Python 2 has a -b command-line option which is undocumented.

It is used by the warnings machinery to decide to warn about (-b) or raise an error (-bb) if someone uses str(bytes) or str(buffer) (explanation courtesy of grep and Python/pythonrun.c).

Are you sure?  (see issue 10471).  I don't see how -b could do anything in python2, since bytes and string are the same there (and, indeed, from command line experimentation it doesn't seem to).

The code definitely is in 2.7.

Python/_warnings.c:861:    if (Py_BytesWarningFlag > 1)
Python/_warnings.c:863:    else if (Py_BytesWarningFlag)
Python/_warnings.c:867:    PyList_SET_ITEM(filters, pos++, create_filter(PyExc_BytesWarning,
Python/sysmodule.c:1257:    SetFlag(Py_BytesWarningFlag);
Python/pythonrun.c:81:int Py_BytesWarningFlag; /* Warn on str(bytes) and str(buffer) */
Include/pydebug.h:14:PyAPI_DATA(int) Py_BytesWarningFlag;
Include/pyerrors.h:178:PyAPI_DATA(PyObject *) PyExc_BytesWarning;
Modules/main.c:285:            Py_BytesWarningFlag++;
Objects/bytearrayobject.c:1003:    if (Py_BytesWarningFlag) {
Objects/bytearrayobject.c:1004:        if (PyErr_WarnEx(PyExc_BytesWarning,
Objects/bytearrayobject.c:1028:        if (Py_BytesWarningFlag && op == Py_EQ) {
Objects/bytearrayobject.c:1029:            if (PyErr_WarnEx(PyExc_BytesWarning,
Objects/exceptions.c:2018: *    BytesWarning extends Warning
Objects/exceptions.c:2020:SimpleExtendsException(PyExc_Warning, BytesWarning,
Objects/exceptions.c:2110:    PRE_INIT(BytesWarning)
Objects/exceptions.c:2178:    POST_INIT(BytesWarning)
Lib/warnings.py:399:    simplefilter(bytes_action, category=BytesWarning, append=1)
Lib/test/test_bytes.py:25:            with test.test_support.check_warnings(('', BytesWarning)):
Lib/test/exception_hierarchy.txt:50:	   +-- BytesWarning
Include/pydebug.h:14:PyAPI_DATA(int) Py_BytesWarningFlag;
Include/pyerrors.h:178:PyAPI_DATA(PyObject *) PyExc_BytesWarning;
Doc/library/warnings.rst:164:* :exc:`BytesWarning` is ignored unless the :option:`-b` option is given once or

rdmurray@hey:~/python/p27>cat temp.py
x = bytes('abc')
print x
print str(x)
rdmurray@hey:~/python/p27>./python -bb temp.py
abc
abc

test_bytes has a branch for -b and passes without it, with -b and with -bb.  I really don’t know.

Even "from __future__ import unicode_literals" doesn't make it do anything.

Perhaps Christian merged it by mistake in [5341b30b1812]?

Nick Coghlan wrote:
> 
> Nick Coghlan <ncoghlan@gmail.com> added the comment:
> 
> Even "from __future__ import unicode_literals" doesn't make it do anything.
> 
> Perhaps Christian merged it by mistake in [5341b30b1812]?

It looks more like some parts were left out in the merge
by accident: The 2.7 code only raises the warning for bytearrays,
not for bytes and buffers, whereas 3.2 issues the warning for
all bytes and bytearray (but not buffers; which makes sense,
since Python 3 no longer has a text buffer interface).

Hi Marc

I tried reproducing this for bytearray using Python 2.7.2 but I can't see a warning.

devel@moses:~$ python --version
Python 2.7.2+
devel@moses:~$ cat test_py.py 
k = range(10)
kb = bytearray(k)
print kb
kb = bytearray("hi")
print kb
devel@moses:~$ python -b test_py.py 
 
hi
devel@moses:~$ python -bb test_py.py 
 
hi

Please correct me if I'm wrong anywhere here. I'd like to work on this.

Try this to trigger a warning:

python2 -b -c 'bytearray("3") == u"3"'
-c:1: BytesWarning: Comparison between bytearray and string

Might be worth making this addition from 3 (I'm not sure how to add this to 2)
-b     : issue warnings about str(bytes_instance), str(bytearray_instance)
         and comparing bytes/bytearray with str. (-bb: issue errors)


Building on Martin's example:

On all of these, python2 is
Python 2.7.6 (default, Apr  6 2014, 23:14:26)
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.


emily-mba:cpython emily$ python2
>>> bytearray("3") == u"3"
False

emily-mba:cpython emily$ python2 -b
>>> bytearray("3") == u"3"
__main__:1: BytesWarning: Comparison between bytearray and string
False

emily-mba:cpython emily$ python2 -bb
>>> bytearray("3") == u"3"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
BytesWarning: Comparison between bytearray and string

Here's an attempt (based on 3's main.c http://hg.python.org/cpython/file/8866ac6f2269/Modules/main.c)

Trouble is, in Python 2 bytes() and str() are the same thing, and most of those conditions don’t apply. Maybe something like this is more correct:

-b     : issue warnings about comparing bytearray with unicode.
         (-bb: issue errors)

As well as the usage string, it is missing from the list of options in the main documentation at Doc/using/cmdline.rst. There are a couple of places (sys.rst and warnings.rst) that reference :option:`-b`, and newer versions of Sphinx complain about this.

I have attached a patch for 2.7 that adds -b and -bb to the command line documentation in Modules/main.c and Doc/library/cmdline.rst, following the suggested text provided by Martin.

Interestingly, unicode(), bytearray(), and str() don't seem to be transitive in 2.7.

>>> u"3" == str(3)
True

>>> str(3) == bytearray("3")
True

>>> u"3" == bytearray("3")
False

Right, the lack of transitivity comes from the fact that:

    >>> u"3" == str(3)
    True

Is really shorthand for:

    >>> u"3" == str(3).decode("ascii")
    True

However, the implicit decoding only triggers for *exactly* bytes instances, so in the bytearray case it needs to be explicit:

    >>> u"3" == bytearray(b"3").decode("ascii")
    True

Added some review comments on the patch. The only critical one was changing a :class:`bytes` reference to :class:`unicode`, but I also suggested it may be worth mentioning that it *doesn't* enable warnings about all binary/text comparisons the way the Python 3 one does.

Thanks for the :class: markup edits. I'm still learning this stuff.

New changeset f6c6d1e2304a06efdf65c60c3e1c6bbe1f192fd0 by Zachary Ware in branch '2.7':
bpo-11681: Document the `-b` and `-bb` options (GH-1562)
https://github.com/python/cpython/commit/f6c6d1e2304a06efdf65c60c3e1c6bbe1f192fd0

Thanks for the patch, Greg!  I implemented Nick's suggestions and merged it.