Python format hex number

Question

I need to send a string via tcp. One of the first sections of the string is the length of the command variable

Example:
command = STATUS?UPDATE

I need to send the following string below

sendCommand = '\x00\x00\x00'+STRINGLENGTH+'\x02'+command+'\x0D\x0A'

My string length is 11 so I need STRINGLENGTH to be the hex equivalent of 11, which is 0xB , except that I need it to output as \x0B

Padding it with the leading 0 is easy, but I cannot get it to output as \x instead of 0x , and if I do a string replace it is treated as text and not as hex, so it doesn't work.

My final hex string should be:

\x00\x00\x00\x0B\x02\x53\x54\x41\x54\x55\x53\x3f\x55\x53\x45\x52\x0D\x0A

I am instead getting:

\x00\x00\x000x0B\x02\x53\x54\x41\x54\x55\x53\x3f\x55\x53\x45\x52\x0D\x0A

Any ideas on how to format it correctly?

Answer 1

So, this is a bit of a round-about fashion, but use a bytes object:

>>> STRINGLENGTH = bytes([11]).decode()
>>> endCommand = '\x00\x00\x00'+STRINGLENGTH+'\x02'
>>> endCommand
'\x00\x00\x00\x0b\x02'

Almost certainly, you are going to want to change your str object back to a bytes object, but the above should get you going.

I suspect what you were doing was using the hex function:

>>> STRINGLENGTH = hex(11)
>>> endCommand = '\x00\x00\x00'+STRINGLENGTH+'\x02'
>>> endCommand
'\x00\x00\x000xb\x02'

The fundamental thing you need to understand is that you aren't working with "hex", you are working with bytes . Hex is just how bytes are traditionally represented. The hex helper function returns a hexadecimal representation, as a string of an integer. But that isn't what you want. You want the byte corresponding to the value 11 .

Note, for the ascii-range, chr(i) might works as well, so

>>> STRINGLENGTH = chr(11)
>>> endCommand = '\x00\x00\x00'+STRINGLENGTH+'\x02'
>>> endCommand
'\x00\x00\x00\x0b\x02'

But be careful, say you wanted the number 129, you have to care about the encoding...

>>> chr(129)
'\x81'

But in bytes, in UTF-8, that's actually represented by two different bytes

>>> chr(129).encode()
b'\xc2\x81'
>>> list(chr(129).encode())
[194, 129]

Which of course, depends on the encoding:

>>> chr(129).encode('latin')
b'\x81'
>>> list(chr(129).encode('latin'))
[129]
>>>

For that reason, I think it is safer to stick with the slightly wordier:

>>> bytes([129])
b'\x81'