chr() equivalent returning a bytes object, in py3k

Question

Python 2.x has chr(), which converts a number in the range 0-255 to a byte string with one character with that numeric value, and unichr(), which converts a number in the range 0-0x10FFFF to a Unicode string with one character with that Unicode codepoint. Python 3.x replaces unichr() with chr(), in keeping with its "Unicode strings are default" policy, but I can't find anything that does exactly what the old chr() did. The 2to3 utility (from 2.6) leaves chr calls alone, which is not right in general :(

(This is for parsing and serializing a file format which is explicitly defined in terms of 8-bit bytes.)

score 56 · Answer 1 · answered Dec 24 '10 at 00:41

56

Try the following:

b = bytes([x])

For example:

>>> bytes([255])
b'\xff'

answered Dec 24 '10 at 00:41

Mark Byers

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I get a little twitchy about throwing around scratch arrays but probably I shouldn't. It does the job, anyway. – zwol Dec 24 '10 at 02:16
4

@Zack: You could use ``bytes((255, ))`` as a variation. – malthe Mar 20 '13 at 14:16
7

bytes((255,)) in Python2 will NOT give you b'\xff' ... it returns '(255,)' instead. – Guido U. Draheim Sep 15 '14 at 00:55
2

@GuidoDraheim2013: it is Python 3 code i.e., don't use `bytes([255])` on Python 2, use `chr(255)` there. – jfs May 31 '16 at 10:18
@malthe: `bytes((255, ))` is still creates a scratch array. – martineau Dec 04 '18 at 11:08

score 24 · Accepted Answer · answered Sep 15 '14 at 01:05

Consider using bytearray((255,)) which works the same in Python2 and Python3. In both Python generations the resulting bytearray-object can be converted to a bytes(obj) which is an alias for a str() in Python2 and real bytes() in Python3.

# Python2
>>> x = bytearray((32,33))
>>> x
bytearray(b' !')
>>> bytes(x)
' !'

# Python3
>>> x = bytearray((32,33))
>>> x
bytearray(b' !')
>>> bytes(x)
b' !'

score 9 · Answer 3 · answered Oct 01 '18 at 16:06

9

Yet another alternative (Python 3.5+):

>>> b'%c' % 65
b'A'

answered Oct 01 '18 at 16:06

Ecir Hana

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Annoying that `b'{:c}'.format(65)` doesn't work as well, but thanks, this could be quite handy for the thing I originally wanted this for (and never got around to finishing). – zwol Oct 01 '18 at 17:07

score 8 · Answer 4 · answered May 30 '16 at 17:26

8

In case you want to write Python 2/3 compatible code, use six.int2byte

answered May 30 '16 at 17:26

youfu

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I don't see why that would be better than Guido's answer, particularly if I have no other need for `six`. – zwol May 31 '16 at 18:21
@zwol: For Python 3.2+, `int2byte = operator.methodcaller("to_bytes", 1, "big")`. According to the comment, this is about 2x faster than `bytes((...))`. Anyway, `int2byte(x)` looks better than `bytes(bytearray((x,)))` for me. – youfu May 31 '16 at 18:29
Speed is good, but in the thing that provoked the original question, *no dependencies outside the standard library* was an overriding concern. – zwol May 31 '16 at 18:33
`bytes` takes a `tuple` for a constructor directly, so you can just use `bytes((x,))`. Only need to use `bytearray` if you want it to be mutable. – Perkins Sep 13 '16 at 08:11

score 7 · Answer 5 · answered Dec 13 '14 at 00:33

7

>>> import struct
>>> struct.pack('B', 10)
b'\n'
>>> import functools
>>> bchr = functools.partial(struct.pack, 'B')
>>> bchr(10)
b'\n'

answered Dec 13 '14 at 00:33

SangYoung Lee

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brilliant. `bchr` is readable and the implementation is efficient. – jfs Jun 04 '23 at 08:07

Glushiator · Answer 6 · 2020-04-11T15:19:40.103

0

simple replacement based on small range memoization (should work on 2 and 3), good performance on CPython and pypy

binchr = tuple([bytes(bytearray((b,))) for b in range(256)]).__getitem__

binchr(1) -> b'\x01'

edited Apr 11 '20 at 15:19

answered Apr 11 '20 at 15:13

Glushiator

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chr() equivalent returning a bytes object, in py3k

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