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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_cache(cache_file, source_file=None):
 """Get cached taxonomy using the cPickle module. No check is done at that stage. :param cache_file: full path to the file holding pickled data :param source_file: if we discover the cache is obsolete, we will build a new cache, therefore we need the source path of the cache :return: (single_keywords, composite_keywords). """

    timer_start = time.clock()

    filestream = open(cache_file, "rb")
    try:
        cached_data = cPickle.load(filestream)
        version_info = cached_data['version_info']
        if version_info['rdflib'] != rdflib.__version__:
            raise KeyError
    except (cPickle.UnpicklingError, ImportError,
            AttributeError, DeprecationWarning, EOFError):
        current_app.logger.warning(
            "The existing cache in %s is not readable. "
            "Removing and rebuilding it." % cache_file
        )
        filestream.close()
        os.remove(cache_file)
        return _build_cache(source_file)
    except KeyError:
        current_app.logger.warning(
            "The existing cache %s is not up-to-date. "
            "Removing and rebuilding it." % cache_file
        )
        filestream.close()
        os.remove(cache_file)
        if source_file and os.path.exists(source_file):
            return _build_cache(source_file)
        else:
            current_app.logger.error(
                "The cache contains obsolete data (and it was deleted), "
                "however I can't build a new cache, the source does not "
                "exist or is inaccessible! - %s" % source_file
            )
    filestream.close()

    single_keywords = cached_data["single"]
    composite_keywords = cached_data["composite"]

    # the cache contains only keys of the composite keywords, not the objects
    # so now let's resolve them into objects
    for kw in composite_keywords.values():
        kw.refreshCompositeOf(single_keywords, composite_keywords)

    current_app.logger.debug(
        "Retrieved taxonomy from cache %s created on %s" %
        (cache_file, time.asctime(cached_data["creation_time"]))
    )

    current_app.logger.debug(
        "%d terms read in %.1f sec." %
        (len(single_keywords) + len(composite_keywords),
         time.clock() - timer_start)
    )

    return (single_keywords, composite_keywords)