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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_placeholder_cache_key(placeholder, language_code):
 """ Return a cache key for an existing placeholder object. This key is used to cache the entire output of a placeholder. """

    return _get_placeholder_cache_key_for_id(
        placeholder.parent_type_id,
        placeholder.parent_id,
        placeholder.slot,
        language_code
    )