143 lines
4.1 KiB
Plaintext
143 lines
4.1 KiB
Plaintext
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PEP: 378
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Title: Format Specifier for Thousands Separator
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Version: $Revision$
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Last-Modified: $Date$
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Author: Raymond Hettinger <python at rcn.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 12-Mar-2009
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Post-History: 12-Mar-2009
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Motivation
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==========
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Provide a simple, non-locale aware way to format a number
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with a thousands separator.
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Adding thousands separators is one of the simplest ways to
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improve the professional appearance and readability of output
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exposed to end users.
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In the finance world, output with commas is the norm. Finance
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users and non-professional programmers find the locale
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approach to be frustrating, arcane and non-obvious.
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It is not the goal to replace locale or to accommodate every
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possible convention. The goal is to make a common task easier
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for many users.
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Current Version of the Mini-Language
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====================================
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* `Python 2.6 docs`_
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.. _Python 2.6 docs: http://docs.python.org/library/string.html#formatstrings
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* PEP 3101 Advanced String Formatting
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Research so far
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===============
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Scanning the web, I've found that thousands separators are
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usually one of COMMA, DOT, SPACE, or UNDERSCORE.
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When a COMMA is the decimal separator, the thousands separator
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is typically a DOT or SPACE (see examples from Denis Spir).
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James Knight observed that Indian/Pakistani numbering systems
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group by hundreds. Ben Finney noted that Chinese group by
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ten-thousands. Eric Smith pointed-out that these are already
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handled by the "n" specifier in the locale module (albiet only
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for integers).
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Visual Basic and its brethren (like MS Excel) use a completely
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different style and have ultra-flexible custom format
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specifiers like: "_($* #,##0_)".
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Proposal I (from Nick Coghlan)
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==============================
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A comma will be added to the format() specifier mini-language:
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[[fill]align][sign][#][0][width][,][.precision][type]
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The ',' option indicates that commas should be included in the
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output as a thousands separator. As with locales which do not
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use a period as the decimal point, locales which use a
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different convention for digit separation will need to use the
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locale module to obtain appropriate formatting.
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The proposal works well with floats, ints, and decimals.
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It also allows easy substitution for other separators.
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For example::
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format(n, "6,f").replace(",", "_")
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This technique is completely general but it is awkward in the
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one case where the commas and periods need to be swapped::
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format(n, "6,f").replace(",", "X").replace(".", ",").replace("X", ".")
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Proposal II (to meet Antoine Pitrou's request)
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==============================================
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Make both the thousands separator and decimal separator user
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specifiable but not locale aware. For simplicity, limit the
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choices to a comma, period, space, or underscore.
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[[fill]align][sign][#][0][width][T[tsep]][dsep precision][type]
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Examples::
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format(1234, "8.1f") --> ' 1234.0'
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format(1234, "8,1f") --> ' 1234,0'
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format(1234, "8T.,1f") --> ' 1.234,0'
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format(1234, "8T .f") --> ' 1 234,0'
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format(1234, "8d") --> ' 1234'
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format(1234, "8T,d") --> ' 1,234'
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This proposal meets mosts needs (except for people wanting
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grouping for hundreds or ten-thousands), but it comes at the
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expense of being a little more complicated to learn and
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remember. Also, it makes it more challenging to write custom
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__format__ methods that follow the format specification
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mini-language.
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No change is proposed for the local module.
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Other Ideas
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===========
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* Lie Ryan suggested a convenience function of the form::
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create_format(self, type='i', base=16, seppos=4, sep=':', \
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charset='0123456789abcdef', maxwidth=32, \
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minwidth=32, pad='0')
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* Eric Smith would like the C version of the mini-language
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parser to be exposed. That would make it easier to write
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custom __format__ methods.
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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