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

This would work:

bk = conn.get_bucket('my_bucket_name')
key = bk.lookup('my_key_name')
print key.size

The lookup method simply does a HEAD request on the bucket for the keyname so it will return all of the headers (including content-length) for the key but will not transfer any of the actual content of the key.

The S3 tutorial mentions this but not very explicitly and not in this exact context. I'll add a section on this to help make it easier to find.

Note: for every old link like http://boto.cloudhackers.com/s3_tut.html that returns a 404, add in "/en/latest" right after the ".com" : http://boto.cloudhackers.com/en/latest/s3_tut.html . (Someone needs to explore mod_rewrite...)

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"

in boto3:

s3.head_object also performs a HEAD request to retrieve the meta data about the object:

s3 = boto3.client('s3')
response = s3.head_object(Bucket='bucketname', Key='keyname')
size = response['ContentLength']