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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 fetch( self, url, filename=None, decompress=False, force=False, timeout=None, use_wget_if_available=True):
 """ Return the local path to the downloaded copy of a given URL. Don't download the file again if it's already present, unless `force` is True. """

        key = (url, decompress)
        if not force and key in self._local_paths:
            path = self._local_paths[key]
            if exists(path):
                return path
            else:
                del self._local_paths[key]
        path = download.fetch_file(
            url,
            filename=filename,
            decompress=decompress,
            subdir=self.subdir,
            force=force,
            timeout=timeout,
            use_wget_if_available=use_wget_if_available)

        self._local_paths[key] = path
        return path