[Python-Dev] RFC: PEP 460: Add bytes % args and bytes.format(args) to Python 3.5

Nick Coghlan ncoghlan at gmail.com
Sun Jan 12 03:53:00 CET 2014

On 12 Jan 2014 03:29, "Ethan Furman" <ethan at stoneleaf.us> wrote:
> On 01/11/2014 12:43 AM, Nick Coghlan wrote:
>> In particular, the bytes type is, and always will be, designed for
>> pure binary manipulation [...]
> I apologize for being blunt, but this is a lie.
> Lets take a look at the methods defined by bytes:
>>>> dir(b'')
> ['__add__', '__class__', '__contains__', '__delattr__', '__dir__',
'__doc__', '__eq__', '__format__', '__ge__', '__getattribute__',
'__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__',
'__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__rmul__', '__setattr__',
'__sizeof__', '__str__', '__subclasshook__', 'capitalize', 'center',
'count', 'decode', 'endswith', 'expandtabs', 'find', 'fromhex', 'index',
'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle',
'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition',
'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip',
'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title',
'translate', 'upper', 'zfill']
> Are you really going to insist that expandtabs, isalnum, isalpha,
isdigit, islower, isspace, istitle, isupper, ljust, lower, lstrip, rjust,
splitlines, swapcase, title, upper, and zfill are pure binary manipulation

Do you think I don't know that? However, those are all *in-place*
modifications. Yes, they assume ASCII compatible formats, but they're a far
cry from encouraging combination of data from potentially different sources.

I'm also on record as considering this a design decision I regret,
precisely because it has resulted in experienced Python 2 developers
failing to understand that the Python 3 text model is *different* and they
may need to  create a new type.

> Let's take a look at the repr of bytes:
>>>> bytes([48, 49, 50, 51])
> b'0123'
> Wow, that sure doesn't look like binary data!
> Py3 did not go from three text models to two, it went to one good one
(unicode strings) and one broken one (bytes).  If the aim was indeed for
pure binary manipulation, we failed.  We left in bunches of methods which
can *only* be interpreted as supporting ASCII manipulation.

No, no, no. We made some concessions in the design of the bytes type to
*ease* development and debugging of ASCII compatible protocols *where we
believed we could do so without compromising the underlying text model

Many experienced Python 2 developers are now suffering one of the worst
cases of paradigm lock I have ever seen as they keep trying to make the
Python 3 text model the same as the Python 2 one instead of actually
learning how Python 3 works and recognising that they may actually need to
create a new type for their use case and then potentially seek core dev
assistance if that type reveals new interoperability bugs in the core types
(or encounters old ones).

> Due to backwards compatibility we cannot now finish yanking those out, so
either we live with a half-dead class screaming "I want be ASCII!  I want
to be ASCII!" or add back the missing functionality.

No, we don't - we treat the core bytes type as PEP 460 does, by adding a
*new* feature proposed by a couple people writing native Python 3 libraries
like asyncio that makes binary formats easier to deal with without carrying
forward even *more* broken assumptions from the Python 2 text model.
(Remember, I'm in favour of Antoine's updated PEP, because it's a real spec
for a new feature, rather than yet another proposal to bolt on even more
text specific formatting features from someone that has never bothered to
understand the reasons for the differences between the two versions).

People that want a full hybrid type back can then pursue the custom
extension type approach.


> --
> ~Ethan~
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