Struct.error: argument for 's' must be a bytes object, already supplied

Question

I've been working on a TCP/IP connection program thingy in Python and came across the need to use Struct. And so I imported the module and after some time came to a very particular issue. I get the error specified in te title when I run the code below, which should be working after I checked some other answers and documentations.

import struct

string = "blab"

s = struct.Struct(b'4s')
packed_data = s.pack(string)

print(packed_data)

As far as I found, the issue should be fixed by prepending the string used in the s variable with 'b' or using the bytes() function parsing 'utf-8' as encoding argument. Tried both, same error.

I have no idea what might be wrong so am I missing something? I could not find relevant information online regarding this issue, so this is why I'm posting here now.

Any help is appreciated and thanks in advance!

Answer 1

One problem is that you put the "b" in the wrong place. You placed it in the format string, when the data to be packed need to be a byte string.

    >>> string = "blab"
    >>> s = struct.Struct('4s')
    >>> packed_data = s.pack(string.encode('utf-8'))
    >>> print(packed_data)
    b'blab'

But even that is problematic. Suppose your string is not in the ascii character set... let's say it's Greek, then the UTF8 encoded string is more than 4 bytes and you write a truncated value

    >>> string = "ΑΒΓΔ"
    >>> s = struct.Struct('4s')
    >>> 
    >>> packed_data = s.pack(string.encode('utf-8'))
    >>> print('utf8len', len(string.encode('utf-8')), 'packedlen', len(packed_data))
    utf8len 8 packedlen 4
    >>> print(packed_data)
    b'\xce\x91\xce\x92'
    >>> print(struct.unpack('4s', packed_data)[0].decode('utf-8'))
    ΑΒ
    >>> 

If you really need to restrict to 4 bytes, then convert the original string using ascii instead of utf-8 so that any unencodable unicode character will raise an exception right away.

    >>> string = "ΑΒΓΔ"
    >>> s = struct.Struct('4s')
    >>> 
    >>> packed_data = s.pack(string.encode('ascii'))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)