import boto3
import re

iam, sts = boto3.client('iam'), boto3.client('sts')
response = sts.get_caller_identity()
role_name, = re.match(r".*assumed-role\/([a-z0-9\-]+)\/.*", response['Arn']).groups()
role_arn = iam.get_role(RoleName=role_name)['Role']['Arn']