append method in python not working as expected in nested for loops

Question

I want to append a list into another list using nested for loops but the code is not working as expected

def apend():
    ls = []
    numbers = [0,0]
    
    for num1 in range(3):
        for num2 in range(2):
            numbers[0] = num1
            numbers[1] = num2
            ls.append(numbers)
    print(ls)

apend()

I expect the output to be: [[0,0],[0,1],[0,2],1,0],[1,1],[1,2]]

but i get this output: [[1, 2], [1, 2], [1, 2], [1, 2], [1, 2], [1, 2]]

I think it's related to docs.python-guide.org/writing/gotchas/#late-binding-closures . Maybe try to instantiate the numbers in your inner for loop. — Tom Wojcik
– Tom Wojcik, Commented Jun 27, 2020 at 15:06
You are appending a reference to the same list in each iteration. Just append [num1, num2] instead. — Mark
– Mark, Commented Jun 27, 2020 at 15:06
ls keeps it as reference to the same numbers but you should create new number before you put it in ls. OR do ls.append([num1, num2]) BTW: run your code on pythontutor.com and you will see references as arrows on visualization - and all arrows with refere to the same list number — furas
– furas, Commented Jun 27, 2020 at 15:20

furas · Accepted Answer · 2020-06-27 18:04:50Z

2

If you run your code on http://pythontutor.com/ then you see

All elements in list ls keep reference to the same list numbers and this makes problem.

You have to create new list numbers = [0,0] inside for-loop

for num1 in range(3):
    for num2 in range(2):
        numbers = [0,0]
        numbers[0] = num1
        numbers[1] = num2
        ls.append(numbers)

Or simply do it without numbers

for num1 in range(3):
    for num2 in range(2):
        ls.append([num1, num2])

edited Jun 27, 2020 at 18:04

answered Jun 27, 2020 at 15:29

furas

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Comments

noob_to_so · Accepted Answer · 2020-06-27 15:21:10Z

1

Just change the numbers to numbers[:] and you will get you output as expected.

ls.append(numbers) means you are appending the reference of the list . so when the list changes , all instace changes . when you do ls.append(numbers[:]) this appends a copy of numbers.

def apend():
    ls = []
    numbers = [0,0]
    
    for num1 in range(3):
        for num2 in range(2):
            numbers[0] = num1
            numbers[1] = num2
            ls.append(numbers[:])
    print(ls)

apend()

answered Jun 27, 2020 at 15:21

noob_to_so

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Comments

hochae · Accepted Answer · 2020-06-27 15:23:09Z

0

You are appending the list named numbers in the loop. Just append a new list object as below.

def apend():
    ls = []
    numbers = [0,0]
    
    for num1 in range(3):
        for num2 in range(2):
            numbers[0] = num1
            numbers[1] = num2
            ls.append(list(numbers)) # append a new list object 
    print(ls)

By the way, I'd like to use list comprehension for this function.

def append():
    ls = [[x, y] for x in range(3) for y in range(2)]
    print(ls)

answered Jun 27, 2020 at 15:23

hochae

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Comments

Prabhu Teja · Accepted Answer · 2020-06-27 15:23:55Z

0

You can do it it in this way :

def apend():
    ls=[]
    numbers=[0,0]
    for num1 in range(3):
        for num2 in range(2):
            numbers[0]=num1
            numbers[1]=num2
            ls.append(num.copy())
    print(ls)
apend()

answered Jun 27, 2020 at 15:23

Prabhu Teja

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