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Code Formatting Conventions and Prettify Tool

Uniform formatting of CP2K source code is enabled by a prettify script that is an almost complete auto-formatter for Fortran 90 source code. As a rule of thumb, developers should not worry about the format of their code and just let prettify do its magic by running make -j pretty.

Formatting conventions

The following formatting conventions are automatically enforced by the make -j pretty command:

  • Sorting and alignment of variable declarations and USE statements, removal of unused list entries.
  • Indentation with a relative width of 3 characters.
  • Line continuations are aligned with the previous opening delimiter (, [ or (/ or with an assignment operator = or =>. If none of the above is present, a default hanging indent of 3 characters is applied.
  • All operators are surrounded by exactly one whitespace character, except for arithmetic operators.
  • Removal of extraneous whitespace and consecutive blank lines.
  • Uppercase notation for all Fortran and OpenMP keywords.

Editor integration

You can optionally integrate prettify into your editor. First install prettify with the script in cp2k/tools/prettify:

./ install


./ install --user

Then configure your editor such that it calls

fprettify --no-normalize-use --no-report-errors

for reformatting. For Vim, this is done by adding the following line to your vimrc:

autocmd Filetype fortran setlocal formatprg=fprettify\ --no-normalize-use\ --no-report-errors

Reformat selected lines with gq and the whole file with gggqG. Note that editor integration only supports whitespace formatting (so you should still run make pretty before a commit).

Manual formatting

The following formatting decisions are still manual and are never changed by prettify:

  • Positions of line breaks (except for variable declarations and USE statements).
  • No indentation of subsequent DO / IF statements that are aligned with each other.

There may be cases where manual alignment is preferred over the automatic formatting conventions. The following options for manual formatting are provided:

  • No automatic realignment of line continuations that are prefixed with an &.
  • No auto-formatting of lines to which a comment starting with !& is attached.
  • No auto-formatting of code blocks enclosed between two comment lines starting with !&< and !&>.


A few examples to illustrate how to deal with cases where auto-formatting produces unsatisfying results:

  • Reduce hanging indent by inserting linebreaks directly after assignment operator and opening delimiter:
! No:
long_result_var_name = long_function_name(arg_1, arg_2, &
                                          arg_3, arg_4, arg_5)+ &
! Yes:
long_result_var_name = &
   long_function_name( &
      arg_1, arg_2, &
      arg_3, arg_4, arg_5)+ &
  • Avoid linebreaks in deeply nested expressions:
! No:
bessj0 = (r1+y*(r2+y*(r3+y*(r4+y* &
                            (r5+y*r6)))))/(s1+y*(s2+y*(s3+y* &
! Yes:
bessj0 = (r1+y*(r2+y*(r3+y*(r4+y*(r5+y*r6)))))/ &
  • Alignment by explicit bracketing:
! No:
foo = bar+foobar(x1, y1, z1)* &
      foobar(x2, y2, z2)* &
      foobar(x3, y3, z3)
! Yes:
foo = bar+(foobar(x1, y1, z1)* &
           foobar(x2, y2, z2)* &
           foobar(x3, y3, z3))
  • Special vertical alignment may require manual formatting:
! Auto-formatting:
align_me = [-1, 10, 0, &
            0, 1000, 0, &
            0, -1, 1]
! Manual alignment (!& disables whitespace formatting):
align_me = [-1,   10, 0, & !&
             0, 1000, 0, & !&
             0,   -1, 1] !&
! Alternatively:
align_me = [-1,   10, 0, &
             0, 1000, 0, &
             0,   -1, 1]
dev/formattingconventions.txt · Last modified: 2016/08/28 15:23 by