creating a reverse method for a python list from scratch

def reverse(data_list):
    return data_list[::-1]

>> reverse([1,2,3,4,5])
[5, 4, 3, 2, 1]

By the time you are half-way through the list, you have swapped all the items; as you continue through the second half, you are swapping them all back to their original locations again.

Instead try

def reverse(lst):
    i = 0            # first item
    j = len(lst)-1   # last item
    while i<j:
        lst[i],lst[j] = lst[j],lst[i]
        i += 1
        j -= 1
    return lst

This can be used in two ways:

a = [1,2,3,4,5]
reverse(a)        # in-place
print a           # -> [5,4,3,2,1]

b = reverse(a[:]) # return the result of reversing a copy of a
print a           # -> [5,4,3,2,1]
print b           # -> [1,2,3,4,5]

You are changing the list that you iterate on it (data_list) because of that it's not working , try like this:

def reverse(data_list):
    length = len(data_list)
    s = length

    new_list = [None]*length

    for item in data_list:
        s = s - 1
        new_list[s] = item
    return new_list