Taking some ideas from the various threads, here's what I'd like to propose:
(Assume that brackets [] means 'optional field')
[:[type][align][sign][[0]minwidth][.precision]][/fill][!r]
Examples:
:f # Floating point number of natural width
:f10 # Floating point number, width at least 10
:f010 # Floating point number, width at least 10, leading zeros
:f.2 # Floating point number with two decimal digits
:8 # Minimum width 8, type defaults to natural type
:d+2 # Integer number, 2 digits, sign always shown
!r # repr() format
:10!r # Field width 10, repr() format
:s10 # String right-aligned within field of minimum width
# of 10 chars.
:s10.10 # String right-aligned within field of minimum width
# of 10 chars, maximum width 10.
:s<10 # String left-aligned in 10 char (min) field.
:d^15 # Integer centered in 15 character field
:>15/. # Right align and pad with '.' chars
:f<+015.5 # Floating point, left aligned, always show sign,
# leading zeros, field width 15 (min), 5 decimal places.
Notes:
-- Leading zeros is different than fill character, although the two
are mutually exclusive. (Leading zeros always go between the sign and
the number, padding does not.)
-- For strings, precision is used as maximum field width.
-- __format__ functions are not allowed to re-interpret '!r'.
I realize that the grouping of things is a little odd - for example, it
would be nice to put minwidth, padding and alignment in their own little
group so that they could be processed independently from __format__.
However:
-- Since minwidth is the most common option, I wanted it to have no
special prefix char.
-- I wanted precision to come after minwidth, since the 'm.n' format
feels intuitive and traditional.
-- I wanted type to come first, since it affects how some attributes
are interpreted.
-- Putting the sign right before the width field also feels right.
The regex for interpreting this, BTW, is something like the following:
"(?:\:([a-z])?(<|>|\^)?(+|-)?(\d+)(\.\d+))(/.)?(!r)?"
(Although it may make more sense to allow the fill and regex fields to
appear in any order. In other words, any field that is identified by a
unique prefix char can be specified in any order.)
-- Talin
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