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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_thumb(self, path, key, format):
 """Get the stored thumbnail if exists. path: path of the source image key: key of the thumbnail format: thumbnail's file extension """

        thumbpath = self.get_thumbpath(path, key, format)
        fullpath = os.path.join(self.out_path, thumbpath)
        if os.path.isfile(fullpath):
            url = self.get_url(thumbpath)
            return Thumb(url, key)
        return Thumb()