Why is swapping using function and swapping using assignment produce in different results?

This is the code for Quick sort in which i used assignment to swap the values.

def partion(arr,low,high):
  partion_index=low
  pivot=arr[partion_index]

  while (low < high):
    while low < len(arr) and (arr[low]<=pivot):
      low+=1
    while (arr[high]>=pivot):
      high-=1
    if (low < high):
      arr[low],arr[high]=arr[high],arr[low]

  pivot,arr[high]=arr[high],pivot
  return high   

def quickSort(arr,low,high):
  if(low <high):
    p=partion(arr,low,high)
    quickSort(arr,low,p-1)
    quickSort(arr,p+1,high)

This is producing the wrong results. When i tried using swap function for the same job it worked.

def swap(a, b, arr):
    if a!=b:
        tmp = arr[a]
        arr[a] = arr[b]
        arr[b] = tmp

def partion(arr,low,high):
  partion_index=low
  pivot=arr[partion_index]

  while (low < high):
    while low < len(arr) and (arr[low]<=pivot):
      low+=1
    while (arr[high]>=pivot):
      high-=1
    if (low < high):
      swap(low, high, arr)
      
  swap(partion_index, high, arr)
  return high

Why is it so?