Question about empty list for define while loop in python

def mergeSort(Data):
# result = []
# This location can't operate result.append(group1(pop(0)) at the while loop.

if len(Data) <= 1:
    return Data

mid = len(Data) // 2

group1 = mergeSort(Data[:mid])
group2 = mergeSort(Data[mid:])

result = []
# Why this postion?

while group1 and group2:
    if group1[0] < group2[0]:
        result.append(group1.pop(0))
    else:
        result.append(group2.pop(0))

while group1:
    result.append(group1.pop(0))
while group2:
    result.append(group2.pop(0))
return result

Hello. I have a doubt about this code. I defined function for making merge sorting algorithm. But the empty list for inserting values couldn't working when I input forward. I don't under reason why this list should be input in front of the while loop. please let me know.