How to cast a string to bytes without encoding

Question

I have a bunch of binary data that comes to python via a char* from some C interface (not under my control) so I have a string of arbitrary binary data (what is normally a byte array). I would like to convert it to a byte array to simplify using it with other python functions but I can't seem to figure out how.

Examples that don't work:

data = rawdatastr.encode() this assumes "utf-8" and mangles the data == BAD

data = rawdatastr.encode('ascii','ignore') strips chars over 127 == BAD

data = rawdatastr.encode('latin1') not sure -- this is the closest so far but I have no proof that it is working for all bytes.

data = array.array('B', [x for x in map(ord,data)]).tobytes() This works but seems like a lot of work to do something simple. Is there something simpler?

I am thinking I need to write my own identity encoding that just passes the bytes along (I think latin1 does this based upon some reading but no proof thus far).

Answer 1

Though I suspect something else is decoding your data for you (a char* in C is usually best represented as bytes, especially if it is binary data):

The latin1 codec can round trip every byte. You can verify this with the following short program:

>>> s = ''.join(chr(i) for i in range(0x100))
>>> s
'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0¡¢£¤¥¦§¨©ª«¬\xad®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ'
>>> s2 = s.encode('latin1').decode('latin1')
>>> s2 == s
True
>>> sb = bytes(range(0x100))
>>> sb
b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff'
>>> sb == s.encode('latin1')
True

Answer 2

Just now I ran into the same problem. This is what I came up with:

import struct

def rawbytes(s):
    """Convert a string to raw bytes without encoding"""
    outlist = []
    for cp in s:
        num = ord(cp)
        if num < 255:
            outlist.append(struct.pack('B', num))
        elif num < 65535:
            outlist.append(struct.pack('>H', num))
        else:
            b = (num & 0xFF0000) >> 16
            H = num & 0xFFFF
            outlist.append(struct.pack('>bH', b, H))
    return b''.join(outlist)

Some examples:

In [34]: rawbytes('this is a test')
Out[34]: b'this is a test'

In [35]: rawbytes('\udc80\udcdf\udcff\udcff\udcff\x7f')
Out[35]: b'\xdc\x80\xdc\xdf\xdc\xff\xdc\xff\xdc\xff\x7f'