Progressive sha1sum in Python

progressive_sha1sum.py

#!/usr/bin/env python

import sys
import os
import hashlib
from argparse import ArgumentParser


def main(args):
    parser = ArgumentParser()
    group = parser.add_mutually_exclusive_group(required=True)
    group.add_argument('--create', metavar='PATH', help='Create manifest with paths from stdin')
    group.add_argument('--update', metavar='PATH')
    parsed = parser.parse_args()
    stdin = getattr(sys.stdin, 'buffer', sys.stdin)
    stdout = getattr(sys.stdout, 'buffer', sys.stdout)
    blank = b' ' * 42
    if parsed.create:
        assert not os.path.exists(parsed.create)
        with open(parsed.create, 'wb') as f:
            f.seek(0, 2)
            for line in stdin:
                f.write(blank)
                f.write(line)
    if parsed.update:
        with open(parsed.update, 'r+b') as f:
            while True:
                line = f.readline()
                if not line:
                    break
                if line.startswith(blank):
                    path = line[len(blank):-len(b'\n')]
                    sha1 = sha1sum(path)
                    f.seek(-len(line), 1)
                    f.write(sha1.encode())
                    f.seek(-len(sha1), 1)
                    stdout.write(f.readline())


def sha1sum(path):
    sha1 = hashlib.sha1()
    with open(path, 'rb') as f:
        for n in iter(lambda: f.read(1024 ** 2), b''):
            sha1.update(n)
    return sha1.hexdigest()


if __name__ == '__main__': main(sys.argv)