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To help writing Python 2/3 code the -b flag should switch on a warning when comparing an int to a bytes object in Python 2. This will help when someone writes something like `b'abcd'[2] == b'c'` and it always returns False thanks to the indexing returning 99 in Python 3.

6 tests failed:
 test_buffer test_poplib test_quopri test_smtpd test_sunau
 test_tokenize
And all of them look as bugs.

Here is a patch that adds required feature and fixes existing bugs in the stdlib and tests.
No one of failed tests was false positive.

> [...] fixes existing bugs in the stdlib and tests.
These changes should probably be backported to 3.4.

LGTM, just one minor comment in review.

I didn't understand the issue.
$ python2
>>> b'A'[0] == 65
False
$ python3
Python 3.4.1 (default, Nov 3 2014, 14:38:10) 
>>> b'A'[0] == 65
True
Oh ok, now I get it: Python 2 and Python 3 behaves differently when comparing a string of a single byte and an integer.
Ok to raise a warning when -b command line option is used.

@Brett: Yes, the patch handles `42 == b'*'` as well.
@Victor: The problem in such code:
x = b'Array'
x[0] == b'A'
Added explicit tests for bytes on the right side of the comparison and replaced b'.'[0] to ord(b'.').

> This will help when someone writes something like `b'abcd'[2] == b'c'`
What if someone writes line[-1] == 0 and line is a Unicode string? Should we emit a warning?
I patched locally PyUnicode_RichCompare() to emit a warning. Hum, there are *many* warnings in argparse, http.client, and many other modules. I don't think that it's a good idea. It looks common to use a string for a sentinel (ex: state = "UNKNOWN") and then store an int (ex: state = 0). So comparison need to check the type (ex: isinstance(state, str) and state == "UNKNOWN") which is verbose and annoying.
So no, we should not emit a warning :-)

I did one final pass on the patch and only had wording comments, so tweak those and it LGTM. Assigned to Serhiy since it's obviously his patch. =)
Only other thing is to either open up a separate bug or at least apply the fixes to the stdlib in Python 3.4 as well as 3.5.

New changeset 104c55bc2276 by Serhiy Storchaka in branch '3.4':
Issue #23681: Fixed Python 2 to 3 poring bugs.
https://hg.python.org/cpython/rev/104c55bc2276
New changeset 817f1f47824c by Serhiy Storchaka in branch 'default':
Issue #23681: Fixed Python 2 to 3 poring bugs.
https://hg.python.org/cpython/rev/817f1f47824c
New changeset 68e9cf0ae93d by Serhiy Storchaka in branch 'default':
Issue #23681: The -b option now affects comparisons of bytes with int.
https://hg.python.org/cpython/rev/68e9cf0ae93d