Given an AWS credentials file that looks like this:

[default]
aws_access_key_id = DEFAULT
aws_secret_access_key = SECRET1

[dev]
aws_access_key_id = DEV
aws_secret_access_key = SECRET2

[prod]
aws_access_key_id = PROD
aws_secret_access_key = SECRET3

You can use any profile, say dev, like this in Python:

import boto3.session

dev = boto3.session.Session(profile_name='dev')

s3 = dev.resource('s3')
for bucket in s3.buckets.all():
    print(bucket.name)
print('')