Can anyone please help me in understanding the following python code

[bhk755 at gmail.com]

Code :
-----
def mergeSort(alist):
    print("Splitting ",alist)
    if len(alist)>1:
        mid = len(alist)//2
        lefthalf = alist[:mid]
        righthalf = alist[mid:]

        mergeSort(lefthalf)
        mergeSort(righthalf)

        i=0
        j=0
        k=0
        while i<len(lefthalf) and j<len(righthalf):
            if lefthalf[i]<righthalf[j]:
                alist[k]=lefthalf[i]
                i=i+1
            else:
                alist[k]=righthalf[j]
                j=j+1
            k=k+1

        while i<len(lefthalf):
            alist[k]=lefthalf[i]
            i=i+1
            k=k+1

        while j<len(righthalf):
            alist[k]=righthalf[j]
            j=j+1
            k=k+1
    print("Merging ",alist)

alist = [54,26,93,17,77,31,44,55,20]
mergeSort(alist)
print(alist)

Output:
-------
('Splitting ', [54, 26, 93, 17, 77, 31, 44, 55, 20])
('Splitting ', [54, 26, 93, 17])
('Splitting ', [54, 26])
('Splitting ', [54])
('Merging ', [54])
('Splitting ', [26])
('Merging ', [26])
('Merging ', [26, 54])
('Splitting ', [93, 17])
('Splitting ', [93])
('Merging ', [93])
('Splitting ', [17])
('Merging ', [17])
('Merging ', [17, 93])
('Merging ', [17, 26, 54, 93])
('Splitting ', [77, 31, 44, 55, 20])
('Splitting ', [77, 31])
('Splitting ', [77])
('Merging ', [77])
('Splitting ', [31])
('Merging ', [31])
('Merging ', [31, 77])
('Splitting ', [44, 55, 20])
('Splitting ', [44])
('Merging ', [44])
('Splitting ', [55, 20])
('Splitting ', [55])
('Merging ', [55])
('Splitting ', [20])
('Merging ', [20])
('Merging ', [20, 55])
('Merging ', [20, 44, 55])
('Merging ', [20, 31, 44, 55, 77])
('Merging ', [17, 20, 26, 31, 44, 54, 55, 77, 93])
[17, 20, 26, 31, 44, 54, 55, 77, 93]


Question:
---------
Function mergeSort is called only once, but it is getting recursively executed after the printing the last statement "print("Merging ",alist)". But don't recursion taking place except at these places "mergeSort(lefthalf), mergeSort(righthalf)"

Sometimes the function execution directly starts from i=0,j=0,k=0 . Not sure how.

Can anyone please help me out here?