[Python-Dev] PEP 460 reboot

Brett Cannon brett at python.org
Mon Jan 13 23:05:10 CET 2014

On Mon, Jan 13, 2014 at 4:51 PM, Guido van Rossum <guido at python.org> wrote:

> Terminology. Let's use the official terminology rather than making stuff
> up.
> The docs at http://docs.python.org/3/library/string.html#formatspec
> use the following terminology:
> Replacement field: {...}; contains field name, conversion, format spec
> in that order, all optional.
> Field name: either a decimal integer (referring to an argument by
> position) or an identifier (by name), or omitted (uses the next
> available position).
> Conversion: !r, !s, !a; these refer to repr(), str(), ascii() to the
> value, and then the format spec applies to the resulting string.
> Format spec: colon, bunch of stuff, type; the type is a letter such as
> d (decimal) or s (string), and the stuff between the colon and the
> type is used to specify field width, alignment, sign, padding and
> such.
> Also. {:b} means binary (i.e. numbers in base 2). I'm not sure what
> this leaves for interpolating bytes if we don't want to use {:s}. The
> docs at
> http://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting
> don't show %b so it could still be used there, but it would be nicer
> to be consistent.

I have been going on the assumption that bytes.format() would change what
'{}' meant for itself and would only interpolate bytes. That convenient
between Python 2 and 3 since it represents what we want it to (str and
bytes under the hood, respectively), so it just falls through. We could
also add a 'b' conversion for bytes() explicitly so as to help people not
accidentally mix up things in bytes.format() and str.format(). But I was
not suggesting adding a specific format spec for bytes but instead making
bytes.format() just do the .encode('ascii') automatically to help with
compatibility when a format spec was present. If people want fancy
formatting for bytes they can always do it themselves before calling
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