How is the variable changed in a function without being called?

Question

def generateNew(data):

    for i in range(len(data)-1):
        data[i].append(data[i+1][2])
    data[-1].append(oP)

    for i in range(len(data)-1):
        data[i].append(data[i+1][0])
        data[i].append(data[i+1][1])
    data[-1].append(hlP[0])
    data[-1].append(hlP[1])

    for i in range(len(data)-1):
        data[i].append(data[i+1][4])
    data[-1].append(volume)

    new = [hlP[0], hlP[1], oP, cP, volume]
    print new
    return new

dataSet = []
for i in f.readlines():
    i = i.split(',')
    x = [float(j) for j in i[1:-1]]
    x.append(int(i[-1]))
    dataSet.append(x)
dataSet.reverse()
mData = dataSet # for next loop, append new set

for i in range(10):
    temp = predictNew(mData)
    print dataSet
    dataSet.append(temp)
    mData = dataSet

Here is my code. dataSet is like [[1,2,3,4],[1,2,3,4]....], then I assign it to mData and pass it into the function generateNew(I remove some of the detail that are not necessary) and the list is now like[[1,2,3,4,5,6],[1,2,3,4,5,6]....]. However, I think neither dataSet nor 'mData' should be changed since all the changes happen inside the function, and the fact is not like that. Can anyone tell me why and improve that? Thank you in advance.

Possible duplicate of [In Python, why can a function modify some arguments as perceived by the caller, but not others?](http://stackoverflow.com/questions/575196/in-python-why-can-a-function-modify-some-arguments-as-perceived-by-the-caller) — CaptainTrunky, Mar 23 '17 at 02:58

score 0 · Accepted Answer · answered Mar 23 '17 at 04:15

If I understand you correctly, you are getting dataSet to change when you don't expect it to change. If this is the problem, then this issue is occurring in this line:

mData = dataSet # for next loop, append new set

To rectify this, I would look into Python's copy and deepcopy. When you 'copy' a list this way, it doesn't actually make another copy, but rather references the original. For example:

x = [1]
y = x
x.append(2)
x     #expected output is [1, 2] because we added 2 to this.
>>>[1, 2]
y     #y changes, even though it wasn't directly modified.
>>>[1, 2]

The easiest way to fix this is to use splice copying: y = x[:] However, this won't solve your issue because you have lists within lists. What you would need (I think) is deepcopy from the copy module. Let me know if I helped!

How is the variable changed in a function without being called?

1 Answers1