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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 cache_key_prefix(request):
 """ Cache key for yacms's cache middleware. Adds the current device and site ID. """

    cache_key = "%s.%s.%s." % (
        settings.CACHE_MIDDLEWARE_KEY_PREFIX,
        current_site_id(),
        device_from_request(request) or "default",
    )
    return _i18n_cache_key_suffix(request, cache_key)