def load_backend(backend_name):
    """
    Return a database backend's "base" module given a fully qualified database
    backend name, or raise an error if it doesn't exist.
    """
    # This backend was renamed in Django 1.9.
    if backend_name == 'django.db.backends.postgresql_psycopg2':
        backend_name = 'django.db.backends.postgresql'

    try:
        return import_module('%s.base' % backend_name)
    except __HOLE__ as e_user:
        # The database backend wasn't found. Display a helpful error message
        # listing all possible (built-in) database backends.
        backend_dir = os.path.join(os.path.dirname(upath(__file__)), 'backends')
        try:
            builtin_backends = [
                name for _, name, ispkg in pkgutil.iter_modules([npath(backend_dir)])
                if ispkg and name not in {'base', 'dummy', 'postgresql_psycopg2'}
            ]
        except EnvironmentError:
            builtin_backends = []
        if backend_name not in ['django.db.backends.%s' % b for b in
                                builtin_backends]:
            backend_reprs = map(repr, sorted(builtin_backends))
            error_msg = ("%r isn't an available database backend.\n"
                         "Try using 'django.db.backends.XXX', where XXX "
                         "is one of:\n    %s\nError was: %s" %
                         (backend_name, ", ".join(backend_reprs), e_user))
            raise ImproperlyConfigured(error_msg)
        else:
            # If there's some other error, this must be an error in Django
            raise
dataset/ETHPy150Open django/django/django/db/utils.py/load_backend