s3_compressed_uncompressed_size.py

from gzip import open as gz_open
from boto3 import client as boto3_client

def get_lengths(bucket, key, open_func=gz_open, s3_client=None, chunk_size=8192):
    s3_client = s3_client or boto3_client('s3')
    resp = s3_client.get_object(Bucket=bucket, Key=key)
    
    uncompressed_length = 0
    with open_func(resp['Body'], mode='rb') as f:
        for chunk in iter(lambda: f.read(chunk_size), b''):
            uncompressed_length += len(chunk)
    
    return resp['ContentLength'], uncompressed_length

get_lengths(bucket, key)