]=a()
g(Initiallist)

Instead of trying to express what I expect in words, I made some diagrams that express what in my mind should happen:

enter image description here

Which would imply that the function inside class a should be called 4 times. However, it only gets called 2 times, so it seems that this is happening:

enter image description here

I don't understand why this is happening or how I should fix it.

Please correct your indentation of the code in the question. — Some programmer dude
– Some programmer dude, Commented Feb 1, 2013 at 12:37
You'll want to correct your formatting, since this is critical in Python for your program to run as intended. It lets people run it more easily and try it out on their systems. — pseudoramble
– pseudoramble, Commented Feb 1, 2013 at 12:39
Yes, I'm sorry. I just fixed it. Thank you for adding the pictures — RobVerheyen
– RobVerheyen, Commented Feb 1, 2013 at 12:43

sloth · Accepted Answer · 2013-02-01 12:44:58Z

5

Not sure, but I think your problem is this line:

newlist = oldlist

I guess you want to copy the list (and not altering oldlist), so you should simply use:

newlist = oldlist[:]

So changing func to

def func(self, oldlist, x):
    newlist = oldlist[:]
    newlist[x] = b()
    print "".join(x.__class__.__name__ for x in newlist)
    g(newlist)

prints

bba
bbb
abb
bbb

answered Feb 1, 2013 at 12:44

sloth

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1 Comment

alvas Over a year ago

remember to tick this as answer if you like it =)

ellak · Accepted Answer · 2013-02-01 12:45:25Z

0

In func you need to create a copy of oldlist, otherwise it will modify the same list.

answered Feb 1, 2013 at 12:45

ellak

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