Wrong list being returned in python

Question

def mainCall(nodeGroup):

    maxScore = -9999999
    maxPart = []
    tempPartition = []
    tempScore = 0.0

    for j in range(2, 1+nodes/2):

        nodeGroup = chooseInitPartition(j, nodeGroup)
        tempScore, tempPartition = runKL(edgeList, nodeGroup, rounds)
        print 'temp score', tempScore
        print 'temp part', tempPartition, "\n"
        if(maxScore < tempScore):
            maxScore = tempScore
            maxPart = tempPartition
            print "max score", maxScore
            print "maxpart", maxPart, "\n"

     print 'before ret max part', maxPart
     return maxScore, maxPart

finalScore, finalPartition = mainCall(nodeGroup)

I am having a problem with the above code. Everything seems fine till the for loop ends, but after that instead of printing the maxPart value at the line print 'before ret max part', it prints the last value of tempPartition (both represent a list of numbers). The print statements confirm that the if condition is not satisfied every time, especially the last few runs in the loop. So I don't see how the value of tempPartition is being transferred to maxPart. Please help me with this. It is driving me crazy. I am sure I am missing something simple. Thanks!

EDIT:

Adding code for runKL

def runKL(edgeList, nodeGroup, rounds):

    nodeSwap = [0]*nodes
    maxLogLScore = 0.0
    edgeListLen = len(edgeList)

    networkPartitionStore = []
    logLScores = []

    #Reset count
    count = 0

    #Start main loop
    for i in range(rounds):
        #mark all vertices as unswapped
        for j in range(len(nodeSwap)):
            nodeSwap[j] = 0


        while(count < 100):
            #Choose one edge uniformly randomly
            randNum = random.uniform(0,1)
            edge = edgeList[int(math.floor(edgeListLen*randNum))]
            node1 = int(edge[0]) - 1
            node2 = int(edge[1]) - 1

            if((nodeGroup[node1] == nodeGroup[node2]) or (nodeSwap[node1] == 1) or (nodeSwap[node2] == 1)):
                count += 1
                continue
            else:
                #swap groups among nodes
                temp = nodeGroup[node1]
                nodeGroup[node1] = nodeGroup[node2]
                nodeGroup[node2] = temp

                #mark vertices as swapped
                nodeSwap[node1] = 1
                nodeSwap[node2] = 1

                #calculate likelihood
                logLScore = logLikelihood(edgeList, nodeGroup)
                logLScores.append(logLScore)

                #store network
                networkPartitionStore.append(nodeGroup)

                #reset count value
                count = 0

        #end while loop

        #Choose the index of the maximum likelihood score
        maxLogLScore = max(logLScores)
        index = logLScores.index(maxLogLScore)
        #print 'max score', modularityScores[index]

        #Choose the corresponding network partition i.e. the way the groups have been assigned to the nodes
        nodeGroup = networkPartitionStore[index]

        #Reset partition storage list and modularity scores
        networkPartitionStore = []
        logLScores = []

        #Store initial network partition structure and modularity score. 
        networkPartitionStore.append(nodeGroup)
        logLScores.append(maxLogLScore)

    return maxLogLScore, nodeGroup

thefourtheye · Accepted Answer · 2013-11-16 14:37:45Z

5

When you say

maxPart = tempPartition

you are not creating a copy of tempPartition but you are making the maxPart also to point to the same list that tempPartition points to. To really make a copy, you can use slicing notation like this

maxPart[:] = tempPartition

or

maxPart = tempPartition[:]

The difference between maxPart[:] = tempPartition and maxPart = tempPartition[:] is that the former mutates the maxPart and copies all the values from tempPartition to maxPart and latter creates a new list with a copy of all the data in tempPartition and makes the maxPart points to the newly created list.

edited Nov 16, 2013 at 14:37

answered Nov 16, 2013 at 14:27

thefourtheye

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10 Comments

Maxime Chéramy Over a year ago

maxPart[:] = tempPartition or maxPart = tempPartition[:] ?

thefourtheye Over a year ago

@Maxime They both work. Please check the explanation I added in the answer.

FieryDragon87 Over a year ago

@thefourtheye Thank you very much for your answer. I used maxPart[:] = tempPartition and it worked.

Blckknght Over a year ago

I'd say that the fix described in this answer shouldn't be necessary. The real bug is in the runKL function, which is returning the same list over and over (with different values in it each time, I assume). I'd fix that instead, rather than working around it.

Blckknght Over a year ago

@thefourtheye: Well, none of the code we can see mutates the list, so it must be getting mutated somehow by the part we can't see. I'm just guessing that it happens in runKL, since that's where we get the list from.

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