Get CN from certificate

def get_crt_common_name(certificate_path=OLD_CERTIFICATE_PATH):
    '''
        Get CN from certificate
    '''
    try:
        certificate_file = open(certificate_path)
        crt = OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_PEM,
                                              certificate_file.read())
        return crt.get_subject().commonName
    except IOError:
        return None