Quick and dirty way to output the sum of all multipart uploads in an s3 bucket, optionally limiting the output to objects that match a given prefix.

bucket_mpu.py

import boto3
from sys import argv, exit


def get_bucket_mpus_size(bucket_name, object_prefix=''):
    aws_session = boto3.session.Session() # Uses either EC2/ECS metadata, ~/.aws/credentials file, or env vars for auth.
    s3 = aws_session.resource('s3')

    bytes_per_gb = 1024 * 1024 * 1024

    bucket = s3.Bucket(bucket_name)
    bucket_mpu_gb = 0
    for mpu in bucket.multipart_uploads.all():
        #print(mpu.initiated)
        if object_prefix in mpu.object_key:
            mpu_size = 0
            for part in mpu.parts.all():
                mpu_size += part.size
            mpu_gb = mpu_size / bytes_per_gb
            print(bucket.name+","+str(mpu.initiated)+','+mpu.object_key+','+str(mpu_gb))

            bucket_mpu_gb += mpu_gb

    print("bucket",bucket.name,bucket_mpu_gb,"GB")


def main():
    if len(argv) == 2:
        get_bucket_mpus_size(argv[1])
    elif len(argv) == 3:
        get_bucket_mpus_size(argv[1], argv[2])
    else:
        print("""
Usage: python3 bucket_mpu.py <bucket_name> [object_prefix]

Example: python3 bucket_mpu.py reonomy-viserion-prd Meridian

This will output stats for each mulipart upload part, as well as the total of every result returned.
""")
        exit(1)


if __name__ in '__main__':
    main()

Borrowed inspiration from here: https://gist.github.com/jonathanwcrane/c26ea5c5daf0a156342b

Nice work @nijine!