A script to calculate adler32 checksum of given files

adler32.py

#!/usr/bin/env python
'''A script to calculate adler32 checksum of given files'''

BLOCKSIZE=256*1024*1024
import sys
from zlib import adler32

for fname in sys.argv[1:]:
 asum = 1
 with open(fname,"rb") as f:
   while True:
     data = f.read(BLOCKSIZE)
     if not data:
       break
     asum = adler32(data, asum)
     if asum < 0:
       asum += 2**32

 print(hex(asum)[2:10].zfill(8).lower(), fname)

Could you please explain this part?

if asum < 0:
       asum += 2**32

EDIT: Is it related to this?

for python3.8

def adler32sum(filepath):
    from zlib import adler32
    BLOCKSIZE = 256*1024*1024
    asum = 1
    with open(filepath, 'rb') as f:
        while (data := f.read(BLOCKSIZE)):
            # print('read len:', len(data))
            asum = adler32(data, asum)
    return asum