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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_image(self, digest, blob, mime_type, index, size=500):
 """Return an image for the given content, only if it already exists in the image cache."""

        # Special case, for now (XXX).
        if mime_type.startswith("image/"):
            return ""

        cache_key = f"img:{index}:{size}:{digest}"
        return self.cache.get(cache_key)