[Python-Dev] Migration from Python 2.7 and bytes formatting

[Glenn Linderman]

On 1/17/2014 2:49 AM, Neil Schemenauer wrote:
> As I see it, there are two separate goals in adding formatting
> methods to bytes.  One is to make it easier to write new programs
> that manipulate byte data.  Another is to make it easier to upgrade
> Python 2.x programs to Python 3.x.  Here is an idea to better
> address these separate goals.
>
> Introduce %-interpolation for bytes.  Support the following format
> codes to aid in writing new code:
>
>      %b: insert arbitrary bytes (via __bytes__ or Py_buffer)
>
>      %[dox]: insert an integer, encoded as ASCII
>
>      %[eEfFgG]: insert a float, encoded as ASCII
>
>      %a: call ascii(), insert result
>
> Add a command-line option, disabled by default, that enables the
> following format codes:
>
>      %s: if the object has __bytes__ or Py_buffer then insert it.
>          Otherwise, call str() and encode with the 'ascii' codec
>
>      %r: call repr(), encode with the 'ascii' codec
>
>      %[iuX]: as per Python 2.x, for backwards compatibility
>
> Introducing these extra codes and the command-line option will
> provide a more gradual upgrade path.  The next step in porting could
> be to examine each %s inside bytes literals and decide if they
> should either be converted to %b or if the literal should be
> converted to a unicode literal.  Any %r codes could likely be safely
> changed to %a.

-1 overall.

Not worth the extra complexity in documentation and command line parameters.

%s, since it cannot be used for strings of characters (str) anyway, 
might as well be used for strings of bytes, and of necessity for 
single-code-base porting, must be usable in that manner.

I would give  +.5 to the idea of supporting %a in Python 3
I would give +.2 for %r as a synonym for %a in Python 3.

%r and %a don't produce fixed-width fields, so are likely used in places 
where the exact length in bytes is flexible, and in ASCII segments of 
the byte stream... supporting them both with the semantics of  %a  might 
be useful.
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