Python 3: obtain struct.pack output that matches Python 2 struct.pack output

Question

In Python 2:

import struct
struct.pack('B', 21)[0] # returns '\x15'

In Python 3:

import struct
struct.pack('B', 21)[0] # returns 21

Is there some way to create the byte string returned by Python 2 in Python 3? Any pointers would be greatly appreciated!

• duhaime: Ooh, chr(21)!
• tdelaney: This is an interesting question. In python 2, pack returns a string and each member of a string is a single character string. In python 3, pack quite sensibly returns a bytes array but individual members of a bytes array are ints < 256. Its a place where 2 to 3 conversion could break. By "member", I mean the value you get when you index an item.
• duhaime: Ya, I'm trying to convert a Python implementation of the original NES synthesizer (written in machine code) to Python 3 and there are lots of breaking changes in the handling of binary objects between Python 2 and 3!
• tdelaney: At the python level, the result of pack is an opaque blob. You get the right stuff back when you unpack. When I wrote my comment I wasn't sure the use case of python ever touching a member of the blob. I'm still not sure I've got one.
• tdelaney: It would bite you if you want to send data a byte at a time. for b in data:myconnection.send(b).

Answer 1

Just don't index the byte string it returns:

>>> struct.pack('B', 21)
b'\x15'

• orlp: @duhaime That's what you wrote, the first byte is 21. Clearly you are looking for something else.
• orlp: @duhaime Are you looking for hex(21) or "{:x}".format(21)?
• orlp: @duhaime Getting the hexadecimal representation of an integer as a string is a way different question that you originally asked though.
• duhaime: I cleaned up some of my comments but wanted to ask--this is a bytes data type, whereas Python 2 returns a string--can one get this object as a string type in Python 3? chr(21) returns the string that's needed
• orlp: @duhaime That is not a valid string for all values you can give to the struct.pack. You will have to change the other code to work with bytes instead of strings.