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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 cat(src_filename, dst_file):
 """Copies the contents of the indicated file to an already opened file."""

    (dev, dev_filename) = get_dev_and_path(src_filename)
    if dev is None:
        with open(dev_filename, 'rb') as txtfile:
            for line in txtfile:
                dst_file.write(line)
    else:
        filesize = dev.remote_eval(get_filesize, dev_filename)
        return dev.remote(send_file_to_host, dev_filename, dst_file, filesize,
                          xfer_func=recv_file_from_remote)