Method to determine your AWS account ID using boto3 for either a user or an ec2 instance or lambda function

01_get_account_id_for_user_ec2instance_role_or_lambda.py

import boto3
print(boto3.client('sts').get_caller_identity()['Account'])

02_get_account_id.bash

aws sts get-caller-identity --query 'Account' --output text

03_deprecated_get_account_id_lambda.py

# This method is no longer needed with the release of the STS GetCallerIdentity method

def get_account_id(context):
  return context.invoked_function_arn.split(':')[4]

def lambda_handler(event, context):
  print("My account ID is %s" % get_account_id(context))

04_deprecated_get_account_id_user_or_ec2_boto3.py

# This method is no longer needed with the release of the STS GetCallerIdentity method

from botocore.vendored import requests
import boto3

def get_account_id():
    try:
        # We're running in an ec2 instance, get the account id from the
        # instance profile ARN
        return requests.get(
            'http://169.254.169.254/latest/meta-data/iam/info/',
            timeout=1).json()['InstanceProfileArn'].split(':')[4]
    except:
        pass

    try:
        # We're not on an ec2 instance but have api keys, get the account
        # id from the user ARN
        return boto3.client('iam').get_user()['User']['Arn'].split(':')[4]
    except:
        pass

    return False

05_deprecated_get_account_id_user_or_ec2_boto3_urllib2.py

# This method is no longer needed with the release of the STS GetCallerIdentity method

import urllib2, json
import boto3

def get_account_id():
    try:
        # We're running in an ec2 instance, get the account id from the
        # instance profile ARN
        return json.loads(urllib2.urlopen(
            'http://169.254.169.254/latest/meta-data/iam/info/', 
            None, 
            1).read())['InstanceProfileArn'].split(':')[4]
    except:
        pass

    try:
        # We're not on an ec2 instance but have api keys, get the account
        # id from the user ARN
        return boto3.client('iam').get_user()['User']['Arn'].split(':')[4]
    except:
        pass

    return False

I was trying to figure out how to get the account number when I assumed a role to access resources in a second account. In this case the get_user() fails: "An error occurred (ValidationError) when calling the GetUser operation: Must specify userName when calling with non-User credentials"

I found that users list_users works cross accounts as long as there is at least one user account created:
account = assumed_session.client('iam').list_users(MaxItems=1)["Users"][0]["Arn"].split(':')[4]

With the release of the new STS GetCallerIdentity method, there's no more need for different processes for users, ec2 instances, roles and lambda. It can all now be done with the example above

import boto3 print(boto3.client('sts').get_caller_identity()['Account'])

This doesn't seem to work now.