python merge sort

def merge(array_1 , array_2):
    temp =  []
    
    while (array_1 and array_2):
        if array_1[0] > array_2[0]:
            temp.append(array_2[0])
            array_2 = array_2[1:]
        else:
            temp.append(array_1[0])
            array_1 =  array_1[1:]
    
    if array_1:
        temp =  temp + array_1
    elif array_2:
        temp = temp + array_2
        
    return temp
        

def merge_sort(nums):
    if not nums:
        raise ValueError("")
    
    if len(nums) == 1:
        return nums
    
    mid = len(nums) //  2
    slice_1 =  nums[0:mid]
    slice_2 =  nums[mid:]
    
    array_1 =  merge_sort(slice_1)
    array_2 =  merge_sort(slice_2)
    
    return merge(array_1, array_2)
    
print(merge_sort([3,1,2,6]))