for loop appends string, but doesn't concatenate

Question

I have this 2d list. I want to derive the cam models with most units sold, and the array has 2 models tied for that. I've defined a function to return the quantity and index of the brands. The quantity and index are getting appended to a new list, but when I try to concatenate the values from the original 2d list, the concatenation is not successful and it is only showing the name of the first model. Can someone explain what's going wrong here?

UnitSold = [['Dash Cam Model', 'SJ Branch', 'PJ Branch', 'KL Branch'], ['RS Pro with GPS', 5, 4, 3],
            ['Transcend Drive Pro', 2, 2, 3], ['H203 1080P', 3, 2, 5], ['Pioneer', 4, 5, 3]]
def maxItem():
    n = 0
    k: list = []
    for i in range(1, len(UnitSold)):
        m = 0
        for j in range(1, len(UnitSold[i])):
            m += UnitSold[i][j]
        if m >= n:
            n = m
            k.append(n)
            k.append(i)
            return k
    return k

This method works and the list has 4 values appended to it when you print this function.

Whereas this method only prints 2 values, the total quantity and the model name which loads earlier.

def maxItem():
    n = 0
    k: list = []
    str = ""
    for i in range(1, len(UnitSold)):
        m = 0
        for j in range(1, len(UnitSold[i])):
            m += UnitSold[i][j]
        if m >= n:
            n = m
            str += UnitSold[i][0]
            return str, n
    return str, n

Answer 1

I'm posting this as an answer because it makes it easier to explain.

Here is your first example with my comments on what is happening.

UnitSold = [['Dash Cam Model', 'SJ Branch', 'PJ Branch', 'KL Branch'], ['RS Pro with GPS', 5, 4, 3],
            ['Transcend Drive Pro', 2, 2, 3], ['H203 1080P', 3, 2, 5], ['Pioneer', 4, 5, 3]]
def maxItem():
    n = 0
    k: list = []

    # this loop iterates from 1 through 4
    for i in range(1, len(UnitSold)):
        m = 0

        # In the first iteration of the above loop
        # when i == 1 it hits this loop 
        # which iterates from 1 through 3
        # counting the integers in this list
        # ['RS Pro with GPS', 5, 4, 3]
        for j in range(1, len(UnitSold[i])):
            m += UnitSold[i][j]

        # after this loop is completed it checks if
        # m (which is equal to 12) is more
        # or greater than n (which is equal to 0)
        if m >= n:
            # it then assigns 12 to n
            # appends 12 and i (which is equal to 1)
            # to an empty list
            # now your list looks like this [12, 1]
            n = m
            k.append(n)
            k.append(i)

            # it then hits this return which ends your function
            # and hands back the k list with 2 elements
            # as it always will on the first iteration of the
            # outer loop
            return k
    # this return is never hit
    return k

print(maxItem())

Here is your second example which essentially does the same thing in a slightly different way. The only difference is you hand back a Tuple rather than a list, and you hand back the value of the first element rather than the first element.

UnitSold = [['Dash Cam Model', 'SJ Branch', 'PJ Branch', 'KL Branch'], ['RS Pro with GPS', 5, 4, 3],
            ['Transcend Drive Pro', 2, 2, 3], ['H203 1080P', 3, 2, 5], ['Pioneer', 4, 5, 3]]
def maxItem():
    n = 0
    k: list = []
    str = ''

    # this loop iterates from 1 through 4
    for i in range(1, len(UnitSold)):
        m = 0

        # In the first iteration of the above loop
        # when i == 1 it hits this loop 
        # which iterates from 1 through 3
        # counting the integers in this list
        # ['RS Pro with GPS', 5, 4, 3]
        for j in range(1, len(UnitSold[i])):
            m += UnitSold[i][j]

        # after this loop is completed it checks if
        # m (which is equal to 12) is more
        # or greater than n (which is equal to 0)
        if m >= n:
            # it then assigns 12 to n
            # concatenates 'RS Pro with GPS' 12 onto an empty string
            # now your string looks like this 'RS Pro with GPS'
            n = m
            str += UnitSold[i][0]

            # it then hits this return which ends your function
            # and hands back an unnamed Tuple with 2 elements
            # so essentially it's doing this
            # first_item = (str, n)
            # return first_item
            return str, n
    # this return is never hit
    return str, n

print(maxItem())

As a side note, you shouldn't name a variable str as that is already used as a builtin type (meaning string).

It would also be helpful in seeing how you are calling this function.