If partition is called with a single byte it works correctly but if called with the equivalent integer it returns the same bytearray with two empty arrays as follows.

py> ba = bytearray(range(8))
py> ba
bytearray(b'\x00\x01\x02\x03\x04\x05\x06\x07')
py> 3 in ba
True
py> ba.find(3) == ba.index(3) == ba.find(b'\x03')
True
py> ba.partition(b'\x03')
(bytearray(b'\x00\x01\x02'), bytearray(b'\x03'), bytearray(b'\x04\x05\x06
\x07'))
py> ba.partition(3)
(bytearray(b'\x00\x01\x02\x03\x04\x05\x06\x07'), bytearray(b''), bytearray
(b''))

More background on the thread starting here https://mail.python.org/pipermail/python-list/2013-December/663111.html which refers to Issue 12170.

Similar bug was in 3.2:

>>> ba = bytearray(range(8))
>>> ba[2:6]
bytearray(b'\x02\x03\x04\x05')
>>> ba[2:6] = 2
>>> ba
bytearray(b'\x00\x01\x00\x00\x06\x07')

Now it is fixed.

Bytearray slice assignment bug was fixed in issue8401.

Whatever the change, bytes and bytearray should act the same.

>>> b = bytes(range(8))
>>> b
b'\x00\x01\x02\x03\x04\x05\x06\x07'
>>> b.partition(3)
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    b.partition(3)
TypeError: expected bytes, bytearray or buffer compatible object

As noted in the thread, ba.partition(a) apparently is executed as ba.partition(bytearray(a)) if a is not a bytearray (or maybe not a buffer object). bytearray(3) == bytearray((0,0,0)) and the latter is not in ba and hence the output given is 'correct'.

I believe that all methods should act the same, but they don't as a result of the work done in issue12170.  E.g. find will accept integer input but split will not.  Given this comment at the top of test_bytes.py "XXX This is a mess.  Common tests should be moved to buffer_tests.py, which itself ought to be unified with string_tests.py (and the latter should be modernized).", it looks like a thorough review of the code and tests is in order.

To answer Mark, even though no longer nosy: In general, sequence methods .count, .index, and .__contains__ take sequence members and only members as arguments.  Unicode sequences are exceptional because codepoints are not Python objects, so string subsequences must be used instead.  Byte-like sequences are also exceptional in that both members and subsequences are accepted for these methods.

String-like sequence methods .split and .partition take subsequences as arguments.  I think the doc should make this clearer.

New changeset a2314283ff87c65e1745a42c2f2b716b1a209128 by Serhiy Storchaka in branch 'master':
bpo-20047: Make bytearray methods partition() and rpartition() rejecting (#4158)
https://github.com/python/cpython/commit/a2314283ff87c65e1745a42c2f2b716b1a209128

New changeset 9ea5a3a45b35d01b602e7e4da4f72b2db407e5c6 by Serhiy Storchaka in branch '3.6':
[3.6] bpo-20047: Make bytearray methods partition() and rpartition() rejecting (GH-4158) (#4162)
https://github.com/python/cpython/commit/9ea5a3a45b35d01b602e7e4da4f72b2db407e5c6

New changeset 107f3cc791d223dc06b7c80f0de672e88ae6a8d1 by Serhiy Storchaka in branch '2.7':
[2.7] bpo-20047: Make bytearray methods partition() and rpartition() rejecting (GH-4158) (#4163)
https://github.com/python/cpython/commit/107f3cc791d223dc06b7c80f0de672e88ae6a8d1

New changeset 0b9fe1734168d45861d6dc3022492387dec5a4a2 by Berker Peksag (Zackery Spytz) in branch '2.7':
[2.7] bpo-20047: Remove Objects/bytesobject.c from 2.7 (GH-9268)
https://github.com/python/cpython/commit/0b9fe1734168d45861d6dc3022492387dec5a4a2