How to concatenate a variable to a string containing escape character

Question

I’m new to Python, and I’m trying to set up a server socket for message exchange (like a telnet) to communicate with a client. The problem I’m facing is that this client has its own protocol, and I need to get the server to send the messages in a compatible way. I need the messages to be sent to the customer in the following format:

    b'\x01\x08\x00Testando'

The problem is that I am not able to produce the signal ' ' as it should be, the second byte ' x08' should be according to the message size (In hexadecimal) and for this I created a variable 'h' to calculate, and then concatenate the string, but I cannot concatenate, Python gives me errors. I tried to concatenate in the following ways:

    '\x01\x'+h+'\x00'   # Deu erro.
    r'\x01\x'+h+'\x00'   # Não deu erro, mas ficou com duas barras '\\'.
    str('\x01\x'+h+'\x00')   # Outra vez um erro.

It did not work, if I put two signs of ' ', then it accepts, but the sending to the customer is wrong, containing two signals and it does not accept this format.

    b'\\x01\\x08\\x00Testando'   # O cliente não aceita assim.

I tried to use Unicode ' u', but it resulted in the same sequence above, with two signs of ' '. I noticed that this error only occurs when I try to concatenate the string with the variable, if I put the value manually as in the first code presented, it accepts without problems, but as I need to do this dynamically it becomes unviable. If I concatenate the variable without the ' ' ' signal in the byte before it, Python does not accuse errors, but if I put the sign it does not accept.

    '\x01x'+h+'\x00'   # Aceitou sem problemas, porém faltando o sinal.

Another thing I noticed is that it requires that after x, you have two characters, if you have less it is error too, for example:

    '\x01\x0\x00Testando'   # Faltando uma casa no segundo byte da erro.
    '\x01\x08\x00Testando'   # Feito manualmente, funciona perfeito.

To send this string to the client, the socket requires it to be in bytes, and I tested two ways:

    b'\x01\x'+h+'\x00'
    bytes('\x01\x'+h+'\x00', 'iso-8859-1')

I tried with and without iso coding. I would like to know if anyone has any idea what I can do to be able to assemble the string in the format that is required by the client. Ah, the encoding the client works with is 'iso-8859-1'.

Thank you very much!

Answer 1

The problem is happening because the data is not a string but a sequence of bytes.

To concatenate the variable h to this sequence, it is necessary to convert the hexadecimal string to the type byte with the method bytes.fromhex(), and then do the concatenation.

Example:

h = '0c'

res = b'\x01' + bytes.fromhex(h) + b'\x00' # Converte em byte e concatena    
res
b'\x01\x0c\x00' # saída