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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 open_file(self, url):
 """Use local file or FTP depending on form of URL."""

        if not isinstance(url, str):
            raise URLError('file error: proxy support for file protocol currently not implemented')
        if url[:2] == '//' and url[2:3] != '/' and url[2:12].lower() != 'localhost/':
            raise ValueError("file:// scheme is supported only on localhost")
        else:
            return self.open_local_file(url)