Simple command line tool for computing S3 ETag

e3tag.py

import hashlib

"""
Calculate Amazon S3 ETag for one or more files.

Example:
    python e3tag.py files/1.txt files/2.txt files/3.txt
"""


def calculate_s3_etag(file_path, chunk_size=8 * 1024 * 1024):
    """Calculate Amazon S3 ETag hash for given file."""
    # https://stackoverflow.com/a/43819225/892534

    md5s = []

    with open(file_path, 'rb') as fp:
        while True:
            data = fp.read(chunk_size)
            if not data:
                break
            md5s.append(hashlib.md5(data))

    if len(md5s) < 1:
        return '"{}"'.format(hashlib.md5().hexdigest())

    if len(md5s) == 1:
        return '"{}"'.format(md5s[0].hexdigest())

    digests = b''.join(m.digest() for m in md5s)
    digests_md5 = hashlib.md5(digests)
    return '{}-{}'.format(digests_md5.hexdigest(), len(md5s))


if __name__ == '__main__':
    import argparse
    import sys

    argp = argparse.ArgumentParser(description='Calculate S3 ETag for file')
    argp.add_argument('filepath', nargs='*', help='Files to hash')
    argp.add_argument('-k', '--chunk-size', type=int, default=8 * 1024 * 1024)
    argp.add_argument(
        '-r', '--reverse', action='store_true', help='Show file path first'
    )

    opts = argp.parse_args()

    for fpath in opts.filepath:
        etag = calculate_s3_etag(fpath, opts.chunk_size)

        if opts.reverse:
            sys.stdout.write(f'{fpath}  {etag}\n')
        else:
            sys.stdout.write(f'{etag}  {fpath}\n')