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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_url_shortener():
 """ Return the selected URL shortener backend. """

    try:
        backend_module = import_module(URL_SHORTENER_BACKEND)
        backend = getattr(backend_module, 'backend')
    except (ImportError, AttributeError):
        warnings.warn('%s backend cannot be imported' % URL_SHORTENER_BACKEND,
                      RuntimeWarning)
        backend = default_backend
    except ImproperlyConfigured as e:
        warnings.warn(str(e), RuntimeWarning)
        backend = default_backend

    return backend