Difference in boto3 between resource, client, and session?

Question

I am using Python 2.7.12 in Ubuntu 16.04 LTS. I'm learning how to use boto3 from the following link: https://boto3.readthedocs.io/en/latest/guide/quickstart.html#using-boto-3. My doubt is when to use resource, client, or session, and their respective functionality.

Answer 1

Here's some more detailed information on what Client, Resource, and Session are all about.

Client:

• low-level AWS service access
• generated from AWS service description
• exposes botocore client to the developer
• typically maps 1:1 with the AWS service API
• all AWS service operations are supported by clients
• snake-cased method names (e.g. ListBuckets API => list_buckets method)

Here's an example of client-level access to an S3 bucket's objects (at most 1000**):

import boto3

client = boto3.client('s3')
response = client.list_objects_v2(Bucket='mybucket')
for content in response['Contents']:
    obj_dict = client.get_object(Bucket='mybucket', Key=content['Key'])
    print(content['Key'], obj_dict['LastModified'])

** you would have to use a paginator, or implement your own loop, calling list_objects() repeatedly with a continuation marker if there were more than 1000.

Resource:

• higher-level, object-oriented API
• generated from resource description
• uses identifiers and attributes
• has actions (operations on resources)
• exposes subresources and collections of AWS resources
• does not provide 100% API coverage of AWS services

Here's the equivalent example using resource-level access to an S3 bucket's objects (all):

import boto3

s3 = boto3.resource('s3')
bucket = s3.Bucket('mybucket')
for obj in bucket.objects.all():
    print(obj.key, obj.last_modified)

Note that in this case you do not have to make a second API call to get the objects; they're available to you as a collection on the bucket. These collections of subresources are lazily-loaded.

You can see that the Resource version of the code is much simpler, more compact, and has more capability (it does pagination for you). The Client version of the code would actually be more complicated than shown above if you wanted to include pagination.

Session:

• stores configuration information (primarily credentials and selected region)
• allows you to create service clients and resources
• boto3 creates a default session for you when needed

A useful resource to learn more about these boto3 concepts is the introductory re:Invent video.