Same as issue #17150: 

>>> pprint.pprint({"a": b"\x00\xff" * 20})
{'a': b'\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff'}

... could be better formatted as:

>>> pprint.pprint({"a": b"\x00\xff" * 20})
{'a': b'\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00'
      b'\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff'
      b'\x00\xff\x00\xff\x00\xff'}

Here's a patch.  I needed to handle the fact that the repr of a single byte can be 1, 2 or 4 characters long and did not want to wrap in the middle of a byte representation.  Note also that bytes literals require a continuation character.  In the pathological case where the wrap size is smaller than the representation of a single byte, I chose to always print at least one byte per line.

As an aside, I also replaced the str wrapping code's calls to len with the cached _len used in the rest of pprint.py

oops, forgot to add some samples:
>>> pprint.pprint(b"\n\n\n\n\n\n", width=5)
b'\n'\
b'\n'\
b'\n'\
b'\n'\
b'\n'\
b'\n'

>>> pprint.pprint({"a": b"\x00\xff" * 20})
{'a': b'\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00'\
      b'\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff'\
      b'\x00\xff\x00\xff\x00\xff'}

>>> pprint.pprint({"a": b"\x00\xff" * 20}, width=20)
{'a': b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'\
      b'\x00\xff'}

>>> pprint.pprint(b'a\x00\n\\x00', width=20)
b'a\x00\n\\x00'

I don't understand why you say that "bytes literals require a continuation character":

>>> (b"x"
...  b"y")
b'xy'
>>> [b"x"
...  b"y"]
[b'xy']

I think the "len caching" is a misoptimization, it's useless here (most CPU time will be sent creating and wrapping the representation).
Also perhaps it would be nice to refactor things a bit, since we have both _str_parts and _bytes_parts used in exactly the same way (but that can also be done later).

As for the doc, the example would probably deserve to be a bit more "meaningful" :-)

- eval expects bytes to have a continuation character and test_str_wrap did an eval check so I figured test_bytes_wrap should as well:

# repr some bytes:
>>> b = b"\x00\xff" * 5
>>> b
b'\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff'
>>> r = repr(b)
>>> r
"b'\\x00\\xff\\x00\\xff\\x00\\xff\\x00\\xff\\x00\\xff'"
>>> eval(r)
b'\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff'

# hand-wrap it without the continuation character and it fails to eval (stuck the 
>>> s = "b'\\x00\\xff\\x00\\xff\\x00'\nb'\\xff\\x00\\xff\\x00\\xff'"
>>> print(s)
b'\x00\xff\x00\xff\x00'
b'\xff\x00\xff\x00\xff'
>>> eval(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 2
    b'\xff\x00\xff\x00\xff'
                          ^
SyntaxError: invalid syntax

# stick the continuation character in, and it evals properly
>>> s = s.replace("\n", "\\\n")
>>> print(s)
b'\x00\xff\x00\xff\x00'\
b'\xff\x00\xff\x00\xff'
>>> eval(s)
b'\x00\xff\x00\xff\x00\xff\x00\xff\x00\xff'

- I agree about the len v _len, but figured this wasn't all that foolish a consistency issue (i.e. the rest of pprint.py used _len)

- I also wanted to refactor _str_parts and _bytes_parts, but couldn't decide on the best course.  I was favoring a helper function to run the common loop since the two "if issubclass..." calls were so different and parameterizing the differences felt like it would obfuscate things too much.

- I also agree on the doc.  I figured I'd see if there weren't any hidden surprises with the patch before I worked on better doc.

Well, but eval works if you put parentheses as required by the grammar:

>>> s = "(b'xy'\nb'za')"
>>> eval(s)
b'xyza'

Yes, _str_parts and _bytes_parts should probably remain separate. It's the higher-level routine that would deserve sharing.
Also, perhaps the other wrapping routines (for dict, list...) could get the same treatment.

Here's a new version.  It looks more like the str_parts patch and uses parentheses instead of continuation as suggested.

Sample output:
>>> pprint.pprint(b"\n\na\x00", width=1)
   (b'\n'
    b'\n'
    b'a'
    b'\x00')

Ok, I hadn't noticed that pretty-printing a single string didn't add the parentheses as desired:

>>> pprint.pprint("abcd " * 6, width=15)
'abcd abcd '
'abcd abcd '
'abcd abcd '

On the other hand, the added parentheses aren't needed when inside a container (line continuations will work without them).

(and of course the same rules should apply to bytes objects :-))

Here is a patch based on current str formatting code (i.e. parenthesis are added only if needed, the space at the right is used more efficiently). It adds pprint support for bytes and bytearrays. Bytes are broken only at positions divisible by 4, so packed 32-bit ints are never broken.

Examples:

>>> pprint.pprint(bytes(range(128)))
(b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13'
 b'\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-./01234567'
 b'89:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{'
 b'|}~\x7f')
>>> pprint.pprint({'abcdefgh': bytes(range(128))})
{'abcdefgh': b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f'
             b'\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f'
             b' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ['
             b'\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\x7f'}
>>> pprint.pprint(bytearray(range(128)))
bytearray(b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f'
          b'\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f'
          b' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_'
          b'`abcdefghijklmnopqrstuvwxyz{|}~\x7f')
>>> pprint.pprint({'abcdefgh': bytearray(range(128))})
{'abcdefgh': bytearray(b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b'
                       b'\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17'
                       b'\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-./0123'
                       b'456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefg'
                       b'hijklmnopqrstuvwxyz{|}~\x7f')}

New changeset 976de10bf731 by Serhiy Storchaka in branch 'default':
Issue #17530: pprint now wraps long bytes objects and bytearrays.
https://hg.python.org/cpython/rev/976de10bf731