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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 valid_service_context(service_context, when=0):
 """ Check if the client_secret has expired :param service_context: A :py:class:`oidcservice.service_context.ServiceContext` instance :param when: A time stamp against which the expiration time is to be checked :return: True if the client_secret is still valid """

    eta = getattr(service_context, 'client_secret_expires_at', 0)
    now = when or utc_time_sans_frac()
    if eta != 0 and eta < now:
        return False
    return True