[Python-ideas] Rough draft: Proposed format specifier for a thousands separator (discussion moved from python-dev)

[Raymond Hettinger]

Motivation:

    Provide a simple, non-locale aware way to format a number
    with a thousands separator.

    Adding thousands separators is one of the simplest ways to
    improve the professional appearance and readability of
    output exposed to end users.

    In the finance world, output with commas is the norm.  Finance users
    and non-professional programmers find the locale approach to be
    frustrating, arcane and non-obvious.

    It is not the goal to replace locale or to accommodate every
    possible convention.  The goal is to make a common task easier
    for many users.


Research so far:

    Scanning the web, I've found that thousands separators are
    usually one of COMMA, PERIOD, SPACE, or UNDERSCORE.  The
    COMMA is used when a PERIOD is the decimal separator.

    James Knight observed that Indian/Pakistani numbering systems
    group by hundreds.   Ben Finney noted that Chinese group by
    ten-thousands.

    Visual Basic and its brethren (like MS Excel) use a completely
    different style and have ultra-flexible custom format specifiers
    like: "_($* #,##0_)".

    

Proposal I (from Nick Coghlan]:
    
    A comma will be added to the format() specifier mini-language:

    [[fill]align][sign][#][0][minimumwidth][,][.precision][type]

    The ',' option indicates that commas should be included in the output as a
    thousands separator. As with locales which do not use a period as the
    decimal point, locales which use a different convention for digit
    separation will need to use the locale module to obtain appropriate
    formatting.

    The proposal works well with floats, ints, and decimals.  It also
    allows easy substitution for other separators.  For example:

        format(n, "6,f").replace(",", "_")

    This technique is completely general but it is awkward in the one
    case where the commas and periods need to be swapped.

        format(n, "6,f").replace(",", "X").replace(".", ",").replace("X", ".")


Proposal II (to meet Antoine Pitrou's request):

    Make both the thousands separator and decimal separator user specifiable
    but not locale aware.  For simplicity, limit the choices to a comma, period, 
    space, or underscore..

    [[fill]align][sign][#][0][minimumwidth][T[tsep]][dsep precision][type]
    
    Examples:

        format(1234, "8.1f")    -->     '  1234.0'
        format(1234, "8,1f")    -->     '  1234,0'        
        format(1234, "8T.,1f")  -->     ' 1.234,0'
        format(1234, "8T .f")   -->     ' 1 234,0'
        format(1234, "8d")      -->     '    1234'
        format(1234, "8T,d")      -->   '   1,234'

    This proposal meets mosts needs (except for people wanting grouping
    for hundreds or ten-thousands), but iIt comes at the expense of
    being a little more complicated to learn and remember.  Also, it makes it
    more challenging to write custom __format__ methods that follow the
    format specification mini-language.

    For the locale module, just the "T" is necessary in a formatting string
    since the tool already has procedures for figuring out the actual
    separators from the local context.

Comments and suggestions are welcome but I draw the line at Mayan
numbering conventions ;-)


Raymond