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Title: Define a binary output formatting mini-language for *.hex()
Type: enhancement Stage: commit review
Components: Interpreter Core Versions: Python 3.9
Status: closed Resolution: fixed
Dependencies: 9951 Superseder:
Assigned To: Nosy List: Arfrever, Christian H, barry, belopolsky, eric.smith, gotgenes, gregory.p.smith, mrh1997, ncoghlan, xtreak
Priority: normal Keywords: patch

Created on 2014-09-10 23:55 by ncoghlan, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Pull Requests
URL Status Linked Edit
PR 13578 merged gregory.p.smith, 2019-05-26 00:54
Messages (18)
msg226733 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2014-09-10 23:55
Inspired by the discussion in issue 9951, I believe it would be appropriate to extend the default handling of the "x" and "X" format characters to accept arbitrary bytes-like objects. The processing of these characters would be as follows:

"x": display a-f as lowercase digits
"X": display A-F as uppercase digits
"#": includes 0x prefix
".precision": chunks output, placing a space after every <precision> bytes
",": uses a comma as the separator, rather than a space

Output order would match binascii.hexlify()


format(b"xyz", "x") -> '78797a'
format(b"xyz", "X") -> '78797A'
format(b"xyz", "#x") -> '0x78797a'

format(b"xyz", ".1x") -> '78 79 7a'
format(b"abcdwxyz", ".4x") -> '61626364 7778797a'
format(b"abcdwxyz", "#.4x") -> '0x61626364 0x7778797a'

format(b"xyz", ",.1x") -> '78,79,7a'
format(b"abcdwxyz", ",.4x") -> '61626364,7778797a'
format(b"abcdwxyz", "#,.4x") -> '0x61626364,0x7778797a'

This approach makes it easy to inspect binary data, with the ability to inject regular spaces or commas to improved readability. Those are the basic features needed to support debugging.

Anything more complicated than that, and we're starting to want something more like the struct module.
msg226746 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2014-09-11 07:23
I think this would need to be implemented by adding bytes.__format__. I can't think of a way to make it work on bytes-like objects in general.
msg226748 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2014-09-11 07:25
> ".precision": chunks output, placing a space after every <precision> bytes

I dislike this option. There is already "%.<precision>s" in Python 2 and Python 3 (and printf of the C language) which truncates the string.

If you need such special output, please write your own function.
msg226749 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2014-09-11 07:34
I'm not particularly wild about the .precision syntax either, but I think the feature is generally useful.

Adding bytes.__format__ is exactly what "special output for bytes" _is_, as far as format() is concerned.

Another option would be to invent a new format specification for bytes. There's no reason it needs to follow the same syntax as for str, int, etc., except for ease of remembering the syntax, and some code reuse.

For example, although it's insane, you could do:

format(b'abcdwxyz', 'use_spaces,grouping=4,add_prefix')
  -> '0x61626364 0x7778797a'
msg226992 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2014-09-17 10:56
Retitled the issue and made it depend on issue 9951.

I now think it's better to tackle this more like strftime and have a method that accepts of particular custom formatting mini-language (in this case, the new hex() methods proposed in issue 9951), and then also support that mini-language in the __format__() method.

It would likely need a PEP to decide on the exact details of the formatting.
msg226993 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2014-09-17 11:01
python-ideas post with a sketch of a possible mini-language:
msg242941 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2015-05-12 05:00
Reviewing the items I had flagged as dependencies of issue 22555 for personal tracking purposes, I suggest we defer further consideration of this idea to 3.6 after folks have had a chance to get some experience with the basic bytes.hex() method.
msg292663 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-05-01 13:34
Copying the amended proposal from that python-ideas thread into here:

Start with a leading base format character (chosen to be orthogonal to the default format characters):

    "h": lowercase hex
    "H": uppercase hex
    "A": ASCII (using "." for unprintable & extended ASCII)

    format(b"xyz", "A") -> 'xyz'
    format(b"xyz", "h") -> '78797a'
    format(b"xyz", "H") -> '78797A'

Followed by a separator and "chunk size":

    format(b"xyz", "h 1") -> '78 79 7a'
    format(b"abcdwxyz", "h 4") -> '61626364 7778797a'

    format(b"xyz", "h,1") -> '78,79,7a'
    format(b"abcdwxyz", "h,4") -> '61626364,7778797a'

    format(b"xyz", "h:1") -> '78:79:7a'
    format(b"abcdwxyz", "h:4") -> '61626364:7778797a'

In the "h" and "H" cases, allow requesting a preceding "0x" on the chunks:

    format(b"xyz", "h#") -> '0x78797a'
    format(b"xyz", "h# 1") -> '0x78 0x79 0x7a'
    format(b"abcdwxyz", "h# 4") -> '0x61626364 0x7778797a'

In the thread, I suggested the section before the format character would use the standard string formatting rules (alignment, fill character, width, precision), but I now think that would be ambiguous and confusing, and would be better left as a post-processing step on the rendered text.
msg292671 - (view) Author: Gregory P. Smith (gregory.p.smith) * (Python committer) Date: 2017-05-01 16:27
Based on the ideas thread it isn't obvious that chunk size means "per byte".  I suggest either specifying the number of base digits per delimiter.  Or using a name other than chunk that indicates it means bytes.

If we're going to do this, it should also be done for octal formatting (the 'o' code) for consistency.

Also, per the python-ideas thread, via parameters to the .hex() method on bytes/bytearray/memoryview.

I'm inclined to leave 'A' printable-ascii formatting out.  Or at least consider that it could also work on unicode str.
msg292699 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2017-05-01 20:07
The Unix "od" command pretty much has all of the possibilities covered.

Although "named characters" might be going a bit far. Float, too.
msg292710 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-05-02 02:10
Minimalist proposal:

    def hex(self, *, bytes_per_group=None, delimiter=" "):
        """B.hex() -> string of hex digits
        B.hex(bytes_per_group=N) -> hex digits in groups separated by *delimeter*
        Create a string of hexadecimal numbers from a bytes object::

        >>> b'\xb9\x01\xef'.hex()
        >>> b'\xb9\x01\xef'.hex(bytes_per_group=1)
        'b9 01 ef'

Alternatively, the grouping could be by digit rather than by byte:

    def hex(self, *, group_digits=None, delimiter=" "):
        """B.hex() -> string of hex digits
        B.hex(group_digits=N) -> hex digits in groups separated by *delimeter*
        Create a string of hexadecimal numbers from a bytes object::

        >>> b'\xb9\x01\xef'.hex()
        >>> b'\xb9\x01\xef'.hex(group_digits=2)
        'b9 01 ef'

One potential advantage of the `group_digits` approach is that it could be fairly readily adapted to the hex/oct/bin builtins (although if we did that, it would make the lack of a "dec" builtin for decimal formatting a bit weird)
msg292871 - (view) Author: Robert (mrh1997) * Date: 2017-05-03 09:54
regarding the proposal for mini format languages for bytes (msg292663):
Wouldn't it be more consistent if the format specifiers are identical to the one of int's (see

I.e. "X" / "x" for hex, "o" for octal, "d" for decimal, "b" for binary, "c" for character (=default). Only 'A' need to be added for printing only ascii characters.

Furthermore I cannot see in how far the format spec in ("h#,1") is more intuitive than in ("#,.4x"), which looks like the existing minilang.

Why does Python need a new format mini lang, if the existing one provides most of the requirements. As developer it is already hard to memorize the details of the existing minilang. Ideally I do not need to learn a similar but different one for bytes...
msg292900 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-05-03 13:19
Re-using an existing minilanguage to mean something completely different wouldn't be a good idea.

Whether or not we should add any bytes specific features for this at all is also still an open question, as one of the points raised in the latest python-ideas thread is that this may be better handled as a general purpose string splitting method that breaks the string up into fixed size units, which can then be rejoined with an arbitrary delimeter. For example:

    >>> digit_groups = b'\xb9\x01\xef'.hex().splitgroups(2)
    >>> ' '.join(digit_groups)
    'b9 01 ef'
msg342888 - (view) Author: Gregory P. Smith (gregory.p.smith) * (Python committer) Date: 2019-05-20 06:31
FYI - micropython added an optional 'sep' second argument to binascii.hexlify() that is a single character separator to insert between every two hex digits.

given the #9951 .hex() methods we have everywhere (and corresponding .fromhex), binascii.hexlify is almost a legacy API.  (but micropython doesn't have those methods yet).  one key difference?  hexlify returns the hex value as a bytes rather than a str.

just adding a couple of parameters to the hex() method seems fine.  a separator string and a number of bytes to separate.

yet another minilanguage would be overkill.  and confusing in the face of the existing numeric formatting mini language ability to insert , or _ separators every four spaces ala f'{value:_x}'.
msg343527 - (view) Author: Gregory P. Smith (gregory.p.smith) * (Python committer) Date: 2019-05-26 00:57
Given that we have f-strings, I don't think a format mini language makes as much sense.  My PR adds support for separators to the .hex() methods (and to binascii.hexlify) via a parameter.  Extending beyond what MicroPython already does in its binascii implementation (a single sep parameter).
msg343910 - (view) Author: Gregory P. Smith (gregory.p.smith) * (Python committer) Date: 2019-05-29 18:47
New changeset 0c2f9305640f7655ba0cd5f478948b2763b376b3 by Gregory P. Smith in branch 'master':
bpo-22385: Support output separators in hex methods. (#13578)
msg343966 - (view) Author: Karthikeyan Singaravelan (xtreak) * (Python committer) Date: 2019-05-30 10:32
This change seems to have created some compile time warnings :

Python/pystrhex.c:18:45: warning: passing argument 1 of ‘PyObject_Size’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
Python/pystrhex.c:60:27: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘Py_ssize_t’ {aka ‘const int’} [-Wsign-compare]
Python/pystrhex.c:90:29: warning: ‘sep_char’ may be used uninitialized in this function [-Wmaybe-uninitialized]
Python/pystrhex.c:90:29: warning: ‘sep_char’ may be used uninitialized in this function [-Wmaybe-uninitialized]
msg343987 - (view) Author: Gregory P. Smith (gregory.p.smith) * (Python committer) Date: 2019-05-30 17:48
thanks, i'll take care of them.
Date User Action Args
2022-04-11 14:58:07adminsetgithub: 66579
2019-10-21 03:21:09gregory.p.smithsetstatus: open -> closed
stage: patch review -> commit review
resolution: fixed
versions: + Python 3.9, - Python 3.8
2019-05-30 17:48:10gregory.p.smithsetmessages: + msg343987
2019-05-30 10:32:55xtreaksetnosy: + xtreak
messages: + msg343966
2019-05-29 18:47:04gregory.p.smithsetmessages: + msg343910
2019-05-26 00:57:40gregory.p.smithsetmessages: + msg343527
2019-05-26 00:54:49gregory.p.smithsetkeywords: + patch
stage: needs patch -> patch review
pull_requests: + pull_request13486
2019-05-20 06:31:33gregory.p.smithsettype: enhancement
stage: needs patch
messages: + msg342888
versions: + Python 3.8, - Python 3.7
2018-06-09 11:03:02ncoghlanunlinkissue22555 dependencies
2017-05-03 14:32:26vstinnersetnosy: - vstinner
2017-05-03 13:19:04ncoghlansetmessages: + msg292900
2017-05-03 09:54:27mrh1997setnosy: + mrh1997
messages: + msg292871
2017-05-02 02:10:58ncoghlansetmessages: + msg292710
2017-05-01 20:07:05eric.smithsetmessages: + msg292699
2017-05-01 16:27:11gregory.p.smithsetnosy: + gregory.p.smith

messages: + msg292671
versions: + Python 3.7, - Python 3.6
2017-05-01 13:34:12ncoghlansetmessages: + msg292663
2017-02-02 22:23:19Christian Hsetnosy: + Christian H
2015-06-04 05:41:01gotgenessetnosy: + gotgenes
2015-05-12 05:00:00ncoghlansetmessages: + msg242941
versions: + Python 3.6, - Python 3.5
2014-10-05 04:09:50ncoghlanlinkissue22555 dependencies
2014-09-23 11:44:35barrysetnosy: + barry
2014-09-19 23:23:14belopolskysetnosy: + belopolsky
2014-09-17 11:01:06ncoghlansetmessages: + msg226993
2014-09-17 10:56:55ncoghlansetdependencies: + introduce bytes.hex method (also for bytearray and memoryview)
messages: + msg226992
title: Allow 'x' and 'X' to accept bytes-like objects in string formatting -> Define a binary output formatting mini-language for *.hex()
2014-09-11 09:44:58Arfreversetnosy: + Arfrever
2014-09-11 07:34:35eric.smithsetmessages: + msg226749
2014-09-11 07:25:21vstinnersetnosy: + vstinner
messages: + msg226748
2014-09-11 07:23:31eric.smithsetmessages: + msg226746
2014-09-11 07:12:17ned.deilysetnosy: + eric.smith
2014-09-10 23:57:42ncoghlansettitle: Allow 'x' and 'X' to accept bytes objects in string formatting -> Allow 'x' and 'X' to accept bytes-like objects in string formatting
2014-09-10 23:55:01ncoghlancreate