A very simple naive VCS in ~130 lines of Python

simplevcs.py

#!/usr/bin/env python


"""Simple VCS"""


from hashlib import sha256
from json import dumps, loads


from argparse import ArgumentParser


def sha256sum(filename, bufsize=(1024 * 1024)):
    checksum = sha256()

    with open(filename, "r") as f:
        checksum.update(f.read(bufsize))

    return checksum.hexdigest()


class DB(object):

    def __init__(self):
        try:
            self.data = loads(open(".db", "r").read())
        except IOError:
            self.data = {}

    def __contains__(self, filename):
        return filename in self.data

    def add(self, filename):
        self.data[filename] = [sha256sum(filename)]

    def remove(self, filename):
        del self.data[filename]

    def update(self, filename):
        self.data[filename].append(sha256sum(filename))

    def status(self):
        for filename, hashes in self.data.iteritems():
            if sha256sum(filename) != hashes[-1]:
                yield "M", filename

    def commit(self):
        for status, filename in self.status():
            self.update(filename)

    def save(self):
        with open(".db", "w") as f:
            f.write(dumps(self.data))


def add(db, args):
    if args.file in db:
        print("File {} already added!".format(args.file))
        return -1

    db.add(args.file)


def status(db, args):
    for status, filename in db.status():
        print("{0:<4} {1:s}".format(status, filename))
    return 0


def commit(db, args):
    db.commit()
    return 0


def parse_args():
    parser = ArgumentParser(description=__doc__)

    parser.add_argument(
        "-q", "--quiet", dest="quiet",
        action="store_true", default=False,
        help="Enable quiet mode",
    )

    subparsers = parser.add_subparsers(
        title="Commands",
        description="Available Commands",
        help="Description"
    )

    # add
    add_parser = subparsers.add_parser(
        "add",
        help="Add a file"
    )
    add_parser.set_defaults(func=add)

    add_parser.add_argument(
        "file", metavar="FILE", type=str,
        help="File to add"
    )

    # status
    status_parser = subparsers.add_parser(
        "status",
        help="Display status"
    )
    status_parser.set_defaults(func=status)

    # commit
    commit_parser = subparsers.add_parser(
        "commit",
        help="Commit changes"
    )
    commit_parser.set_defaults(func=commit)

    return parser.parse_args()


def main():
    args = parse_args()

    db = DB()

    try:
        raise SystemExit(args.func(db, args))
    finally:
        db.save()


if __name__ == "__main__":
    main()

Demo:

$ ./simplevcs.py status
$ echo "foo" > foo.txt
$ ./simplevcs.py add foo.txt
$ ./simplevcs.py status
$ echo "bar" >> foo.txt
$ ./simplevcs.py status
M    foo.txt
$ ./simplevcs.py commit
$ ./simplevcs.py status

cat .db:

{"foo.txt": ["b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c", "d78931fcf2660108eec0d6674ecb4e02401b5256a6b5ee82527766ef6d198c67"]}

This in response to SO question How to compare two versions of same file (old and new), and detect if there were some changes?