如何获得boto3 Collection的大小？

Question

The way I have been using is to transform the Collection into a List and query the length:

s3 = boto3.resource('s3')
bucket = s3.Bucket('my_bucket')
size = len(list(bucket.objects.all()))

However, this forces resolution of the whole collection and obviates the benefits of using a Collection in the first place. Is there a better way to do this?

Answer 1

There is no way to get the count of keys in a bucket without listing all the objects this is a limitation of AWS S3 (see https://forums.aws.amazon.com/thread.jspa?messageID=164220 ).

Getting the Object Summaries (HEAD) doesn't get the actual data so should be a relatively inexpensive operation and if you are just discarding the list then you could do:

size = sum(1 for _ in bucket.objects.all())

Which will give you the number of objects without constructing a list.

Answer 2

Borrowing from a similar question , one option to retrieve the complete list of object keys from a bucket + prefix is to use recursion with the list_objects_v2 method.

This method will recursively retrieve the list of object keys, 1000 keys at a time.

Each request to list_objects_v2 uses the StartAfter argument to continue listing keys after the last key from the previous request.

import boto3

if __name__ == '__main__':

    client = boto3.client('s3',
        aws_access_key_id     = 'access_key',
        aws_secret_access_key = 'secret_key'
    )

    def get_all_object_keys(bucket, prefix, start_after = '', keys = []):
        response = client.list_objects_v2(
            Bucket     = bucket,
            Prefix     = prefix,
            StartAfter = start_after
        )

        if 'Contents' not in response:
            return keys

        key_list = response['Contents']
        last_key = key_list[-1]['Key']

        keys.extend(key_list)

        return get_all_object_keys(bucket, prefix, last_key, keys)

    object_keys = get_all_object_keys('your_bucket', 'prefix/to/files')

    print(len(object_keys))

Answer 3

For my use case, I just needed to know whether the folder is empty or not.

s3 = boto3.client('s3')
response = s3.list_objects(
        Bucket='your-bucket',
        Prefix='path/to/your/folder/',
)
print(len(response['Contents']))

This was enough to know whether the folder is empty. Note that a folder, if manually created in the S3 console, can count as a resource itself. In this case, if the length shown above is greater than 1, then the S3 "folder" is empty.