Format for MESA defaults files

This document defines a format for MESA defaults files. Because these files are compiled, documenation must be in commments. After leading spaces and ! are stripped, this file should be valid reStructuredText (reST) so that it can rendered in the Sphinx docs. This file is written in the format it describes.

The stripping procedure is performed via a custom Sphinx extension defaults2rst (located in docs/source/_ext) and occurs when the files are read from disk (using the Sphinx source-read hook). The defaults files in MESA modules should be symlinked into the docs tree in an appropriate location. The Sphinx docnames that should undergo this transformation are specified in When a new defaults file is added, it must be added to that list.


Comment lines begin with a bang, followed by a single space, and then have at least one non-whitespace character. Empty comments are treated like blank lines.

Regular Comments

Most comments will be paragraphs of text.

In reST, a paragraph is chunks of text separated by one or more blank lines. indentation is significant in reST, so all lines of the same paragraph must be left-aligned to the same level of indentation.

Each block of comments is treated as a paragraph, and so the lengths of the lines will not be preserved. (This feature allow us to nicely display much of the content in a non-fixed width font!) For example, the following two blocks will look similar when rendered for the web.

This block has no line breaks.

This block has several line breaks

Formatted Comments

More complicated comments benefit greatly from additional formatting.

You can trigger emphasis using one asterisk for italics or two asterisks for bold. The use of backquotes lets you put fixed-width font material inline.

It is frequently helpful to make a list

  • foo

  • bar

  • baz

If you’re writing code in your comments, then you probably want things to appear in a fixed-width font in code blocks. To do so, end a paragraph with the special marker :: (double colon). Then write your code as an indented block surrounded by blank lines. (Remember, a comment begins a bang & space, so you must have at least 2 spaces after the ! to indent.)

foo =   1
bar =  10
baz = 100

By default, blocks trigger fortran syntax highlighting:

subroutine foo(bar)
  implicit none
  integer, intent(in) :: bar
  bar = bar + 1
end subroutine foo

If you know where you want the line breaks to be, but don’t want to render things as code (in a fixed-width font), you can force a line break by starting lines with a | (pipe).

These lines are
broken exactly like in
the source file.

Options & Defaults

Single Options

Most MESA options stand alone and have their own documentation.


Number options are great. MESA is good at taking numbers and making more numbers for you. Often you’ll want to set number_option > 0, but sometimes you want to set number_option <= 0.

number_option = 1


String options are great. You can tell MESA what you want it to do.

string_option = 'make me a sandwich'


Boolean options are great. They can take one of two values:

  • .true.

  • .false.

boolean_option = .true.

Multiple Options



Sometimes there are options that need to grouped together, because they share documentation. If options are to be grouped, both their headings and their default values should be adjacent.

xa_average_lower_limit_species(1) = ''
xa_average_lower_limit(1) = 0

Whitespace Style

An indent is 3 spaces. No tabs are used. Almost all whitespace at the beginning of lines is strictly cosmetic and is not used by the parser in determining the structure of the file. Whitespace at the end of lines should be avoided.