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Python Enhancement Proposals

PEP 313 – Adding Roman Numeral Literals to Python

Mike Meyer <mwm at>
Standards Track

Table of Contents


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 consensus 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.


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, Super Bowls 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) 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) 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, the code is already broken, it just wasn’t generating exceptions. This proposal corrects that oversight in the language.


Last modified: 2023-09-09 17:39:29 GMT