Eric V. Smith added the comment:
So it sounds like the use case is (as Glyph said in msg180432):
- Provide a transition for users of 2.7's of str %-formatting into a style
that's compatible with both str in 2.7 and bytes in 3.4.
In that case the only options I see are to implement __mod__ or .format for
bytes in 3.4. I'd of course prefer to use .format, although __mod__ would
probably make the transition easier (no need to move to .format first). It
would probably also make the implementation easier, since there's so much less
code in str.__mod__. But let's assume we're using .format [1].
Given the restricted use case, and assuming we using .format, the
implementation would not need to support:
- Types other than bytes, int, float.
- Subclasses of these types with custom formatting.
- !s, !r, or !a (none of the ! conversions). [2]
But it would support all of the specifiers for formatting strs (except now for
bytes), floats, and ints.
I haven't looked through the str.format or {str,int,float}.__format__ code
since the PEP 393 work, so I'm not really sure if we could stringlib-ify the
code again, or if it would just be easier to reimplement it as separate
bytes-only code.
[1] It's open for debate whether .format or .__mod__ is preferable.
[2] Since %-formatting supports %r and %s, this point is arguable.
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