import os

assert os.environ['PGUSER'] != '', os.environ

connection_string = \
    'postgresql://{user}:{password}@{host}/{database}'.format(
        user=os.environ['PGUSER'],
        password=os.environ['PGPASSWORD'],
        host=os.environ['PGHOST'],
        database=os.environ['PGDATABASE'])

from sqlalchemy import create_engine

engine = create_engine(connection_string)