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There are currently no dedicated docs for the bytes and bytearray methods - the relevant section just refers back to the str methods. This isn't sufficient, since the str methods cover of lot of stuff related to Unicode that isn't relevant to the binary sequence types, and it doesn't cleanly cover the differences either (like the fact that several methods accept integers).
I've started work on a patch that documents the binary APIs explicitly, although bytes and bytearray still share docs. The methods are grouped into three categories:
- work with arbitrary binary data
- assume ASCII compatibility by default, but can still be used with arbitrary binary data when given suitable arguments
- can only be used safely with data in an ASCII compatible format
I've worked through and updated the new entries for the first category, but the latter two categories are still just copy-and-paste from the str docs.

v2 patch converts the second category of functions. This conversion highlighted the lack of good examples in the str.split() docs, as well as some over and underspecification in the behaviour of the centering and justification methods (guarantees about object identity that don't hold for bytearray, failure to note that the default fill character is specifically an ASCII space - Unicode has more than one space type), so I also fixed those.
Added Guido to the nosy list - Guido, if you could cast your eye over this and at least give a +1 to the general approach, that would be great, otherwise I'll just go ahead and merge it some time after I finish converting the final category (which I expect will be no later than the PyCon AU sprints in early August, and potentially sooner)

Why are you removing guarantees like these from the str docs:
"The original string is returned if *width* is less than or equal to ``len(s)``."
?
This doesn't seem to have anything to do with documenting bytes and bytearrays.

On 13 Jul 2014 18:39, "Marc-Andre Lemburg" <report@bugs.python.org> wrote:
>
>
> Marc-Andre Lemburg added the comment:
>
> Why are you removing guarantees like these from the str docs:
>
> "The original string is returned if *width* is less than or equal to
``len(s)``."
Because it's untrue for bytearray, and possible object reuse is a general
characteristic of immutability for str and bytes. If another implementation
makes a copy for some reason, it would still be considered "Python".
Since the sentence thus conveys no useful information, I removed it from
both the text and binary variants rather than coming up with appropriate
wording to indicate that the behaviour of returning a new reference to the
existing object when no content changes are needed doesn't apply to the
mutable bytearray.
>
> ?
>
> This doesn't seem to have anything to do with documenting bytes and
bytearrays.
>
> ----------
> nosy: +lemburg
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue21777>
> _______________________________________

On Sun, Jul 13, 2014 at 7:00 PM, Nick Coghlan <report@bugs.python.org>
wrote:
>
> Nick Coghlan added the comment:
>
> On 13 Jul 2014 18:39, "Marc-Andre Lemburg" <report@bugs.python.org> wrote:
> >
> >
> > Marc-Andre Lemburg added the comment:
> >
> > Why are you removing guarantees like these from the str docs:
> >
> > "The original string is returned if *width* is less than or equal to
> ``len(s)``."
>
> Because it's untrue for bytearray, and possible object reuse is a general
> characteristic of immutability for str and bytes. If another implementation
> makes a copy for some reason, it would still be considered "Python".
>
> Since the sentence thus conveys no useful information, I removed it from
> both the text and binary variants rather than coming up with appropriate
> wording to indicate that the behaviour of returning a new reference to the
> existing object when no content changes are needed doesn't apply to the
> mutable bytearray.
>
That feels like overreacting. It *is* useful to know about this guarantee,
and it would be better if we could somehow require it rather than claim it
doesn't matter. And before you counter with examples of other CPython
behaviors that *shouldn't* be guaranteed across implementations, I am
talking about this specific case, and every case needs to be examined on
its merits separately. It is possible that in the end we'll decide this
particular guarantee is not worth having -- but I think that should not be
decided by a refactoring of the docs.

3rd in progress draft - converted most of the "inherently assumes ASCII" docs now. I think this set of changes really makes it clear how non-trivial it actually is to infer the binary domain behaviour from the str docs, which have all sorts of Unicode complications. You can't easily infer the behaviour from the Python 2 docs either, since these operations were locale dependent for Python 2 str objects.

Note I haven't added back the immutability guarantees yet - I'll do that before declaring this ready for final review.

OK, I've completed the initial pass through all the methods. Remaining items:
* add back the guarantees where str will return the same object, add those guarantees for bytes where applicable
* address the review comments from Zach and Ezio
There are a couple of review comments about removing duplication that I'd like to skip addressing for now. I think they're reasonable ideas, but I also think it's a lot easier to go wrong with DRY in docs than it is in code. Indeed, this whole matter of not documenting the bytes behaviour in the first place was a matter of assuming folks could just infer the binary behaviour from the text behaviour.

v5 has all the review comments I accepted as being in scope addressed, including the restoration/addition of the notes about returning the object unchanged for center(), ljust(), rjust() and zfill() when the field width is less than or equal to the length of the string.

I think this is done now - absent any major objections, I'll push it live in a couple of days time.

New changeset e750d2b44c1d by Nick Coghlan in branch '3.4':
Issue #21777: separate docs for binary sequence methods
http://hg.python.org/cpython/rev/e750d2b44c1d
New changeset e205bce4cc0a by Nick Coghlan in branch 'default':
Merge #21777 from 3.4
http://hg.python.org/cpython/rev/e205bce4cc0a 

Merged after reviews from Zach & Ezio.
Zach, Ezio - if there are any other refactorings from the reviews that you'd like to pursue, consider pulling them out to separate issues so we don't forget about them.

> Zach, Ezio - if there are any other refactorings from the reviews that
> you'd like to pursue, consider pulling them out to separate issues so
> we don't forget about them.
See #23560.