Simple python code snippet to get the list of all the running ec2 instances across regions. The code will dynamically pick the list of available regions. So this will work perfectly without a code change even if a new region gets added.

GetEC2Instances.py

import boto3

access_key = "XXXXXXXXXXXXXXXXXX"
secret_key = "XXXXXXXXXXXXXXXXXX"

client = boto3.client('ec2', aws_access_key_id=access_key, aws_secret_access_key=secret_key,
                                  region_name='us-east-1')

ec2_regions = [region['RegionName'] for region in client.describe_regions()['Regions']]

for region in ec2_regions:
    conn = boto3.resource('ec2', aws_access_key_id=access_key, aws_secret_access_key=secret_key,
                   region_name=region)
    instances = conn.instances.filter()
    for instance in instances:
        if instance.state["Name"] == "running":
            print (instance.id, instance.instance_type, region)