packing and unpacking variable length array/string using the struct module in python

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I am trying to get a grip around the packing and unpacking of binary data in Python 3. Its actually not that hard to understand, except one problem:

what if I have a variable length textstring and want to pack and unpack this in the most elegant manner?

As far as I can tell from the manual I can only unpack fixed size strings directly? In that case, are there any elegant way of getting around this limitation without padding lots and lots of unnecessary zeroes?

struct python-3.x binary python
7 Answers

Another silly but very simple approach: (PS:as others mentioned there is no pure pack/unpack support for that, with that in mind)

import struct


def pack_variable_length_string(s: str) -> bytes:
    str_size_bytes = struct.pack('!Q', len(s))
    str_bytes = s.encode('UTF-8')
    return str_size_bytes + str_bytes


def unpack_variable_length_string(sb: bytes, offset=0) -> (str, int):
    str_size_bytes = struct.unpack('!Q', sb[offset:offset + 8])[0]
    return sb[offset + 8:offset + 8 + str_size_bytes].decode('UTF-8'), 8 + str_size_bytes + offset


if __name__ == '__main__':
    b = pack_variable_length_string('Worked maybe?') + \
        pack_variable_length_string('It seems it did?') + \
        pack_variable_length_string('Are you sure?') + \
        pack_variable_length_string('Surely.')
    next_offset = 0
    for i in range(4):
        s, next_offset = unpack_variable_length_string(b, next_offset)
        print(s)
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