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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 pdf(cls, uuid):
 """Return a PDF of the invoice identified by the UUID This is a raw string, which can be written to a file with: ` with open('invoice.pdf', 'w') as invoice_file: invoice_file.write(recurly.Invoice.pdf(uuid)) ` """

        url = urljoin(base_uri(), cls.member_path % (uuid,))
        pdf_response = cls.http_request(url, headers={'Accept': 'application/pdf'})
        return pdf_response.read()