An uncompromising Fortran formatter, inspired by Black.

"So it goes." — Kurt Vonnegut, Slaughterhouse-Five

Sable enforces one consistent style for modern free-form Fortran, so you can focus on code instead of formatting. It also supports code checks beyond formatting to identify code issues.

Installation

pip install sable-fortran

Quick Start

# Format in place (default command)
sable src/

# Equivalent explicit form
sable format src/

# Check formatting only (no writes)
sable format --check src/

# Preview formatting diff
sable format --diff src/

# Safer migration pass (layout-focused)
sable format --safe src/

Recommended CI formatting gate:

sable format --check src/

Formatting

format rewrites source into Sable's canonical style. This includes:

• Relational operator normalization (.EQ. -> ==, etc.)
• END keyword normalization (endif -> end if, configurable)
• Declaration normalization (integer x -> integer :: x)
• Deterministic spacing, indentation, wrapping, and trailing newline handling

Example:

! Before
IF(A .EQ. B)THEN
CALL compute(argument_alpha,argument_beta,argument_gamma)
ENDIF

! After
if (A == B) then

   call compute(argument_alpha, &
                argument_beta, &
                argument_gamma)

end if

Checking

check reports rule diagnostics and can apply rule fixes.

# Run all rules (style + lint)
sable check src/

# Focus on policy/lint only
sable check --rule-set lint src/

# Formatter-adjacent style rules only
sable check --rule-set style src/

# Apply safe fixes, then re-check
sable check --fix src/

# Allow unsafe fixes too
sable check --fix --unsafe-fixes src/

Rule-set notes:

• --rule-set all (default): style + lint
• --select takes precedence over --rule-set
• --fix applies safe fixes
• --unsafe-fixes enables unsafe fixes (only with --fix)

Current lint rules:

• SBL101: Program/module is missing implicit none
• SBL102: Procedure is missing implicit none
• SBL103: Dummy argument is missing intent(in|out|inout)

Suppressions

Inline:

if (a .eq. b) then ! sable: ignore SBL001
end if

File-wide:

! sable: ignore-file SBL001,SBL004

Configuration

Formatter options are CLI flags:

sable format --line-length 88 src/

Option	Default	Values	Applies to
--line-length	100	integer	format, check
--indent-width	3	integer	format, check
--keyword-case	lower	lower, upper	format, check
--end-keyword-form	spaced	spaced, compact	format, check
--no-normalize-operators	off	flag	format, check
rule_set / --rule-set	all	style, lint, all	check
select / --select	[]	rule codes	check
ignore / --ignore	[]	rule codes	check
output_format / --output-format	text	text, json, sarif, gitlab-codequality	check
baseline / --baseline	unset	path	check
fix / --fix	false	boolean	check
unsafe_fixes / --unsafe-fixes	false	boolean	check
generate_baseline / --generate-baseline	false	boolean	check

Example pyproject.toml:

[tool.sable.check]
rule_set = "all"  # style | lint | all
select = []
ignore = []
output_format = "text"   # text | json | sarif | gitlab-codequality
baseline = ".sable-baseline.json"
fix = false
unsafe_fixes = false
generate_baseline = false

For check, CLI flags override pyproject.toml defaults.

Badge

[![Code style: Sable](https://raw.githubusercontent.com/eirik-kjonstad/sable/main/assets/sable-badge.svg)](https://github.com/eirik-kjonstad/sable)

License

MIT