>>> import boto3
>>> account_id = boto3.resource('iam').CurrentUser().arn.split(':')[4]

>>> timeit.timeit("boto3.resource('iam').CurrentUser().arn",
... 'import boto3', number=10)
4.8895583080002325

>>> import boto3
>>> boto3.client('sts').get_caller_identity().get('Account')
'012345678901'