Formatting Rules

tox-toml-fmt is an opinionated formatter, much like black is for Python code. The tool intentionally provides minimal configuration options because the goal is to establish a single standard format that all tox.toml files follow.

Benefits of this approach:

  • Less time configuring tools

  • Smaller diffs when committing changes

  • Easier code reviews since formatting is never a question

While a few key options exist (column_width, indent, table_format, sub_table_spacing, separate_root_table), the tool does not expose dozens of toggles. You get what the maintainers have chosen to be the right balance of readability, consistency, and usability.

General Formatting

These rules apply uniformly across the entire tox.toml file.

String Quotes

All strings use double quotes by default. Single quotes are only used when the value contains double quotes:

# Before
description = 'Run tests'
commands = ["echo \"hello\""]

# After
description = "Run tests"
commands = ['echo "hello"']

Key Quotes

TOML keys are normalized to the simplest valid form. Keys that are valid bare keys (containing only A-Za-z0-9_-) have redundant quotes stripped. Single-quoted (literal) keys that require quoting are converted to double-quoted (basic) strings with proper escaping. This applies to all keys: table headers, key-value pairs, and inline table keys:

# Before
[env.'my-env']
"description" = "run tests"
pass_env = [{ "else" = "no" }]

# After
[env."my-env"]
description = "run tests"
pass_env = [{ else = "no" }]

Backslashes and double quotes within literal keys are escaped during conversion.

Array Formatting

Arrays are formatted based on line length, trailing comma presence, and comments:

# Short arrays stay on one line
env_list = ["py312", "py313", "lint"]

# Long arrays that exceed column_width are expanded and get a trailing comma
deps = [
    "pytest>=7",
    "pytest-cov>=4",
    "pytest-mock>=3",
]

# Trailing commas signal intent to keep multiline format
deps = [
    "pytest>=7",
]

# Arrays with comments are always multiline
deps = [
    "pytest>=7",  # testing framework
    "coverage>=7",
]

Multiline formatting rules:

An array becomes multiline when any of these conditions are met:

  1. Trailing comma present - A trailing comma signals intent to keep multiline format

  2. Exceeds column width - Arrays longer than column_width are expanded (and get a trailing comma added)

  3. Contains comments - Arrays with inline or leading comments are always multiline

String Wrapping

Long strings that exceed column_width are wrapped using TOML multiline basic strings with line-ending backslashes:

# Before
description = "A very long description string that exceeds the column width limit set for this project"

# After (with column_width = 40)
description = """\
  A very long description \
  string that exceeds the \
  column width limit set \
  for this project\
  """

Specific keys can be excluded from wrapping using skip_wrap_for_keys. Patterns support wildcards (e.g. *.commands skips wrapping for commands under any table).

Table Formatting

Sub-tables can be formatted in two styles controlled by table_format:

Short format (default, collapsed to dotted keys):

[env.test]
description = "run tests"
sub.value = 1

Long format (expanded to table headers):

[env.test]
description = "run tests"

[env.test.sub]
value = 1

Individual tables can override the default using expand_tables and collapse_tables.

Table spacing:

By default, different table groups are separated by a blank line, while sub-tables within the same group are kept compact. You can control this with sub_table_spacing and separate_root_table. Each option takes a string of \n characters where each \n adds one blank line. For example, setting sub_table_spacing = "\n" adds a blank line between sub-tables within the same environment.

See Configuration for how to control this behavior.

Environment tables are always expanded:

Regardless of the table_format setting, [env.*] tables are never collapsed into dotted keys under [env]. Each environment always gets its own [env.NAME] table section:

# This is always the output format, even in short mode:
[env.fix]
description = "fix"

[env.test]
description = "test"

# Dotted keys under [env] are automatically expanded:
# [env]
# fix.description = "fix"    →    [env.fix]
#                                  description = "fix"

Sub-tables within an environment (e.g. [env.test.sub]) still follow the table_format setting.

Comment Preservation

All comments are preserved during formatting:

  • Inline comments - Comments after a value on the same line stay with that value

  • Leading comments - Comments on the line before an entry stay with the entry below

  • Block comments - Multi-line comment blocks are preserved

Inline comment alignment:

Inline comments within arrays are aligned independently per array, based on that array’s longest value:

# Before - comments at inconsistent positions
deps = [
  "pytest", # testing
  "pytest-cov",  # coverage
  "pytest-mock", # mocking
]

# After - comments align to longest value in this array
deps = [
  "pytest",       # testing
  "pytest-cov",   # coverage
  "pytest-mock",  # mocking
]

Table-Specific Handling

Beyond general formatting, tables have specific key ordering, value normalization, and sorting rules.

Table Ordering

Tables are reordered into a consistent structure:

  1. Root-level keys (min_version, requires, env_list, etc.)

  2. [env_run_base]

  3. [env_pkg_base]

  4. [env_base.*] sections (shared base configurations)

  5. [env.NAME] sections ordered by env_list if specified

  6. Any remaining [env.*] sections not in env_list, sorted alphabetically

  7. [env] (catch-all environment table, if present)

# env_list determines the order of [env.*] sections
env_list = ["lint", "type", "py312", "py313"]

[env_run_base]
deps = ["pytest>=7"]

[env_pkg_base]
# ...

[env_base.ci]
# shared base config

# Environments appear in env_list order:
[env.lint]
# ...

[env.type]
# ...

[env.py312]
# ...

[env.py313]
# ...

Environments not listed in env_list are placed at the end, sorted alphabetically.

Alias Normalization

Legacy INI-style key names are renamed to their modern tox 4 TOML equivalents. This applies automatically to the root table, [env_run_base], [env_pkg_base], and all [env.*] tables.

Root table aliases:

# Before
envlist = ["py312", "py313"]
minversion = "4.2"
skipsdist = true

# After
env_list = ["py312", "py313"]
min_version = "4.2"
no_package = true

Full list: envlist → env_list, toxinidir → tox_root, toxworkdir → work_dir, skipsdist → no_package, isolated_build_env → package_env, setupdir → package_root, minversion → min_version, ignore_basepython_conflict → ignore_base_python_conflict

Environment table aliases:

# Before
[env_run_base]
basepython = "python3.12"
setenv.PYTHONPATH = "src"
passenv = ["HOME"]

# After
[env_run_base]
base_python = "python3.12"
set_env.PYTHONPATH = "src"
pass_env = ["HOME"]

Full list: setenv → set_env, passenv → pass_env, envdir → env_dir, envtmpdir → env_tmp_dir, envlogdir → env_log_dir, changedir → change_dir, basepython → base_python, usedevelop → use_develop, sitepackages → system_site_packages, alwayscopy → always_copy

Root Key Ordering

Keys in the root table are reordered into a consistent sequence:

min_version → requires → provision_tox_env → env_list → labels → base → package_env → package_root → no_package → skip_missing_interpreters → ignore_base_python_conflict → work_dir → temp_dir → tox_root

# Before
env_list = ["py312", "lint"]
requires = ["tox>=4.2"]
min_version = "4.2"

# After
min_version = "4.2"
requires = ["tox>=4.2"]
env_list = ["py312", "lint"]

Environment Key Ordering

Keys within [env_run_base], [env_pkg_base], and [env.*] tables are reordered to group related settings:

factors → runner → description → base_python → default_base_python → system_site_packages → always_copy → download → virtualenv_spec → package → package_env → wheel_build_env → package_tox_env_type → package_root → skip_install → use_develop → meta_dir → pkg_dir → pip_pre → install_command → list_dependencies_command → deps → dependency_groups → pylock → constraints → constrain_package_deps → use_frozen_constraints → extras → recreate → recreate_commands → parallel_show_output → skip_missing_interpreters → fail_fast → pass_env → disallow_pass_env → set_env → change_dir → platform → args_are_paths → ignore_errors → commands_retry → ignore_outcome → extra_setup_commands → commands_pre → commands → commands_post → allowlist_externals → labels → suicide_timeout → interrupt_timeout → terminate_timeout → depends → env_dir → env_tmp_dir → env_log_dir

# Before
[env_run_base]
commands = ["pytest"]
deps = ["pytest>=7"]
description = "run tests"

# After
[env_run_base]
description = "run tests"
deps = ["pytest>=7"]
commands = ["pytest"]

requires Normalization

Dependencies in the root requires array are normalized per PEP 508 (canonical package names, consistent spacing around specifiers) and sorted alphabetically by package name:

# Before
requires = ["tox >= 4.2", "tox-uv"]

# After
requires = ["tox>=4.2", "tox-uv"]

env_list Sorting

The env_list array is sorted with a specific ordering:

  1. Pinned environments come first, in the order specified by --pin-env

  2. CPython versions (matching py3.12, py312, 3.12, etc.) sorted descending (newest first)

  3. PyPy versions (matching pypy3.10, pypy310, etc.) sorted descending

  4. Named environments (lint, type, docs, etc.) sorted alphabetically

Inline table entries (such as { product = ... }) in env_list are excluded from sorting and remain in their original positions.

Compound environment names separated by - are classified by their first recognized part:

# Before
env_list = ["lint", "py38", "py312", "docs", "py310-django"]

# After
env_list = ["py312", "py310-django", "py38", "docs", "lint"]

Use --pin-env to pin specific environments to the start:

# With --pin-env fix,type
env_list = ["fix", "type", "py313", "py312", "docs", "lint"]

See Configuration for how to set pin-env via the config file or CLI.

use_develop Upgrade

The legacy use_develop = true setting is automatically converted to the modern package = "editable" equivalent. If use_develop = false, the key is left as-is. If a package key already exists, only the use_develop key is removed:

# Before
[env_run_base]
use_develop = true

# After
[env_run_base]
package = "editable"

Array Sorting

Certain arrays within environment tables are sorted automatically:

Sorted by canonical PEP 508 package name:

  • deps, constraints — dependencies normalized and sorted by package name

Pip file references (-r, -c), editable installs (-e), local paths (./, ../, /), and entries containing tox substitution variables ({tox_root}, etc.) are preserved as-is without PEP 508 normalization, but still participate in sorting by their lowercased value:

# Before
deps = ["Pytest >= 7", "-r requirements.txt", "coverage", "-e ./my-pkg[test]"]

# After
deps = ["-e ./my-pkg[test]", "-r requirements.txt", "coverage", "pytest>=7"]

Sorted alphabetically:

  • dependency_groups, allowlist_externals, extras, labels, depends

Special handling for ``pass_env``:

Replacement objects (inline tables like { replace = "default", ... }) are pinned to the start, then string entries are sorted alphabetically:

# Before
pass_env = ["TERM", "CI", { replace = "default", ... }, "HOME"]

# After
pass_env = [{ replace = "default", ... }, "CI", "HOME", "TERM"]

Arrays NOT sorted:

  • commands, commands_pre, commands_post — execution order matters

  • base_python — first entry takes priority

Inline Table Key Reordering

Keys within inline tables are reordered into a consistent order based on the inline table’s type. The type is detected by the presence of a discriminator key:

  • ``replace`` — replace → condition → of → env → key → name → pattern → then → else → default → extend → marker

  • ``prefix`` — prefix → start → stop

  • ``product`` — product → exclude

  • ``value`` — value → marker

Keys not listed in the schema are appended at the end in their original order.

# Before
pass_env = [{ default = ".", replace = "default", extend = true }]
env_list = [{ exclude = ["py38-django50"], product = ["py38", "py310", "django42", "django50"] }]

# After
pass_env = [{ replace = "default", default = ".", extend = true }]
env_list = [{ product = ["py38", "py310", "django42", "django50"], exclude = ["py38-django50"] }]

This reordering applies to all inline tables in the file, including those nested inside arrays.

Other Tables

Any unrecognized tables are preserved and reordered according to standard table ordering rules. Keys within unknown tables are not reordered or normalized.