Method to determine the AWS account ID of your account using boto

01_get_aws_account_id.py

#!/usr/bin/env python

import boto, boto.jsonresponse
conn = boto.connect_sts()
e = boto.jsonresponse.Element()
boto.jsonresponse.XmlHandler(e, conn).parse(conn.make_request('GetCallerIdentity',{},'/','POST').read())
e['GetCallerIdentityResponse']['GetCallerIdentityResult']['Account']

02_get_aws_account_id.py

#!/usr/bin/env python

import boto

metadata = boto.utils.get_instance_metadata(timeout=1, num_retries=1)
if 'iam' in metadata:
    # We're running in an ec2 instance, get the account id from the
    # instance profile ARN
    return metadata['iam']['info']['InstanceProfileArn'].split(':')[4]
else:
    try:
        # We're not on an ec2 instance but have api keys, get the account
        # id from the user ARN
        return boto.connect_iam().get_user().arn.split(':')[4]
    except:
        # We don't have IAM or user credentials
        return False

I've added a new approach above which uses the new STS GetCallerIdentity method. This will work for users, roles, lambda, everything except an ec2 instance with no IAM role.