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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()
Examples:
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.

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.

> ".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.

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'

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.

python-ideas post with a sketch of a possible mini-language: https://mail.python.org/pipermail/python-ideas/2014-September/029352.html 

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.

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.

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.

The Unix "od" command pretty much has all of the possibilities covered.
https://linuxconfig.org/od-1-manual-page
Although "named characters" might be going a bit far. Float, too.

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()
 'b901ef'
 >>> 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()
 'b901ef'
 >>> 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)

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 https://docs.python.org/3/library/string.html#format-specification-mini-language).
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 http://bugs.python.org/issue22385#msg292663 ("h#,1") is more intuitive than in http://bugs.python.org/issue22385#msg226733 ("#,.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...

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'

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}'.

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).

New changeset 0c2f9305640f7655ba0cd5f478948b2763b376b3 by Gregory P. Smith in branch 'master':
bpo-22385: Support output separators in hex methods. (#13578)
https://github.com/python/cpython/commit/0c2f9305640f7655ba0cd5f478948b2763b376b3

This change seems to have created some compile time warnings : https://buildbot.python.org/all/#/builders/103/builds/2544/steps/3/logs/warnings__6_
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]

thanks, i'll take care of them.