How to assume a role in boto3

boto3_with_role.py

def boto3_with_role(role_arn, session_prefix, external_id, **kwargs):
    """
    Create a partially applied session to assume a role with an external id.
    A unique session_name will be generated by {session_prefix}_{time}
    `session` can be passed, otherwise the default sesion will be used
    see: http://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_use_switch-role-api.html
    """
    res = sts.assume_role(
        RoleArn         = role_arn,
        RoleSessionName = '{}_{}'.format(session_prefix, int(time.time())),
        ExternalId      = external_id,
        **kwargs
        )
    creds = res['Credentials']
    return partial(boto3.session.Session,
        aws_access_key_id     = creds['AccessKeyId'],
        aws_secret_access_key = creds['SecretAccessKey'],
        aws_session_token     = creds['SessionToken']
        )
 
# use like this
Session = boto3_with_role('arn:aws:iam::1234567890:role/RoleName', 'MyPrefix', '12345')
my_session = Session()
ec2 = my_session.resource('ec2')
# or connect to a different region (which is the whole point to partially apply Session so we can use with multiple regions)
s3 = Session(region_name='us-east-2').resource('s3')