set_s3_acl.py

#!/usr/bin/env python
import argparse
import sys

import boto3

client = boto3.client('s3')

def main(args):
    bucket = args.bucket
    prefix = args.prefix
    kwargs = {'Bucket': bucket, 'Prefix': prefix}
    failures = []
    while_true = True
    while while_true:
        resp = client.list_objects_v2(**kwargs)
        for obj in resp['Contents']:
            try:
                print(obj['Key'])
                set_acl(bucket=bucket, key=obj['Key'])
                kwargs['ContinuationToken'] = resp['NextContinuationToken']
            except KeyError:
                while_true = False
            except Exception:
                failures.append(obj["Key"])
                continue

    print "failures :", failures

def set_acl(bucket, key):
    client.put_object_acl(
        GrantFullControl="id=%s" % get_account_canonical_id,
        Bucket=bucket,
        Key=key
    )

def get_account_canonical_id():
    return client.list_buckets()["Owner"]["ID"]

if __name__ == "__main__":
    parser = argparse.ArgumentParser(
        description="Set S3 ACL on bucket to current AWS account owner"
    )
    parser.add_argument(
        "--bucket",
        help="<required> S3 Bucket name.",
        required=True
    )
    parser.add_argument(
        "--prefix",
        help="<required> S3 prefix to set permissions recursively on.",
        required=True
    )
    main(parser.parse_args())

Thank you for this! It gave me the starting point I needed.