Abstract

    This PEP (also known as PEP CCCXIII) proposes adding Roman
    numerals as a literal type.  It also proposes the new built-in
    function "roman", which converts an object to an integer, then
    converts the integer to a string that is the Roman numeral literal
    equivalent to the integer.

BDFL Pronouncement

    This PEP is rejected.  While the majority of Python users deemed this
    to be a nice-to-have feature, the community was unable to reach a
    concensus on whether nine should be represented as IX, the modern
    form, or VIIII, the classic form.  Likewise, no agreement was
    reached on whether MXM or MCMXC would be considered a well-formed
    representation of 1990.  A vocal minority of users has also requested
    support for lower-cased numerals for use in (i) powerpoint slides,
    (ii) academic work, and (iii) Perl documentation.

Rationale

    Roman numerals are used in a number of areas, and adding them to
    Python as literals would make computations in those areas easier.
    For instance, Superbowls are counted with Roman numerals, and many
    older movies have copyright dates in Roman numerals.  Further,
    LISP provides a Roman numerals literal package, so adding Roman
    numerals to Python will help ease the LISP-envy sometimes seen in
    comp.lang.python.  Besides, the author thinks this is the easiest
    way to get his name on a PEP.

Syntax for Roman literals

    Roman numeral literals will consist of the characters M, D, C, L,
    X, V and I, and only those characters.  They must be in upper
    case, and represent an integer with the following rules:

    1.  Except as noted below, they must appear in the order M, D, C,
    L, X, V then I.  Each occurrence of each character adds 1000, 500,
    100, 50, 10, 5 and 1 to the value of the literal, respectively.

    2.  Only one D, V or L may appear in any given literal.

    3.  At most three each of Is, Xs and Cs may appear consecutively
    in any given literal.

    4.  A single I may appear immediately to the left of the single V,
    followed by no Is, and adds 4 to the value of the literal.

    5.  A single I may likewise appear before the last X, followed by
    no Is or Vs, and adds 9 to the value.

    6.  X is to L and C as I is to V and X, except the values are 40
    and 90, respectively.

    7.  C is to D and M as I is to V and X, except the values are 400
    and 900, respectively.

    Any literal composed entirely of M, D, C, L, X, V and I characters
    that does not follow this format will raise a syntax error,
    because explicit is better than implicit.

Built-In "roman" Function

    The new built-in function "roman" will aide the translation from
    integers to Roman numeral literals.  It will accept a single
    object as an argument, and return a string containing the literal
    of the same value.  If the argument is not an integer or a
    rational (see PEP 239 [1]) it will passed through the existing
    built-in "int" to obtain the value.  This may cause a loss of
    information if the object was a float.  If the object is a
    rational, then the result will be formatted as a rational literal
    (see PEP 240 [2]) with the integers in the string being Roman
    numeral literals.

Compatibility Issues

    No new keywords are introduced by this proposal.  Programs that
    use variable names that are all upper case and contain only the
    characters M, D, C, L, X, V and I will be affected by the new
    literals.  These programs will now have syntax errors when those
    variables are assigned, and either syntax errors or subtle bugs
    when those variables are referenced in expressions.  Since such
    variable names violate PEP 8 [3], the code is already broken, it
    just wasn't generating exceptions. This proposal corrects that
    oversight in the language.

References

    [1] PEP 239, Adding a Rational Type to Python
        http://www.python.org/peps/pep-0239.html

    [2] PEP 240, Adding a Rational Literal to Python
        http://www.python.org/peps/pep-0240.html

    [3] PEP 8, Style Guide for Python Code
        http://www.python.org/peps/pep-0008.html

Copyright

    This document has been placed in the public domain.