LilyPond Contributor's Guide: 13.5.4.1 GOP-PROP 1 - python formatting

#### 13.5.4.1 GOP-PROP 1 - python formatting

We will follow the indentation described in PEP-8. http://www.python.org/dev/peps/pep-0008/

- use 4 spaces per indentation level
- never mix tabs and spaces (for indentation)
- Code indented with a mixture of tabs and spaces should be converted to using spaces exclusively

  Once this is done, we should add `python -tt` to the build system to avoid such errors in the future.

There should be absolutely no tab characters for indentation in any `.py` file in lilypond git. All such files should be converted to use spaces only.

#### Discussions

    https://lists.gnu.org/archive/html/lilypond-devel/2011-06/msg00060.html
    https://lists.gnu.org/archive/html/lilypond-devel/2011-06/msg00084.html
    https://lists.gnu.org/archive/html/lilypond-devel/2011-06/msg00310.html
    https://lists.gnu.org/archive/html/lilypond-devel/2011-06/msg00574.html

#### LilyPond — Contributor’s Guide v2.22.2 (stable-branch).