python-peps/pep-0378.txt

PEP: 378
Title: Format Specifier for Thousands Separator
Version: $Revision$
Last-Modified: $Date$
Author: Raymond Hettinger <python@rcn.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 12-Mar-2009
Python-Version: 2.7 and 3.1
Post-History: 12-Mar-2009


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
humanize a program's output, improving its professional appearance
and readability.

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

The locale module presents two other challenges.  First, it is
a global setting and not suitable for multi-threaded apps that
need to serve-up requests in multiple locales.  Second, the
name of a relevant locale (such as "de_DE") can vary from
platform to platform or may not be defined at all.  The docs
for the locale module describe these and `many other challenges`_
in detail.

.. _`many other challenges`:  http://docs.python.org/library/locale.html#background-details-hints-tips-and-caveats

It is not the goal to replace the locale module, to perform
internationalization tasks, or accommodate every possible
convention.  Such tasks are better suited to robust tools like
`Babel`_ . Instead, the goal is to make a common, everyday
task easier for many users.

.. _`Babel`: http://babel.edgewall.org/


Current Version of the Mini-Language
====================================

* `Python 2.6 docs`_

  .. _Python 2.6 docs: http://docs.python.org/library/string.html#formatstrings

* PEP 3101 Advanced String Formatting


Research so far
===============

Scanning the web, I've found that thousands separators are
usually one of COMMA, DOT, SPACE, APOSTROPHE or UNDERSCORE.  

Visual Basic and its brethren (like `MS Excel`_) use a completely
different style and have ultra-flexible custom format
specifiers like::

    "_($* #,##0_)".

.. _`MS Excel`: http://www.brainbell.com/tutorials/ms-office/excel/Create_Custom_Number_Formats.htm

`COBOL`_ uses picture clauses like::

    PICTURE $***,**9.99CR

.. _`COBOL`: http://en.wikipedia.org/wiki/Cobol#Syntactic_features

`Common Lisp`_ uses a COLON before the ``~D`` decimal type specifier to
emit a COMMA as a thousands separator.  The  general form of ``~D`` is
``~mincol,padchar,commachar,commaintervalD``.  The *padchar* defaults
to SPACE.  The *commachar* defaults to COMMA.  The *commainterval*
defaults to three.

::

    (format nil "~:D" 229345007)   =>   "229,345,007"

.. _`Common Lisp`: http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node200.html

`C-Sharp`_ provides both styles (picture formatting and type specifiers).
The type specifier approach is locale aware.  The picture formatting only
offers a COMMA as a thousands separator::

    String.Format("{0:n}", 12400)     ==>    "12,400"
    String.Format("{0:0,0}", 12400)   ==>    "12,400"

.. _`C-Sharp`: http://blog.stevex.net/index.php/string-formatting-in-csharp/


Proposal I (from Nick Coghlan)
==============================

A comma will be added to the format() specifier mini-language::

[[fill]align][sign][#][0][width][,][.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,d").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", ".")

The *width* argument means the total length including the commas
and decimal point::

  format(1234, "08,d")     -->    '0001,234'
  format(1234.5, "08,.1f") -->    '01,234.5'


Proposal II (from Eric Smith)
=============================

Make both the thousands separator and decimal separator user
specifiable but not locale aware.  For simplicity, limit the
choices to a COMMA, DOT, SPACE, APOSTROPHE or UNDERSCORE.
The SPACE can be either U+0020 or U+00A0.

Whenever a separator is followed by a precision, it is a
decimal separator and an optional separator preceding it is a
thousands separator.  When the precision is absent, a lone
specifier means a thousands separator::

[[fill]align][sign][#][0][width][tsep][dsep precision]][type]

Examples::

  format(1234, "8.1f")     -->    '  1234.0'
  format(1234, "8,1f")     -->    '  1234,0'
  format(1234, "8.,1f")    -->    ' 1.234,0'
  format(1234, "8 ,f")     -->    ' 1 234,0'
  format(1234, "8d")       -->    '    1234'
  format(1234, "8,d")      -->    '   1,234'
  format(1234, "8_d")      -->    '   1_234'

This proposal meets mosts needs, but it comes at the expense
of taking a bit more effort to parse.  Not every possible
convention is covered, but at least one of the options (spaces
or underscores) should be readable, understandable, and useful
to folks from many diverse backgrounds.

As shown in the examples, the *width* argument means the total
length including the thousands separators and decimal separators.

No change is proposed for the locale module.


Comparison
==========

The difference between the two proposals is that the first is hard-wired
to a COMMA for a thousands separator and a DOT as a decimal separator.
The second allows either separator to be one of several possibilities.



Other Ideas
===========

* Lie Ryan suggested a convenience function of the form::

    create_format(self, type='i', base=16, seppos=4, sep=':',
                  charset='0123456789abcdef', maxwidth=32,
                  minwidth=32, pad='0')

* Eric Smith would like the C version of the mini-language
  parser to be exposed with hooks that would make it easier
  to write custom *__format__* methods.  That way, methods like
  *Decimal.__format__* would not have to be written from scratch.

* Antoine Pitrou noted that the provision for a SPACE separator
  should also allow a non-breaking space (U+00A0).

* A poster on the newgroup, Wolfgang Rohdewald, noted that a
  convention in Switzerland is to use an APOSTROPHE as a
  thousands separator, ``12`000.99``.

* The `ADA language`_ allows UNDERSCORES in its numeric literals.

.. _`ADA language`: http://archive.adaic.com/standards/83lrm/html/lrm-02-04.html


Commentary
==========

* Some commenters do not like the idea of format strings at all
  and find them to be unreadable.  Suggested alternatives include
  the COBOL style PICTURE approach or a convenience function with
  keyword arguments for every possible combination.

* Some newsgroup respondants think there is no place for any
  scripts that are not internationalized and that it is a step
  backwards to provide a simple way to hardwire a particular choice
  (thus reducing incentive to use a locale sensitive approach).

* Another thought is that embedding some particular convention in
  individual format strings makes it hard to change that convention
  later.  No workable alternative was suggested but the general idea
  is to set the convention once and have it apply everywhere (others
  commented that locale already provides a way to do this).                                                              

* There are some precedents for grouping digits in the fractional
  part of a floating point number, but this PEP does not venture into
  that territory.  Only digits to the left of the decimal point are
  grouped.  This does not preclude future extensions; it just focuses
  on a single, generally useful extension to the formatting language.

* James Knight observed that Indian/Pakistani numbering systems
  group by hundreds.   Ben Finney noted that Chinese group by
  ten-thousands.  Eric Smith pointed-out that these are already
  handled by the "n" specifier in the locale module (albeit only
  for integers).  This PEP does not attempt to support all of those
  possibilities.  It focuees on a single, relatively common grouping
  convention that offers a quick way to improve readability in many
  (though not all) contexts.


Copyright
=========

This document has been placed in the public domain.



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