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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_rendering_cache_key(placeholder_name, contentitem):
 """ Return a cache key for the content item output. .. seealso:: The :func:`ContentItem.clear_cache() <fluent_contents.models.ContentItem.clear_cache>` function can be used to remove the cache keys of a retrieved object. """

    if not contentitem.pk:
        return None
    return "contentitem.@{0}.{1}.{2}".format(
        placeholder_name,
        contentitem.plugin.type_name,  # always returns the upcasted name.
        contentitem.pk,                # already unique per language_code
    )