How do I get the file / key size in boto S3?

Question

There must be an easy way to get the file size (key size) without pulling over a whole file. I can see it in the Properties of the AWS S3 browser. And I think I can get it

Answer 1

in boto3 using an S3 resource:

boto3.resource('s3').Bucket(bucketname).Object(keyname).content_length

The head_object call of the S3 client returned me an http "403 Forbidden"