Rough draft: Proposed format specifier for a thousands separator

Raymond Hettinger python at rcn.com
Thu Mar 12 08:30:51 CET 2009


If anyone here is interested, here is a proposal I posted on the
python-ideas list.

The idea is to make numbering formatting a little easier with the new
format() builtin
in Py2.6 and Py3.0:  http://docs.python.org/library/string.html#formatspec


-------------------------------------------------------------


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 it 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 supporting
Mayan numbering conventions ;-)


Raymond



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