Rearranged and updated to reflect Guido's comments.
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Raymond Hettinger
parent
51a6726ee3
commit
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200
pep-0378.txt
200
pep-0378.txt
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@ -33,7 +33,7 @@ platform to platform or may not be defined at all. The docs
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for the locale module describe these and `many other challenges`_
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in detail.
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.. _`many other challenges`: http://docs.python.org/library/locale.html#background-details-hints-tips-and-caveats
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.. _`many other challenges`: http://www.python.org/doc/2.6.1/library/locale.html#background-details-hints-tips-and-caveats
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It is not the goal to replace the locale module, to perform
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internationalization tasks, or accommodate every possible
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@ -44,95 +44,8 @@ task easier for many users.
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.. _`Babel`: http://babel.edgewall.org/
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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://www.python.org/doc/2.6.1/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, APOSTROPHE or UNDERSCORE.
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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::
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"_($* #,##0_)".
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.. _`MS Excel`: http://www.brainbell.com/tutorials/ms-office/excel/Create_Custom_Number_Formats.htm
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`COBOL`_ uses picture clauses like::
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PICTURE $***,**9.99CR
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.. _`COBOL`: http://en.wikipedia.org/wiki/Cobol#Syntactic_features
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`Common Lisp`_ uses a COLON before the ``~D`` decimal type specifier to
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emit a COMMA as a thousands separator. The general form of ``~D`` is
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``~mincol,padchar,commachar,commaintervalD``. The *padchar* defaults
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to SPACE. The *commachar* defaults to COMMA. The *commainterval*
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defaults to three.
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::
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(format nil "~:D" 229345007) => "229,345,007"
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.. _`Common Lisp`: http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node200.html
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`C-Sharp`_ provides both styles (picture formatting and type specifiers).
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The type specifier approach is locale aware. The picture formatting only
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offers a COMMA as a thousands separator::
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String.Format("{0:n}", 12400) ==> "12,400"
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String.Format("{0:0,0}", 12400) ==> "12,400"
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.. _`C-Sharp`: http://blog.stevex.net/index.php/string-formatting-in-csharp/
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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,d").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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The *width* argument means the total length including the commas
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and decimal point::
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format(1234, "08,d") --> '0001,234'
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format(1234.5, "08,.1f") --> '01,234.5'
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The ',' option is defined as shown above for types 'd', 'f',
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and 'F'. It is undefined for other types (binary, octal, hex,
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character, exponential, general, percentage, etc.)
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Proposal II (from Eric Smith)
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=============================
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Main Proposal (from Eric Smith)
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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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@ -168,45 +81,106 @@ length including the thousands separators and decimal separators.
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No change is proposed for the locale module.
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The thousands separator is defined as shown above for types
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'd', 'f', and 'F'. It is undefined for other types (binary,
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octal, hex, character, exponential, general, percentage, etc.)
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'd', 'e', 'f', 'g', 'E', 'G'and 'F'. To allow future extensions, it is
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undefined for other types: binary, octal, hex, character, etc.
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Comparison
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==========
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Current Version of the Mini-Language
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====================================
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The difference between the two proposals is that the first is hard-wired
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to a COMMA for a thousands separator and a DOT as a decimal separator.
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The second allows either separator to be one of several possibilities.
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* `Python 2.6 docs`_
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The PEP author recommends Proposal II.
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.. _Python 2.6 docs: http://www.python.org/doc/2.6.1/library/string.html#formatstrings
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* PEP 3101 Advanced String Formatting
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Other Ideas
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===========
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Research into what Other Languages Do
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=====================================
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* Lie Ryan suggested a convenience function of the form::
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Scanning the web, I've found that thousands separators are
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usually one of COMMA, DOT, SPACE, APOSTROPHE or UNDERSCORE.
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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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`C-Sharp`_ provides both styles (picture formatting and type specifiers).
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The type specifier approach is locale aware. The picture formatting only
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offers a COMMA as a thousands separator::
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* Eric Smith would like the C version of the mini-language
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parser to be exposed with hooks that would make it easier
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to write custom *__format__* methods. That way, methods like
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*Decimal.__format__* would not have to be written from scratch.
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String.Format("{0:n}", 12400) ==> "12,400"
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String.Format("{0:0,0}", 12400) ==> "12,400"
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* Antoine Pitrou noted that the provision for a SPACE separator
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should also allow a non-breaking space (U+00A0).
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.. _`C-Sharp`: http://blog.stevex.net/index.php/string-formatting-in-csharp/
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`Common Lisp`_ uses a COLON before the ``~D`` decimal type specifier to
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emit a COMMA as a thousands separator. The general form of ``~D`` is
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``~mincol,padchar,commachar,commaintervalD``. The *padchar* defaults
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to SPACE. The *commachar* defaults to COMMA. The *commainterval*
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defaults to three.
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::
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(format nil "~:D" 229345007) => "229,345,007"
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.. _`Common Lisp`: http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node200.html
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* A poster on the newgroup, Wolfgang Rohdewald, noted that a
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convention in Switzerland is to use an APOSTROPHE as a
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thousands separator, ``12`000.99``.
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* The `ADA language`_ allows UNDERSCORES in its numeric literals.
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.. _`ADA language`: http://archive.adaic.com/standards/83lrm/html/lrm-02-04.html
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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::
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"_($* #,##0_)".
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.. _`MS Excel`: http://www.brainbell.com/tutorials/ms-office/excel/Create_Custom_Number_Formats.htm
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`COBOL`_ uses picture clauses like::
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PICTURE $***,**9.99CR
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.. _`COBOL`: http://en.wikipedia.org/wiki/Cobol#Syntactic_features
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Alternative Proposal (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,d").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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The *width* argument means the total length including the commas
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and decimal point::
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format(1234, "08,d") --> '0001,234'
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format(1234.5, "08,.1f") --> '01,234.5'
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The ',' option is defined as shown above for types 'd', 'e',
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'f', 'g', 'E', 'G'and 'F'. To allow future extensions, it is
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undefined for other types: binary, octal, hex, character,
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etc.
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This alternative proposal has the virtue of being simpler
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than the main proposal but is much less flexible and meets
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the needs of fewer users right out of the box.
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Commentary
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==========
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