Configuration

tox configuration can be split into two categories: core and environment specific. Core settings are options that can be set once and used for all tox environments, while environment options are applied to the given tox environment only.

System requirements

tox works with the following Python interpreter implementations:

• CPython versions 3.10, 3.11, 3.12, 3.13, 3.14

This means tox works on the latest patch version of each of these minor versions. Previous patch versions are supported on a best effort approach.

Discovery and file types

With regards to the configuration format, at the moment we support the following formats:

• INI.

• TOML.

Out of box tox supports five configuration locations prioritized in the following order:

        flowchart TD
    search[tox searches current directory] --> tox_ini
    tox_ini[tox.ini — INI] -- not found --> setup_cfg[setup.cfg — INI]
    setup_cfg -- not found --> pyproject_native[pyproject.toml — tool.tox]
    pyproject_native -- not found --> pyproject_legacy[pyproject.toml — legacy_tox_ini]
    pyproject_legacy -- not found --> tox_toml[tox.toml — TOML]

    classDef ini fill:#dbeafe,stroke:#3b82f6,stroke-width:2px,color:#1e3a5f
    classDef toml fill:#dcfce7,stroke:#22c55e,stroke-width:2px,color:#14532d
    classDef start fill:#f3f4f6,stroke:#9ca3af,stroke-width:2px,color:#374151

    class tox_ini,setup_cfg,pyproject_legacy ini
    class pyproject_native,tox_toml toml
    class search start
    

1. tox.ini (INI),

2. setup.cfg (INI),

3. Native pyproject.toml under the tool.tox table (TOML),

4. pyproject.toml with the tool.tox table, having legacy_tox_ini key (containing INI),

5. tox.toml (TOML).

Historically, the INI format was created first, and TOML was added in 2024. TOML is the recommended format for new projects – it is more robust, has proper type support, and avoids ambiguities inherent in INI parsing (e.g. multi-line values, comment escaping). INI remains supported and is more concise for some patterns.

Format comparison

Both INI and TOML support the same features with different syntax. The only INI feature without a TOML equivalent is generative section names (see below).

Conditional factors – Both INI and TOML support filtering by factor name. A factor is any dash-separated segment in the environment name (e.g. py313-django50 has factors py313 and django50). Additionally, the current platform (sys.platform value like linux, darwin, win32) is automatically available as an implicit factor.

TOML

[env_run_base]
deps = [
    "pytest",
    { replace = "if", condition = "factor.django50", then = ["Django>=5.0,<5.1"], extend = true },
    { replace = "if", condition = "factor.django42", then = ["Django>=4.2,<4.3"], extend = true },
    { replace = "if", condition = "not factor.lint", then = ["coverage"], extend = true },
]
commands = [
    { replace = "if", condition = "factor.linux", then = [["python", "-c", "print('on linux')"]], extend = true },
    { replace = "if", condition = "factor.darwin", then = [["python", "-c", "print('on mac')"]], extend = true },
    { replace = "if", condition = "factor.win32", then = [["python", "-c", "print('on windows')"]], extend = true },
]

Use replace = "if" with factor.NAME conditions. Supports boolean operations (and, or, not) and can combine with environment variable checks (env.VAR).

INI

[testenv]
deps =
    pytest
    django50: Django>=5.0,<5.1
    django42: Django>=4.2,<4.3
    !lint: coverage
commands =
    linux: python -c 'print("on linux")'
    darwin: python -c 'print("on mac")'
    win32: python -c 'print("on windows")'

Supports simple factors (django50:), multiple factors (py313,py312:), and negation (!lint:). Any multi-line setting (deps, commands, set_env, etc.) can use factor conditions.

Platform factors work in any environment without requiring the platform name in the environment name.

Generative environment lists – Both INI and TOML support generating environment lists from factor combinations.

TOML

env_list = [
    { product = [["py312", "py313", "py314"], ["django42", "django50"]] },
]

The product dict computes the Cartesian product of its factor groups and joins each combination with -. Range dicts ({ prefix = "py3", start = 12, stop = 14 }) and literal strings can be mixed in the same list. An optional exclude key skips specific combinations.

INI

[tox]
env_list = py3{12,13,14}-django{42,50}

Curly-brace expressions expand into the Cartesian product of all combinations. This generates 6 environments: py312-django42, py312-django50, …, py314-django50. Ranges are also supported: py3{12-14} expands to py312, py313, py314. Open-ended ranges expand to the supported CPython versions at the time of the tox release: py3{10-} expands up to the latest supported version, py3{-13} expands down to the oldest supported one.

Generative section names – INI section headers can use the same curly-brace expansion:

[testenv:py3{12,13}-{x86,x64}]
envdir =
    x86: .venv-x86
    x64: .venv-x64

TOML does not support generative section names, but you can use env_base templates (see Environment base templates) to generate multiple environments from a single definition:

[env_base.build]
factors = [["py312", "py313"], ["x86", "x64"]]
env_dir = { replace = "if", condition = "factor.x86", then = ".venv-x86", "else" = ".venv-x64" }
commands = [["python", "-c", "print('ok')"]]

This generates build-py312-x86, build-py312-x64, build-py313-x86, build-py313-x64 – each inheriting from the template and resolving factor conditions per environment.

Environment base templates

env_base sections define named templates that generate multiple environments from factor combinations. Each template requires a factors key – a list of factor groups whose Cartesian product determines which environments are created. The template name is joined with each factor combination using -:

[env_base.test]
factors = [["3.13", "3.14"]]
deps = ["pytest>=8"]
commands = [["pytest"]]

This generates test-3.13 and test-3.14. For multi-dimensional matrices:

[env_base.django]
factors = [["py312", "py313"], ["django42", "django50"]]
deps = [
    "pytest",
    { replace = "if", condition = "factor.django42", then = ["Django>=4.2,<4.3"], extend = true },
    { replace = "if", condition = "factor.django50", then = ["Django>=5.0,<5.1"], extend = true },
]
commands = [["pytest"]]

This generates 4 environments: django-py312-django42, django-py312-django50, django-py313-django42, django-py313-django50.

Inheritance chain: settings resolve in this order: [env.{name}] (explicit overrides) → [env_base.{template}] (template defaults) → [env_run_base] (global defaults).

Factor groups can be:

• A list of strings: ["a", "b"]

• A range dict: { prefix = "py3", start = 12, stop = 14 } (generates py312, py313, py314)

• A labeled dict: { ecosystem = ["oci", "python"] } (same as a list, but registers the label for substitution)

• Mixed in the same factors list for Cartesian products

The template name itself does not appear as a runnable environment – only the generated names do.

Factor label substitution

Added in version 4.49.0.

Factor groups can be labeled by using a single-key dict instead of a plain list. The label enables {factor:label} substitution in any string value (descriptions, commands, deps, set_env, etc.), resolving to whichever value from the labeled group is an active factor in the current environment. Every factor group also receives a positional label (0, 1, …) automatically.

Named labels – use a single-key dict to name a factor group:

[env_base.sync]
factors = [{ecosystem = ["oci", "python", "js"]}, {target = ["pw", "tt"]}]
description = "Sync {factor:ecosystem} artifacts to {factor:target}"
commands = [["sync-tool", "--ecosystem", "{factor:ecosystem}"]]

For sync-oci-pw, the description resolves to Sync oci artifacts to pw and the command receives --ecosystem oci.

Positional labels – plain lists automatically get index-based labels:

[env_base.task]
factors = [["oci", "python"], ["pw", "tt"]]
description = "Run {factor:0} on {factor:1}"

For task-oci-pw, the description resolves to Run oci on pw. Labeled dicts also support positional access: {factor:0} and {factor:ecosystem} both resolve to oci when the labeled dict is in position 0.

In product matrices – labeled factor groups work in env_list product dicts too:

env_list = [
    { product = [["sync"], {ecosystem = ["oci", "python"]}, {target = ["pw", "tt"]}] },
]

[env_run_base]
description = "Sync {factor:ecosystem} to {factor:target}"

Defaults – {factor:label:fallback} uses fallback when the label is unknown, following the same convention as {env:VAR:default}.

Comparison with conditionals – {factor:label} replaces nested replace = "if" chains when the substituted value equals the factor name itself. For values that don’t match factor names (e.g., mapping pw to production-west-cluster), use replace = "if" with factor.NAME conditions.

Reserved names – the following cannot be used as label names: env, posargs, tty, glob, factor.

tox.ini

This configuration file uses:

• tox section to host core configuration,

• testenv:{env_name} section to host environment configuration,

• testenv section as base configuration for run environments (fallback location for missing values for a test/run environment),

• pkgenv section as base configuration for package environments (fallback location for missing values for a package environment).

For example:

[tox]
requires =
    tox >= 4.20
env_list =
    3.14t
    3.14
    3.13
    3.12
    type

[testenv]
deps = pytest
commands = pytest tests

[testenv:type]
deps = mypy
commands = mypy src

setup.cfg

This configuration file uses:

• tox:tox section to host core configuration,

• testenv:{env_name} section to host environment configuration,

• testenv section as base configuration for run environments (fallback location for missing values for a test/run environment),

• pkgenv section as base configuration for package environments (fallback location for missing values for a package environment).

[tox:tox]
requires =
    tox >= 4.0
env_list =
    3.14t
    3.14
    3.13
    3.12
    type

[testenv]
deps = pytest
commands = pytest tests

[testenv:type]
deps = mypy
commands = mypy src

pyproject.toml - INI

This configuration file is equivalent to tox.ini format, with the difference that the text is stored instead inside the pyproject.toml file under the tool.tox table and legacy_tox_ini key:

[tool.tox]
legacy_tox_ini = """
    [tox]
    requires =
        tox >= 4.0
    env_list =
        3.14
        3.13
        3.12
        type

    [testenv]
    deps = pytest
    commands = pytest tests

    [testenv:type]
    deps = mypy
    commands = mypy src
"""

pyproject.toml - native

We support native TOML configuration via the pyproject.toml files tool.tox table. This configuration file uses:

• tool.tox table to host core configuration,

• tool.tox.env.{env_name} table to host environment configuration,

• tool.tox.env_run_base table as base configuration for run environments (fallback location for missing values for a test/run environment),

• tool.tox.env_pkg_base table as base configuration for package environments (fallback location for missing values for a package environment).

[tool.tox]
requires = ["tox>=4.19"]
env_list = ["3.14t", "3.14", "3.13", "3.12", "type"]

[tool.tox.env_run_base]
description = "Run test under {base_python}"
commands = [["pytest"]]

[tool.tox.env.type]
description = "run type check on code base"
deps = ["mypy==1.18.2", "types-cachetools>=5.5.0.20240820", "types-chardet>=5.0.4.6"]
commands = [["mypy", "src{/}tox"], ["mypy", "tests"]]

tox.toml

This configuration file is equivalent to pyproject.toml - native with the difference that it lives in a separate dedicated files and accordingly the tool.tox sub-table is no longer required.

For example:

requires = ["tox>=4.19"]
env_list = ["3.14t", "3.14", "3.13", "3.12", "type"]

[env_run_base]
description = "Run test under {base_python}"
commands = [["pytest"]]

[env.type]
description = "run type check on code base"
deps = ["mypy==1.18.2", "types-cachetools>=5.5.0.20240820", "types-chardet>=5.0.4.6"]
commands = [["mypy", "src{/}tox"], ["mypy", "tests"]]

Core

The following options are set in the [tox] section of tox.ini, the [tox:tox] section of setup.cfg, or the top level of tox.toml. Placing these options in an environment section (e.g. [testenv]) has no effect. Run tox config or tox run -v to check for misplaced keys.

⚙️ requires with default value of <empty list> 📢 added in 3.2.0

Specify a list of PEP 508 compliant dependencies that must be satisfied in the Python environment hosting tox when running the tox command. If any of these dependencies are not satisfied will automatically create a provisioned tox environment that does not have this issue, and run the tox command within that environment. See provision_tox_env for more details.

TOML

[tool.tox]
requires = [
  "tox>=4",
  "virtualenv>20.2",
]

INI

[tox]
requires =
    tox>=4
    virtualenv>20.2

⚙️ min_version minversion with default value of <current version of tox> 📢 added in 2.4 ⚠️ deprecated in 4.28.0

DEPRECATED Prefer requiring a minimum tox version via requires.

A string to define the minimal tox version required to run. If the host’s tox version is less than this, it will automatically create a provisioned tox environment that satisfies this requirement. See provision_tox_env for more details.

⚙️ provision_tox_env with default value of .tox 📢 added in 3.8.0

Name of the tox environment used to provision a valid tox run environment.

Note

The provisioning environment does not inherit settings from env_run_base (TOML) or [testenv] (INI). It must be explicitly configured if you need to customize it (e.g. env.".tox" in TOML or [testenv:.tox] in INI).

Changed in version 3.23.0: When tox is invoked with the --no-provision flag, the provision won’t be attempted, tox will fail instead.

⚙️ env_list envlist with default value of <empty list> 📢 added in 1.0

A list of environments to run by default (when the user does not specify anything during the invocation).

Changed in version 3.4.0: Which tox environments are run during the tox invocation can be further filtered via the operating system environment variable TOX_SKIP_ENV regular expression (e.g. py27.* means don’t evaluate environments that start with the key py27). Skipped environments will be logged at level two verbosity level.

⚙️ skip_missing_interpreters with default value of config 📢 added in 1.7.2

Setting this to true will force tox to return success even if some of the specified environments were missing. This is useful for some CI systems or when running on a developer box, where you might only have a subset of all your supported interpreters installed but don’t want to mark the build as failed because of it. As expected, the command line switch always overrides this setting if passed on the invocation. Setting it to config means that the value is read from the config file.

⚙️ tox_root toxinidir 📢 added in 1.6.1

The root directory for the tox project (where the configuration file is found).

⚙️ work_dir toxworkdir with default value of {tox_root}/.tox 📢 added in 1.0

Directory for tox to generate its environments into, will be created if it does not exist.

⚙️ temp_dir with default value of {work_dir}/.tmp 📢 added in 3.5

Directory where to put tox temporary files. For example: we create a hard link (if possible, otherwise new copy) in this directory for the project package. This ensures tox works correctly when having parallel runs (as each session will have its own copy of the project package - e.g. the source distribution).

⚙️ no_package skipsdist with default value of false 📢 added in 1.6

Flag indicating to perform the packaging operation or not. Set it to true when using tox for an application, instead of a library.

⚙️ package_env isolated_build_env with default value of .pkg 📢 added in 3.3.0

Default name of the virtual environment used to create a source distribution from the source tree.

⚙️ package_root setupdir with default value of {tox_root} 📢 added in 1.0

Indicates where the packaging root file exists (historically setup.py file or pyproject.toml now).

⚙️ labels with default value of <empty dictionary> 📢 added in 4.0

A mapping of label names to environments it applies too. For example:

TOML

[tool.tox]
labels = { test = ["3.14t", "3.14", "3.13", "3.12"], static = ["ruff", "mypy"] }

INI

[tox]
labels =
     test = 3.14t, 3.14, 3.13, 3.12
     static = ruff, mypy

■ on_platform 📢 added in 4.17

A constant holding the platform of the tox runtime environment.

Python language core options

⚙️ ignore_base_python_conflict ignore_basepython_conflict with default value of False 📢 added in 3.1

Added in version 3.1.0.

tox allows setting the Python version for an environment via the base_python setting. If that’s not set tox can set a default value from the environment name (e.g. 3.10 or py310 implies Python 3.10). Matching up the Python version with the environment name has became expected at this point, leading to surprises when some configs don’t do so. To help with sanity of users, an error will be raised whenever the environment name version does not match up with this expectation.

Furthermore, we allow hard enforcing this rule by setting this flag to true. In such cases we ignore the base_python and instead always use the base Python implied from the Python name. This allows you to configure base_python in the base section without affecting environments that have implied base Python versions.

This flag handles two types of conflicts:

1. Explicit setting vs. environment name: When base_python is explicitly set but conflicts with the version extracted from the environment name. Setting this to true uses the version from the environment name.

2. Multiple version factors in environment name (since 4.47.3): When the environment name itself contains multiple factors that look like Python versions (e.g., unit-py3.10-2.16 where both py3.10 and 2.16 match Python version patterns). Setting this to true falls back to default_base_python instead of raising an error. To avoid this ambiguity, prefix non-Python version factors (e.g. use ac2.16 instead of 2.16).

tox environment

These are configuration for the tox environments (either packaging or run type). Set these in [testenv] (INI), env_run_base (TOML), or per-environment sections. Placing these options in the core [tox] section has no effect. Run tox config or tox run -v to check for misplaced keys.

Base options

■ envname env_name 📢 added in 4.0

The name of the tox environment.

⚙️ env_dir envdir with default value of {work_dir}/{env_name} 📢 added in 1.5

Directory assigned to the tox environment. If not absolute it would be treated as relative to tox_root.

⚙️ env_tmp_dir envtmpdir with default value of {work_dir}/{env_name}/tmp 📢 added in 0.5

A folder that is always reset at the start of the run.

⚙️ env_log_dir envlogdir with default value of {work_dir}/{env_name}/log 📢 added in 0.5

A folder containing log files about tox runs. It’s always reset at the start of the run. Currently contains every process invocation in the format of <index>-<run name>.log, and details the execution request (command, environment variables, current working directory, etc.) and its outcome (exit code and standard output/error content).

⚙️ platform 📢 added in 2.0

Run on platforms that match this regular expression (empty means any platform). If a non-empty expression is defined and does not match against the sys.platform string the entire test environment will be skipped and none of the commands will be executed. Running tox -e <platform_name> will run commands for a particular platform and skip the rest.

⚙️ pass_env passenv with default value of <empty list> 📢 added in 2.0

Environment variables to pass on to the tox environment. The values are evaluated as UNIX shell-style wildcards, see fnmatch If a specified environment variable doesn’t exist in the tox invocation environment it is ignored. The list of environment variable names is not case sensitive, for example: passing A or a will pass through both A and a.

Some variables are always passed through to ensure the basic functionality of standard library functions or tooling like pip. This is also not case sensitive on all platforms except Windows.

Environment Variables per Operating System

Environment Variable	Linux	MacOS	Windows
https_proxy	✅	✅	✅
http_proxy	✅	✅	✅
no_proxy	✅	✅	✅
LANG	✅	✅	✅
LANGUAGE	✅	✅	✅
CURL_CA_BUNDLE	✅	✅	✅
SSL_CERT_FILE	✅	✅	✅
CC	✅	✅	✅
CFLAGS	✅	✅	✅
CCSHARED	✅	✅	✅
CXX	✅	✅	✅
CPPFLAGS	✅	✅	✅
LD_LIBRARY_PATH	✅	✅	✅
LDFLAGS	✅	✅	✅
HOME	✅	✅	✅
FORCE_COLOR	✅	✅	✅
NO_COLOR	✅	✅	✅
TMPDIR	✅	✅	❌
TEMP	❌	❌	✅
TMP	❌	❌	✅
USERPROFILE	❌	❌	✅
PATHEXT	❌	❌	✅
MSYSTEM	❌	❌	✅
WINDIR	❌	❌	✅
APPDATA	❌	❌	✅
LOCALAPPDATA	❌	❌	✅
PROGRAMDATA	❌	❌	✅
PROGRAMFILES(x86)	❌	❌	✅
SYSTEMDRIVE	❌	❌	✅
SYSTEMROOT	❌	❌	✅
COMSPEC	❌	❌	✅
PROCESSOR_ARCHITECTURE	❌	❌	✅
NUMBER_OF_PROCESSORS	❌	❌	✅
PIP_*	✅	✅	✅
VIRTUALENV_*	✅	✅	✅
NETRC	✅	✅	✅
NIX_LD*	✅	✅	❌
NIX_LD_LIBRARY_PATH	✅	✅	❌
PYTHON_GIL	✅	✅	✅
SSH_AGENT_PID	✅	✅	❌
SSH_AUTH_SOCK	✅	✅	❌

If the environment variable CI is present, __TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI will be set to the value of CI. The CI variable itself will not be passed through.

More environment variable-related information can be found in Environment variable substitutions.

⚙️ disallow_pass_env with default value of <empty list> 📢 added in 4.36

Environment variable patterns to exclude after pass_env glob expansion. Uses the same fnmatch wildcard syntax as pass_env. Applied after pass_env patterns are resolved against the host environment and before set_env values are applied:

TOML

[env_run_base]
pass_env = ["FOO_*"]
disallow_pass_env = ["FOO_SECRET"]

INI

[testenv]
pass_env = FOO_*
disallow_pass_env = FOO_SECRET

In this example, all environment variables matching FOO_* are passed through except FOO_SECRET.

⚙️ set_env setenv 📢 added in 1.0

A dictionary of environment variables to set when running commands in the tox environment.

In addition, there is an option to include an existing environment file. See the different syntax for TOML and INI below.

TOML

[tool.tox.env_run_base]
set_env = { file = "conf{/}local.env", TEST_TIMEOUT = "30" }

INI

[testenv]
set_env = file|conf{/}local.env
     TEST_TIMEOUT = 30

The env file path may include previously defined tox variables:

TOML

[tool.tox.env_run_base]
set_env = { file = "{env:variable}" }

INI

[testenv]
set_env = file|{env:variable}

Note

Environment files are processed using the following rules:

• blank lines are ignored,

• lines starting with the # character are ignored,

• each line is in KEY=VALUE format; both the key and the value are stripped,

• there is no special handling of quotation marks, they are part of the key or value.

Conditional environment variables

Added in version 4.33.

Environment variables can be set conditionally based on PEP-508 environment markers. If the marker evaluates to false, the variable will not be set.

Note

Markers are evaluated against the Python interpreter running tox (the host Python), not the target environment’s Python interpreter. This means markers like python_version reflect tox’s Python version, not the target’s. For platform-specific conditions (e.g., sys_platform, os_name), this is typically the desired behavior since the target environment runs on the same platform.

TOML

[tool.tox.env_run_base]
set_env.LINUX_VAR = { value = "1", marker = "sys_platform == 'linux'" }
set_env.WIN_VAR = { value = "1", marker = "sys_platform == 'win32'" }

# Can also be combined with replace directives
set_env.CONDITIONAL = { replace = "env", name = "MY_VAR", default = "fallback", marker = "sys_platform == 'linux'" }

INI

[testenv]
set_env =
    LINUX_VAR = 1; sys_platform == 'linux'
    DARWIN_VAR = 1; sys_platform == 'darwin'
    WIN_ONLY = 1; sys_platform == 'win32'

Available markers include sys_platform, os_name, platform_machine, platform_system, python_version, implementation_name, and others as defined in PEP-508.

For conditional settings that should differ per target Python environment (e.g., based on Python version), use tox’s conditional settings mechanism with environment factors instead.

More environment variable-related information can be found in Environment variable substitutions.

Injected environment variables

tox automatically injects several environment variables into the test environment when running commands. These are always set regardless of the pass_env or set_env configuration and cannot be overridden by the user.

Variable	Description
TOX_ENV_NAME	The name of the current tox environment (e.g. 3.12, lint).
TOX_WORK_DIR	The tox working directory (by default .tox under the project root).
TOX_ENV_DIR	The directory of the current tox environment (e.g. .tox/3.12).
PYTHONIOENCODING	Always set to utf-8 to ensure consistent encoding for standard I/O.
PIP_USER	Always set to 0 to prevent pip from attempting --user installs inside virtualenvs, which would fail because user site-packages aren’t visible. Only set when using virtualenv-based environments.
TOX_PACKAGE	The path(s) to the built package artifact(s), joined by os.pathsep if there are multiple. Only set in run environments where a package has been built. See Reference the built package path in commands for usage examples.
VIRTUAL_ENV	The path to the virtual environment directory. Only set when using virtualenv-based environments (the default).
__TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI	The value of the CI environment variable from the host. Only set when the CI variable is present in the host environment.

Note

In tox 3, TOX_PARALLEL_ENV was injected during parallel runs. This variable is no longer injected in tox 4.

⚙️ parallel_show_output with default value of False 📢 added in 3.7

If set to True the content of the output will always be shown when running in parallel mode.

⚙️ recreate with default value of False 📢 added in 0.9

Always recreate virtual environment if this option is true, otherwise leave it up to tox.

⚙️ allowlist_externals with default value of <empty list> 📢 added in 1.5

Each line specifies a command name (in glob-style pattern format) which can be used in the commands section even if it’s located outside of the tox environment. For example: if you use the unix rm command for running tests you can list allowlist_externals=rm or allowlist_externals=/usr/bin/rm. If you want to allow all external commands you can use allowlist_externals=* which will match all commands (not recommended).

⚙️ labels with default value of <empty list> 📢 added in 4.0

A list of labels to apply for this environment. For example:

TOML

[tool.tox.env_run_base]
labels = ["test", "core"]
[tool.tox.env.flake8]
labels = ["mypy"]

INI

[testenv]
labels = test, core
[testenv:flake8]
labels = mypy

Execute

⚙️ suicide_timeout with default value of 0.0 📢 added in 3.15.2

When an interrupt is sent via Ctrl+C or the tox process is killed with a SIGTERM, a SIGINT is sent to all foreground processes. The suicide_timeout gives the running process time to cleanup and exit before receiving (in some cases, a duplicate) SIGINT from tox.

⚙️ interrupt_timeout with default value of 0.3 📢 added in 3.15

When tox is interrupted, it propagates the signal to the child process after suicide_timeout seconds. If the process still hasn’t exited after interrupt_timeout seconds, its sends a SIGTERM.

⚙️ terminate_timeout with default value of 0.2 📢 added in 3.15

When tox is interrupted, after waiting interrupt_timeout seconds, it propagates the signal to the child process, waits interrupt_timeout seconds, sends it a SIGTERM, waits terminate_timeout seconds, and sends it a SIGKILL if it hasn’t exited.

Run

⚙️ base with default value of testenv 📢 added in 4.0.0

Inherit missing keys from these sections.

⚙️ runner with default value of 📢 added in 4.0.0

The tox execute used to evaluate this environment. Defaults to Python virtual environments, however may be overwritten by plugins.

⚙️ description with default value of <empty string> 📢 added in 2.7

A short description of the environment, this will be used to explain the environment to the user upon listing environments.

⚙️ depends with default value of <empty list> 📢 added in 3.7

tox environments that this environment depends on (must be run after those). Supports glob patterns using fnmatch syntax to match environment names dynamically:

• * matches everything (e.g. 3.* matches 3.12, 3.13, 3.14)

• ? matches any single character (e.g. py? matches py3 but not py314)

• [seq] matches any character in seq (e.g. lint-[ab] matches lint-a and lint-b)

• [!seq] matches any character not in seq

TOML

[env.coverage]
depends = ["3.*"]

INI

[testenv:coverage]
depends = 3.*

This matches all environments whose name starts with 3. (e.g. 3.12, 3.13, 3.14). Glob patterns are resolved at runtime against the set of environments being run, so adding a new environment to env_list automatically includes it in the dependency without updating depends. Self-matches are excluded, so depends = * will depend on all other environments without creating a cycle.

Warning

depends does not pull in dependencies into the run target, for example if you select 3.13,3.12,coverage via the -e tox will only run those three (even if coverage may specify as depends other targets too - such as 3.13, 3.12, 3.11). This is solely meant to specify dependencies and order in between a target run set.

⚙️ extra_setup_commands with default value of <empty list> 📢 added in 4.37

Commands to execute after the setup phase (dependencies and package installation) but before test commands. These commands run during the --notest phase, making them useful for separating environment setup from test execution. All evaluation and configuration logic applies from commands.

This is particularly useful when you want to use tox run --notest to set up the environment and install additional tools or perform setup tasks, while keeping the actual test execution separate.

For example, to install pre-commit hooks during the setup phase:

TOML

[tool.tox.env_run_base]
deps = ["pre-commit"]
extra_setup_commands = [
  ["pre-commit", "install-hooks"],
]
commands = [
  ["pre-commit", "run", "--all-files"],
]

INI

[testenv]
deps = pre-commit
extra_setup_commands = pre-commit install-hooks
commands = pre-commit run --all-files

When running tox run --notest, the environment will be created, dependencies installed, and extra_setup_commands executed, but commands will be skipped. When running tox run without --notest, all commands including extra_setup_commands will execute.

⚙️ recreate_commands with default value of <empty list> 📢 added in 4.42

Commands to run before the environment directory is removed during recreation (tox run -r). These commands execute inside the existing environment, so tools installed there are available. Useful for cleaning external caches managed by tools like pre-commit. Failures are logged as warnings but never block recreation.

TOML

[env_run_base]
deps = ["pre-commit"]
recreate_commands = [["{env_python}", "-Im", "pre_commit", "clean"]]

INI

[testenv]
deps = pre-commit
recreate_commands = {env_python} -Im pre_commit clean

These commands do not run on first creation (the environment directory does not exist yet) or on normal re-runs without -r.

⚙️ commands_pre with default value of <empty list> 📢 added in 3.4

Commands to run before running the commands. All evaluation and configuration logic applies from commands.

⚙️ commands with default value of <empty list> 📢 added in 0.5

The commands to be called for testing. Only execute if commands_pre succeed. Each line is interpreted as one command; however a command can be split over multiple lines by ending the line with the \ character.

Commands will execute one by one in sequential fashion until one of them fails (their exit code is non-zero) or all of them succeed. The exit code of a command may be ignored (meaning they are always considered successful) by prefixing the command with a dash (-) - this is similar to how make recipe lines work. The outcome of the environment is considered successful only if all commands (these + setup + teardown) succeeded (exit code ignored via the - or success exit code value of zero).

Note

The virtual environment binary path (see env_bin_dir) is prepended to the PATH environment variable, meaning commands will first try to resolve to an executable from within the virtual environment, and only after that outside of it. Therefore python translates as the virtual environments python (having the same runtime version as the base_python), and pip translates as the virtual environments pip.

Note

shlex POSIX-mode quoting rules are used to split the command line into arguments on all supported platforms as of tox 4.4.0.

The backslash \ character can be used to escape quotes, whitespace, itself, and other characters (except on Windows, where a backslash in a path will not be interpreted as an escape). Unescaped single quote will disable the backslash escape until closed by another unescaped single quote. For more details, please see shlex parsing rules.

Note

Inline scripts can be used, however note these are discovered from the project root directory, and is not influenced by change_dir (this only affects the runtime current working directory). To make this behavior explicit we recommend that you make inline scripts absolute paths by prepending {tox_root}, instead of path/to/my_script prefer {tox_root}{/}path{/}to{/}my_script. If your inline script is platform dependent refer to Configure platform-specific settings on how to select different script per platform.

⚙️ commands_post with default value of <empty list> 📢 added in 3.4

Commands to run after running the commands. Execute regardless of the outcome of both commands and commands_pre. All evaluation and configuration logic applies from commands.

⚙️ change_dir changedir with default value of {tox_root} 📢 added in 0.5

Change to this working directory when executing the test command. If the directory does not exist yet, it will be created (required for Windows to be able to execute any command).

⚙️ args_are_paths with default value of False 📢 added in 0.5

Treat positional arguments passed to tox as file system paths and - if they exist on the filesystem and are in relative format - rewrite them according to the current and change_dir working directory. This handles automatically transforming relative paths specified on the CLI to relative paths respective of the commands executing directory.

⚙️ ignore_errors with default value of False 📢 added in 2.0

When executing the commands keep going even if a sub-command exits with non-zero exit code. The overall status will be “commands failed”, i.e. tox will exit non-zero in case any command failed. It may be helpful to note that this setting is analogous to the -k or --keep-going option of GNU Make.

⚙️ commands_retry with default value of 0 📢 added in 4.41

Number of times to retry a failed command before giving up. A value of N means each command can be attempted up to N + 1 times total. Applies to commands_pre, commands, and commands_post. Commands prefixed with - (ignore exit code) are never retried since their failures are already ignored.

TOML

[env_run_base]
commands_retry = 2
commands = [["pytest", "tests"]]

INI

[testenv]
commands_retry = 2
commands = pytest tests

⚙️ ignore_outcome with default value of False 📢 added in 2.2

If set to true a failing result of this test environment will not make tox fail (instead just warn).

⚙️ fail_fast with default value of False 📢 added in 4.36.0

If set to true, tox will stop executing remaining environments when this environment fails. This can also be enabled globally via the --fail-fast (-x) CLI flag. The behavior respects ignore_outcome – environments with ignore_outcome = true will not trigger fail-fast even if they fail.

⚙️ skip_install with default value of False 📢 added in 1.9

Skip installation of the package. This can be used when you need the virtualenv management but do not want to install the current package into that environment. To also skip dependency installation at runtime, use the --skip-env-install CLI flag (see Reuse an environment without network).

⚙️ package_env with default value of {package_env} 📢 added in 4.0.0

Name of the virtual environment used to create a source distribution from the source tree for this environment.

⚙️ package_tox_env_type with default value of virtualenv-pep-517 📢 added in 4.0.0

tox package type used to package.

Packaging environment configuration

Packaging environments (such as .pkg and .pkg-cpython314) are tox environments used to build your project’s package. Unlike run environments, packaging environments do not inherit from the env_run_base table (TOML) or [testenv] section (INI). This is intentional — test environment settings often conflict with packaging settings.

Instead, packaging environments inherit from the env_pkg_base table (TOML) or [pkgenv] section (INI). This allows you to define common packaging settings in one central place, while still overriding them for specific packaging environments when needed.

TOML

[env_pkg_base]
pass_env = ["PKG_CONFIG", "PKG_CONFIG_PATH", "PKG_CONFIG_SYSROOT_DIR"]

[env.".pkg-cpython311"]
pass_env = ["PKG_CONFIG", "PKG_CONFIG_PATH", "PKG_CONFIG_SYSROOT_DIR", "IS_311"]

INI

[pkgenv]
pass_env =
    PKG_CONFIG
    PKG_CONFIG_PATH
    PKG_CONFIG_SYSROOT_DIR

[testenv:.pkg-cpython311]
pass_env =
    {[pkgenv]pass_env}
    IS_311 = yes

Note

Specific packaging environments are defined under [testenv:.pkg] and not [pkgenv:.pkg]. The [pkgenv] section (or env_pkg_base in TOML) serves only as the base/fallback for settings shared across all packaging environments.

Changed in version 4.2: Packaging environments now inherit from the [pkgenv] / env_pkg_base section.

Python options

⚙️ base_python basepython with default value of <{env_name} python factor> or <python version of tox> 📢 added in 0.5

Name or path to a Python interpreter which will be used for creating the virtual environment, first one found wins. This determines in practice the Python for what we’ll create a virtual isolated environment. Use this to specify the Python version for a tox environment. If not specified, the virtual environments factors (e.g. name part) will be used to automatically set one. For example, py310 means python3.10, py3 means python3 and py means python. If the name does not match this pattern, default_base_python is consulted, and if that is also not set, the same Python version tox is installed into will be used. A base interpreter ending with t means that only free threaded Python implementations are accepted.

The preferred naming convention is N.M (e.g. 3.14, 3.13) rather than pyNMM (e.g. py314, py313). The dotted form is clearer, avoids ambiguity for Python versions >= 3.10, and reads more naturally in environment lists and CI output.

Warning

Any factor matching the pattern 2.N or 3.N (where N is one or more digits) is treated as a Python version specifier. This means non-Python versions like 2.16 in an environment name such as unit-py3.10-2.16 will conflict with the py3.10 factor. To avoid this, prefix non-Python version numbers (e.g. ac2.16), or set ignore_base_python_conflict to true.

Added in version 4.46:

You can append a CPU architecture (ISA) suffix to constrain the interpreter to a specific machine type. The architecture is derived from sysconfig.get_platform() and normalized by virtualenv (e.g. amd64 becomes x86_64, aarch64 becomes arm64). Examples:

• cpython3.12-64-arm64 — CPython 3.12, 64-bit, ARM

• cpython3.13-64-x86_64 — CPython 3.13, 64-bit, x86-64

If the discovered interpreter’s architecture does not match the requested one, tox raises a failure, just as it does for version mismatches. This is useful on machines with multi-architecture support (e.g. Apple Silicon running both arm64 and x86_64 via Rosetta, or Linux with aarch64 and x86_64 via qemu-user).

Changed in version 3.1: After resolving this value if the interpreter reports back a different version number than implied from the name a warning will be printed by default. However, if ignore_basepython_conflict is set, the value is ignored and we force the base_python implied from the factor name.

Note

Leaving this unset will cause an error if the package under test has a different Python requires than tox itself and tox is installed into a Python that’s not supported by the package. For example, if your package requires Python 3.10 or later, and you install tox in Python 3.9, when you run a tox environment that has left this unspecified tox will use Python 3.9 to build and install your package which will fail given it requires 3.10.

⚙️ default_base_python with default value of <python version of tox> 📢 added in 4.42

Fallback Python interpreter used when the environment name contains no Python factor and no explicit base_python is set. Accepts a list of specifications – the first one found wins. This provides a way to pin a default Python version for reproducibility across different machines without conflicting with pyXY factor-named environments.

The resolution order for an environment’s Python interpreter is:

1. Python factor extracted from the environment name (e.g. 3.13, py313)

2. Explicit base_python setting on the environment

3. base_python_file — read from a version file

4. default_base_python (this setting)

5. The Python running tox (sys.executable)

TOML

[env_run_base]
default_base_python = ["3.14", "3.13"]

INI

[testenv]
default_base_python = 3.14, 3.13

⚙️ base_python_file with default value of <empty string> 📢 added in 4.51

A path to a file containing the Python version to use for the environment (e.g. .python-version). The first non-blank, non-comment line in the file is used as the Python version specifier. This is useful for tool environments (such as mypy or coverage) where you want a single source of truth for the default Python version without hardcoding it in the tox configuration.

This option is only consulted when base_python is not explicitly set and the environment name does not contain a Python factor (e.g. py314). See default_base_python for the full resolution order.

TOML

[env.mypy]
base_python_file = [".python-version", ".python-version-default"]

INI

[testenv:mypy]
base_python_file = .python-version, .python-version-default

The file format is the same as used by pyenv and GitHub Actions’ python-version-file: one version per file, lines starting with # are comments, blank lines are ignored.

■ env_site_packages_dir envsitepackagesdir 📢 added in 1.4.3

The Python environments site package - where pure-python packages are installed (the purelib folder path).

■ env_site_packages_dir_plat envsitepackagesdir_plat 📢 added in 4.42

The Python environments platform-specific site package (the platlib folder path). On most platforms this is the same as env_site_packages_dir, but on some Linux distributions (Fedora, RHEL) platlib resolves to lib64 instead of lib.

■ env_bin_dir envbindir 📢 added in 0.5

The binary folder where console/gui scripts are generated during installation.

■ env_python envpython 📢 added in 0.5

The Python executable from within the tox environment.

■ py_dot_ver 📢 added in 4.0.10

Major.Minor version of the Python interpreter in the tox environment (e.g., 3.14).

■ py_impl 📢 added in 4.0.10

Name of the Python implementation in the tox environment in lowercase (e.g., cpython, pypy).

■ py_free_threaded 📢 added in 4.26

True if the Python interpreter in the tox environment is a free-threaded CPython build, else False.

Python run

⚙️ dependency_groups with default value of <empty list> 📢 added in 4.22

A list of names of dependency groups (as defined by PEP 735) to install into this Python environment. The installation will happen before installing the package or any of its dependencies.

For example:

TOML

[dependency-groups]
test = [
   "pytest>=8",
]

[tool.tox.env_run_base]
dependency_groups = [
  "test",
]

INI

[testenv]
dependency_groups =
    test

[dependency-groups]
test = [
   "pytest>=8",
]

⚙️ pylock with default value of <empty string> 📢 added in 4.44

Path to a PEP 751 pylock.toml lock file to install locked dependencies from. Mutually exclusive with deps. Each package in the lock file is filtered by evaluating its environment markers against the target Python interpreter and the configured extras and dependency_groups, then converted to a pinned requirement (name==version) and installed via pip with --no-deps. The path is resolved relative to the package_root (or tox_root if no package root is configured). Change detection is automatic: adding, removing, or changing packages triggers environment recreation as needed. See Lock file installation (PEP 751) for details.

For example:

TOML

[tool.tox.env_run_base]
pylock = "pylock.toml"

INI

[testenv]
pylock = pylock.toml

⚙️ deps with default value of <empty list> 📢 added in 0.5

Python dependencies. Installed into the environment prior to project after environment creation, but before package installation. All installer commands are executed using the tox_root as the current working directory. Each value must be one of:

• a Python dependency as specified by PEP 440,

• a requirement file when the value starts with -r (followed by a file path or URL),

• a constraint file when the value starts with -c (followed by a file path or URL).

If you are only defining PEP 508 requirements (aka no pip requirement files), you should use dependency_groups instead.

For example:

TOML

[tool.tox.env_run_base]
deps = [
  "pytest>=8",
  "-r requirements.txt",
  "-c constraints.txt",
]

INI

[testenv]
deps =
    pytest>=7,<8
    -r requirements.txt
    -c constraints.txt

Note

constraints is the preferred way to specify constraints files since they will apply to package dependencies also.

⚙️ constraints with default value of <empty list> 📢 added in 4.28.0

Constraints files to use during package and dependency installation. Provided constraints files will be used when installing package dependencies and any additional dependencies specified in deps, but will not be used when installing the package itself. Each value must be a file path or URL.

⚙️ use_develop usedevelop with default value of false 📢 added in 1.6

Install the current package in development mode using PEP 660. This means that the package will be installed in-place and editable.

Note

package = editable is the preferred way to enable development/editable mode. See the details in package.

Note

PEP-660 introduced a standardized way of installing a package in development mode, providing the same effect as if pip install -e was used.

⚙️ package 📢 added in 4.0

When option can be one of wheel, sdist, sdist-wheel, editable, editable-legacy, deps-only, skip, or external. If use_develop is set this becomes a constant of editable. If skip_install is set this becomes a constant of skip.

When deps-only is selected, tox installs the package’s dependencies (including any requested extras) but does not build or install the package itself. This is useful for environments that need the same dependencies as the package without the package, such as coverage combining, documentation building, or linting. For projects with static PEP 621 metadata in pyproject.toml, dependencies are read directly without creating a packaging environment. For dynamic dependencies or non-PEP-621 projects, the packaging environment is used to extract metadata.

When sdist-wheel is selected, tox first builds a source distribution and then builds a wheel from that sdist (rather than directly from the source tree). This is useful for verifying that the sdist is complete and that the package can be correctly built from it — catching missing files or packaging errors early.

When editable is selected and the build backend supports PEP 660, tox will use the standardized editable install mechanism. If the backend does not support PEP 660, tox will automatically fall back to editable-legacy mode (equivalent to pip install -e) and print a message suggesting you make this setting explicit in your configuration. To ensure your backend supports the standardized method, verify it meets the minimum version requirement (e.g. setuptools>=64).

⚙️ wheel_build_env with default value of <package_env>-<python-flavor-lowercase><python-version-no-dot> 📢 added in 4.0

If package is set to wheel or sdist-wheel this will be the tox Python environment in which the wheel will be built. For sdist-wheel, the sdist is first built in the package_env environment, then the wheel is built from the extracted sdist in this environment. The value is generated to be unique per Python flavor and version, and prefixed with package_env value. This is to ensure the target interpreter and the generated wheel will be compatible. If you have a wheel that can be reused across multiple Python versions set this value to the same across them (to avoid building a new wheel for each one of them).

⚙️ extras with default value of <empty list> 📢 added in 2.4

A list of “extras” from the package to be installed. For example, extras = testing is equivalent to [testing] in a pip install command.

External package builder

tox supports operating with externally built packages. External packages might be provided in two ways:

• explicitly via the –installpkg CLI argument,

• setting the package to external and using a tox packaging environment named <package_env>_external (see package_env) to build the package. The tox packaging environment takes all configuration flags of a python environment, plus the following:

⚙️ deps with default value of <empty list> 📢 added in 0.5

Name of the Python dependencies as specified by PEP 440. Installed into the environment prior running the build commands. All installer commands are executed using the tox_root as the current working directory.

⚙️ commands with default value of <empty list> 📢 added in 0.5

Commands to run that will build the package. If any command fails the packaging operation is considered failed and will fail all environments using that package.

⚙️ ignore_errors with default value of False 📢 added in 2.0

When executing the commands keep going even if a sub-command exits with non-zero exit code. The overall status will be “commands failed”, i.e. tox will exit non-zero in case any command failed. It may be helpful to note that this setting is analogous to the -k or --keep-going option of GNU Make.

⚙️ change_dir changedir with default value of {tox_root} 📢 added in 0.5

Change to this working directory when executing the package build command. If the directory does not exist yet, it will be created (required for Windows to be able to execute any command).

⚙️ package_glob with default value of {envtmpdir}{/}dist{/}* 📢 added in 4.0

A glob that should match the wheel/sdist file to install. If no file or multiple files is matched the packaging operation is considered failed and will raise an error.

Python virtual environment

⚙️ system_site_packages sitepackages with default value of False 📢 added in 0.8

Create virtual environments that also have access to globally installed packages. Note the default value may be overwritten by the VIRTUALENV_SYSTEM_SITE_PACKAGES environment variable.

Warning

In cases where a command line tool is also installed globally you have to make sure that you use the tool installed in the virtualenv by using python -m <command line tool> (if supported by the tool) or {env_bin_dir}/<command line tool>. If you forget to do that you will get an error.

⚙️ always_copy alwayscopy with default value of False 📢 added in 2.6

Force virtualenv to always copy rather than symlink. Note the default value may be overwritten by the VIRTUALENV_COPIES or VIRTUALENV_ALWAYS_COPY (in that order) environment variables. This is useful for situations where hardlinks don’t work (e.g. running in VMS with Windows guests).

⚙️ download with default value of False 📢 added in 3.10

True if you want virtualenv to upgrade pip/wheel/setuptools to the latest version. Note the default value may be overwritten by the VIRTUALENV_DOWNLOAD environment variable. If (and only if) you want to choose a specific version (not necessarily the latest) then you can add VIRTUALENV_PIP=20.3.3 (and similar) to your set_env.

⚙️ virtualenv_spec with default value of "" 📢 added in 4.42

A PEP 440 version specifier for virtualenv (e.g. virtualenv<20.22.0). When set, tox bootstraps the specified virtualenv version into an isolated environment and drives it via subprocess, instead of using the imported virtualenv library. This enables environments targeting Python versions that are incompatible with the virtualenv installed alongside tox.

The bootstrap environment is cached under .tox/.virtualenv-bootstrap/ (keyed by a hash of the spec string) and reused across runs. Concurrent access is protected by a file lock. When the spec is empty (the default), tox uses the imported virtualenv with zero overhead.

TOML

[env.legacy]
base_python = ["python3.6"]
virtualenv_spec = "virtualenv<20.22.0"
commands = [["python", "-c", "import sys; print(sys.version)"]]

[env.modern]
base_python = ["python3.15"]
commands = [["python", "-c", "import sys; print(sys.version)"]]

INI

[testenv:legacy]
base_python = python3.6
virtualenv_spec = virtualenv<20.22.0
commands = python -c 'import sys; print(sys.version)'

[testenv:modern]
base_python = python3.15
commands = python -c 'import sys; print(sys.version)'

Changing this value triggers automatic environment recreation (the spec is included in the cache key).

See Virtualenv version pinning for background on when and why to use this setting.

Python virtual environment packaging

⚙️ meta_dir with default value of {env_dir}/.meta 📢 added in 4.0.0

Directory where to put the project metadata files.

⚙️ pkg_dir with default value of {env_dir}/.dist 📢 added in 4.0.0

Directory where to put project packages.

⚙️ config_settings_get_requires_for_build_sdist 📢 added in 4.11

Config settings (dict[str, str]) passed to the get_requires_for_build_sdist backend API endpoint.

⚙️ config_settings_build_sdist 📢 added in 4.11

Config settings (dict[str, str]) passed to the build_sdist backend API endpoint.

⚙️ config_settings_get_requires_for_build_wheel 📢 added in 4.11

Config settings (dict[str, str]) passed to the get_requires_for_build_wheel backend API endpoint.

⚙️ config_settings_prepare_metadata_for_build_wheel 📢 added in 4.11

Config settings (dict[str, str]) passed to the prepare_metadata_for_build_wheel backend API endpoint.

⚙️ config_settings_build_wheel 📢 added in 4.11

Config settings (dict[str, str]) passed to the build_wheel backend API endpoint.

⚙️ config_settings_get_requires_for_build_editable 📢 added in 4.11

Config settings (dict[str, str]) passed to the get_requires_for_build_editable backend API endpoint.

⚙️ config_settings_prepare_metadata_for_build_editable 📢 added in 4.11

Config settings (dict[str, str]) passed to the prepare_metadata_for_build_editable backend API endpoint.

⚙️ config_settings_build_editable 📢 added in 4.11

Config settings (dict[str, str]) passed to the build_editable backend API endpoint.

⚙️ fresh_subprocess with default value of True if build backend is setuptools otherwise False 📢 added in 4.14.0

A flag controlling if each call to the build backend should be done in a fresh subprocess or not (especially older build backends such as setuptools might require this to discover newly provisioned dependencies).

Pip installer

⚙️ install_command with default value of python -I -m pip install {opts} {packages} 📢 added in 1.6

Determines the command used for installing packages into the virtual environment; both the package under test and its dependencies (defined with deps). Must contain the substitution key {packages} which will be replaced by the package(s) to install. You should also accept {opts} – it will contain index server options such as --pre (configured as pip_pre).

Note

You can also provide arbitrary commands to the install_command. Please take care that these commands can be executed on the supported operating systems. When executing shell scripts we recommend to not specify the script directly but instead pass it to the appropriate shell as argument (e.g. prefer bash script.sh over script.sh).

⚙️ list_dependencies_command with default value of python -m pip freeze --all 📢 added in 2.4

The list_dependencies_command setting is used for listing the packages installed into the virtual environment. This command will be executed only if executing on Continuous Integrations is detected (for example set environment variable CI=1) or if journal is active. In TOML configurations, reference this Command value using {replace = "ref"} with extend = true rather than string interpolation.

⚙️ pip_pre with default value of false 📢 added in 1.9

If true, adds --pre to the opts passed to install_command. This will cause it to install the latest available pre-release of any dependencies without a specified version. If false, pip will only install final releases of unpinned dependencies.

⚙️ constrain_package_deps with default value of false 📢 added in 4.4.0

If constrain_package_deps is true, then tox will create and use {env_dir}{/}constraints.txt when installing package dependencies during install_package_deps stage. When this value is set to false, any conflicting package dependencies will override explicit dependencies and constraints passed to deps.

⚙️ use_frozen_constraints with default value of false 📢 added in 4.4.0

When use_frozen_constraints is true, then tox will use the list_dependencies_command to enumerate package versions in order to create {env_dir}{/}constraints.txt. Otherwise the package specifications explicitly listed under deps (or in requirements / constraints files referenced in deps) will be used as the constraints. If constrain_package_deps is false, then this setting has no effect.

User configuration

tox allows creation of user level config-file to modify default values of the CLI commands. It is located in the OS-specific user config directory under tox/config.ini path, see tox --help output for exact location. It can be changed via TOX_USER_CONFIG_FILE environment variable. Example configuration:

[tox]
skip_missing_interpreters = true

Set CLI flags via environment variables

All configuration can be overridden via environment variables too, the naming convention here is TOX_<option>. E.g. TOX_WORK_DIR sets the --workdir flag, or TOX_OVERRIDE sets the --override flag. For flags accepting more than one argument, use the ; character to separate these values:

All configuration inside the configuration file may be overwritten via the TOX_OVERRIDE, note in this case the configuration file and its access (section/table + key) are needed. Here we demonstrate with a tox.ini file:

# set FOO and bar as passed environment variable
$ env 'TOX_OVERRIDE=testenv.pass_env=FOO,BAR' tox c -k pass_env -e py
[testenv:py]
pass_env =
  BAR
  FOO
  <default pass_envs>

# append FOO and bar as passed environment variable to the list already defined in
# the tox configuration
$ env 'TOX_OVERRIDE=testenv.pass_env+=FOO,BAR' tox c -k pass_env -e py
[testenv:py]
pass_env =
  BAR
  FOO
  <pass_envs defined in configuration>
  <default pass_envs>

# set httpx and deps to and 3.12 as base_python
$ env 'TOX_OVERRIDE=testenv.deps=httpx;testenv.base_python=3.12' .tox/dev/bin/tox c \
      -k deps base_python -e py
[testenv:py]
deps = httpx
base_python = 3.12

Overriding configuration from the command line

You can override options in the configuration file, from the command line. For example, given this config:

TOML

 # tox.toml
[env_run_base]
deps = ["pytest"]
set_env = { foo = "bar" }
commands = [[ "pytest", "tests" ]]

INI

[testenv]
deps = pytest
set_env =
  foo=bar
commands = pytest tests

You could enable ignore_errors by running:

TOML

tox --override env_run_base.ignore_errors=True

INI

tox --override testenv.ignore_errors=True

You could add additional dependencies by running:

TOML

tox --override env_run_base.deps+=pytest-xdist

INI

tox --override testenv.deps+=pytest-xdist

You could set additional environment variables by running:

TOML

tox --override env_run_base.set_env+=baz=quux

INI

tox --override testenv.set_env+=baz=quux

You can specify overrides multiple times on the command line to append multiple items:

TOML

tox -x env_run_base.set_env+=foo=bar -x env_run_base.set_env+=baz=quux
tox -x testenv_run_baseenv.deps+=pytest-xdist -x env_run_base.deps+=pytest-covt

INI

tox -x testenv.set_env+=foo=bar -x testenv.set_env+=baz=quux
tox -x testenv.deps+=pytest-xdist -x testenv.deps+=pytest-covt

Or reset override and append to that (note the first override is = and not +=):

TOML

tox -x env_run_base.deps=pytest-xdist -x env_run_base.deps+=pytest-cov

INI

tox -x testenv.deps=pytest-xdist -x testenv.deps+=pytest-cov

TOML only

These additional rules are active for native TOML configuration files.

String elements (excluding keys) will be transformed according to the Substitutions section.

String substitution references

Added in version 4.21.

Within strings values from other sections can be referred to via {[<table>]<key>}:

which you can use to avoid repetition of config values. You can put default values in one section and reference them in others to avoid repeating the same values:

[extra]
ok = "o"
[.env.B]
description = "{[extra]ok}"

If the target table is one of the tox environments variable substitution will be applied on the replaced value, otherwise the text will be inserted as is (e.g., here with extra).

Configuration reference

Added in version 4.21.

You can reference other configurations via the ref replacement. This can either be of type:

• env, in this case the configuration is loaded from another tox environment, where string substitution will happen in that environments scope:

  [env.src]
  extras = ["A", "{env_name}"]
  [env.dest]
  extras = [{ replace = "ref", env = "src", key = "extras", extend = true }, "B"]
  

  In this case dest environments extras will be A, src, B.

• raw, in this case the configuration is loaded as raw, and substitution executed in the current environments scope:

  [env.src]
  extras = ["A", "{env_name}"]
  [env.dest]
  extras = [{ replace = "ref", of = ["env", "src", "extras"], extend = true }, "B"]
  

  In this case dest environments extras will be A, dest, B.

The extend flag controls if after replacement the value should be replaced as is in the host structure (when flag is false – by default) or be extended into. This flag only operates when the host is a list.

When referencing Command-type configuration values (like list_dependencies_command), the reference automatically extracts the command’s argument list, making it compatible with TOML’s structured commands format. For example:

[tool.tox.env.a]
package = "skip"
commands = [[{ replace = "ref", env = "a", key = "list_dependencies_command", extend = true }]]

This expands the Command’s args into the command list, avoiding the need for string interpolation which doesn’t work properly with Command values in TOML.

Positional argument reference

Added in version 4.21.

You can reference positional arguments via the posargs replacement:

[env.A]
commands = [["python", { replace = "posargs", default = ["a", "b"], extend = true } ]]

If the positional arguments are not set commands will become python a b, otherwise will be python posarg-set. The extend option instructs tox to unroll the positional arguments within the host structure. Without it the result would become ["python", ["a", "b"] which would be invalid.

Note that:

[env.A]
commands = [["python", "{posargs}" ]]

Differs in sense that the positional arguments will be set as a single argument, while in the original example they are passed through as separate.

Empty commands groups will be ignored:

[env.A]
commands = [[], ["pytest"]]

will only invoke pytest. This is especially useful together with posargs allowing you to opt out of running a set of commands:

[env.A]
commands = [
    { replace = "posargs", default = ["python", "patch.py"]},
    ["pytest"]
]

When running tox run -e A it will invoke python patch.py followed by pytest. When running tox run -e A -- it will invoke only pytest.

Environment variable reference

Added in version 4.21.

You can reference environment variables via the env replacement:

[env.A]
set_env.COVERAGE_FILE = { replace = "env", name = "COVERAGE_FILE", default = "ok" }

If the environment variable is set the the COVERAGE_FILE will become that, otherwise will default to ok.

References within set_env

Added in version 4.21.1.

When you want to inherit set_env from another environment you can use the feature that if you pass a list of dictionaries to set_env they will be merged together, for example:

[tool.tox.env_run_base]
set_env = { A = "1", B = "2"}

[tool.tox.env.magic]
set_env = [
    { replace = "ref", of = ["tool", "tox", "env_run_base", "set_env"]},
    { C = "3", D = "4"},
]

Here the magic tox environment will have both A, B, C and D environments set.

Glob pattern reference

Added in version 4.40.

You can expand file system glob patterns via the glob replacement. Matched paths are sorted for deterministic output, and relative patterns are resolved against tox_root. The ** wildcard matches any number of directories recursively.

[env.A]
commands = [["twine", "upload", { replace = "glob", pattern = "dist/*.whl", extend = true }]]

When used with extend = true the matched files are expanded as separate arguments in the host list. Without extend the matches are joined as a single space-separated string.

If no files match and a default is provided it will be used as fallback:

[env.A]
commands = [["twine", "upload", { replace = "glob", pattern = "dist/*.whl", default = ["fallback.whl"], extend = true }]]

When no files match and no default is given, the result is an empty string (or empty list with extend).

Conditional value reference

Added in version 4.40: Conditional value replacement with env.VAR lookups.

Changed in version 4.42: Added factor.NAME lookups for environment name factors and platform.

Changed in version 4.50: Added factor['NAME'] and env['VAR'] subscript syntax and env_name variable.

You can conditionally select values based on environment variables and factors via the if replacement. The condition field accepts an expression language that supports env.VAR_NAME lookups for environment variables, factor.NAME lookups for environment name factors and platform, ==/!= comparisons, and and/or/not boolean logic.

Check if an environment variable is set (non-empty):

[env.A]
set_env.MATURITY = { replace = "if", condition = "env.TAG_NAME", then = "production", "else" = "testing" }

If TAG_NAME is set and non-empty, MATURITY becomes production, otherwise testing.

Compare an environment variable to a value:

[env.A]
set_env.MODE = { replace = "if", condition = "env.CI == 'true'", then = "ci", "else" = "local" }

Combine conditions with boolean logic:

[env.A]
set_env.DEPLOY_TARGET = { replace = "if", condition = "env.CI and env.DEPLOY", then = "production", "else" = "none" }

[env.B]
pass_env = [{ replace = "if", condition = "env.CI or env.LOCAL", then = ["AWS_*"], "else" = [], extend = true }]

[env.C]
commands = [["pytest", { replace = "if", condition = "not env.CI", then = ["--pdb"], "else" = [], extend = true }]]

[env.D]
set_env.LOG_LEVEL = { replace = "if", condition = "env.MODE != 'prod'", then = "DEBUG", "else" = "INFO" }

Omitting the else clause defaults to an empty string:

[env.A]
set_env.DEPLOY_FLAG = { replace = "if", condition = "env.DEPLOY", then = "1" }

Nested substitutions in then/else values are processed normally:

[env.A]
set_env.TEST_ENV = { replace = "if", condition = "env.DEPLOY", then = "{env_name}", "else" = "local" }

With extend in list contexts:

[env.A]
commands = [["pytest", { replace = "if", condition = "env.VERBOSE", then = ["--verbose", "--debug"], "else" = ["--quiet"], extend = true }]]

Check if a factor is present:

[env_run_base]
deps = [
    "pytest",
    { replace = "if", condition = "factor.django50", then = ["Django>=5.0,<5.1"], extend = true },
]

If the environment name contains django50 (e.g., py313-django50), the Django dependency is added.

Check for platform factors:

[env_run_base]
commands = [
    { replace = "if", condition = "factor.linux", then = [["pytest", "--numprocesses=auto"]], extend = true },
    { replace = "if", condition = "not factor.linux", then = [["pytest"]], extend = true },
]

The current platform (sys.platform value like linux, darwin, win32) is automatically available as a factor without requiring it in the environment name.

Combine factor conditions with environment variables:

[env_run_base]
commands = [["pytest", { replace = "if", condition = "factor.linux and env.CI", then = ["--numprocesses=auto"], "else" = [], extend = true }]]

Check factors with non-identifier names:

Factor names like 3.14 are not valid Python identifiers, so factor.3.14 would be a syntax error. Use subscript syntax instead:

[env."test-3.14"]
commands = [
    ["pytest", { replace = "if", condition = "factor['3.14']", then = ["--strict-markers"], "else" = [], extend = true }],
]

In this example, the environment name test-3.14 has factors test and 3.14. The subscript syntax also works for environment variables: env['CI'] is equivalent to env.CI.

Check the environment name:

Use env_name to match the full environment name as a string:

[env_run_base]
set_env.CUSTOM_FLAG = { replace = "if", condition = "env_name == 'test-3.14'", then = "enabled", "else" = "" }

Condition expression reference:

• env.VAR – value of environment variable VAR (empty string if unset); truthy when non-empty

• env['VAR'] – same as env.VAR

• factor.NAME – True if NAME is a factor in the environment name or platform; False otherwise

• factor['NAME'] – same as factor.NAME, for names that aren’t valid Python identifiers (e.g. 3.14)

• env_name – the full environment name as a string

• ==, != – string comparison

• and, or, not – boolean logic

• 'string' – string literal

INI only

These additional rules are active for native INI configuration.

The value for each setting in an INI configuration will be transformed according to the Substitutions section.

Substitution for values from other sections

Added in version 1.4.

Values from other sections can be referred to via:

{[sectionname]valuename}

which you can use to avoid repetition of config values. You can put default values in one section and reference them in others to avoid repeating the same values:

[base]
deps =
    pytest
    mock
    pytest-xdist

[testenv:dulwich]
deps =
    dulwich
    {[base]deps}

[testenv:mercurial]
deps =
    mercurial
    {[base]deps}

Conditional settings

• Configurations may be set conditionally within the tox.ini file. If a line starts with an environment name or names, separated by a comma, followed by : the configuration will only be used if the environment name(s) matches the executed tox environment. For example:

  [testenv]
  deps =
     pip
     format: black
     py310,py39: pytest
  

  Here pip will be always installed as the configuration value is not conditional. black is only used for the format environment, while pytest is only installed for the 3.10 and 3.9 environments.

• Negative factors: prefixing a factor name with ! inverts the match – the line is included only when the factor is not present in the environment name:

  [testenv]
  deps =
      !lint: pytest
      !lint: coverage
      lint: ruff
  

  Here pytest and coverage are installed for every environment except lint, while ruff is only installed for lint.

• Multi-factor conditions with negation: you can combine positive and negative factors:

  [testenv]
  deps =
      # installed only when both "django" and "mysql" factors are present
      django-mysql: PyMySQL
      # installed for all envs except those with the "slow" factor
      !slow: pytest-timeout
      # installed when either "3.12" or "3.13" factor is present, but not "minimal"
      !minimal-3.12,!minimal-3.13: hypothesis
  

• Factor-conditional commands: factor conditions work in any multi-line setting, not just deps:

  [testenv]
  commands =
      pytest tests
      coverage: coverage report --fail-under=80
  

  The coverage report command only runs in environments whose name contains the coverage factor (e.g. 3.13-coverage).

• Default fallback: when all lines in a setting are conditional and none match the current environment, the setting falls back to its default value. For example:

  [testenv]
  base_python =
      py312: python3.12
      py313: python3.13
  

  If you run an environment named lint (which contains neither the py312 nor py313 factor), base_python falls back to the default – derived from the environment name – rather than becoming an empty string. This applies to any configuration key, not just base_python.

Generative environment list

If you have a large matrix of dependencies, python versions and/or environments you can use a generative env_list and conditional settings to express that in a concise form:

TOML

env_list = [
    "lint",
    { product = [
        { prefix = "py3", start = 9, stop = 11 },
        ["django41", "django40"],
        ["sqlite", "mysql"],
    ] },
]

[env_run_base]
deps = [
    { replace = "if", condition = "factor.django41", then = ["Django>=4.1,<4.2"], extend = true },
    { replace = "if", condition = "factor.django40", then = ["Django>=4.0,<4.1"], extend = true },
    { replace = "if", condition = "factor.py311 and factor.mysql", then = ["PyMySQL"], extend = true },
    { replace = "if", condition = "factor.py311 or factor.py310", then = ["urllib3"], extend = true },
    { replace = "if", condition = "(factor.py311 or factor.py310) and factor.sqlite", then = ["mock"], extend = true },
]

INI

[tox]
env_list = py3{9-11}-django{41,40}-{sqlite,mysql}

[testenv]
deps =
    django41: Django>=4.1,<4.2
    django40: Django>=4.0,<4.1
    py311-mysql: PyMySQL
    py311,py310: urllib3
    py{311,310}-sqlite: mock

This will generate the following tox environments (plus lint):

> tox l
default environments:
py39-django41-sqlite  -> [no description]
py39-django41-mysql   -> [no description]
py39-django40-sqlite  -> [no description]
py39-django40-mysql   -> [no description]
py310-django41-sqlite -> [no description]
py310-django41-mysql  -> [no description]
py310-django40-sqlite -> [no description]
py310-django40-mysql  -> [no description]
py311-django41-sqlite -> [no description]
py311-django41-mysql  -> [no description]
py311-django40-sqlite -> [no description]
py311-django40-mysql  -> [no description]

INI expansion syntax

Both enumerations ({1,2,3}) and numerical ranges ({1-3}) are supported, and can be mixed together:

[tox]
env_list = py3{8-10, 11, 13-14}

[testenv]
deps =
    py{310,311-314}: urllib3
setenv =
    py{310,311-314}: FOO=bar

will create the following envs:

> tox l
default environments:
py38  -> [no description]
py39  -> [no description]
py310 -> [no description]
py311 -> [no description]
py313 -> [no description]
py314 -> [no description]

Negative ranges will also be expanded ({3-1} -> {3,2,1}). Non-numerical open ranges such as {a-} and {-b} will not be expanded.

Open-ended numerical ranges use bounds derived from the supported CPython versions at the time of the tox release:

• {10-} expands up to the latest supported CPython minor version (currently 14)

• {-13} expands down from the oldest supported CPython minor version (currently 10)

For example, py3{10-} is equivalent to py3{10,11,12,13,14} and py3{-13} is equivalent to py3{10,11,12,13}. These bounds are updated with each tox release as new CPython versions become supported or old ones reach end-of-life.

Caution

Be conscious of the number of significant digits in your range endpoints. A range like py{39-314} will not do what you may expect. (It expands to 275 environment names, py39, py40, py41 … py314.) Instead, use py3{9-14}.

TOML product syntax

TOML uses a product dict instead of curly-brace expansion. Each factor group is an array of strings or a range dict. The Cartesian product of all groups is computed and each combination is joined with -.

Range dicts accept prefix, start, and stop. Omit stop to expand up to the latest supported CPython minor version (currently 14), or omit start to expand down from the oldest (currently 10):

env_list = [
    { product = [{ prefix = "py3", start = 10 }, ["django42"]] },
]

The range { prefix = "py3", start = 10 } is equivalent to py3{10-} in INI.

An exclude key removes specific combinations from the product – a capability not available in INI:

env_list = [
    { product = [["py312", "py313"], ["django42", "django50"]], exclude = ["py312-django50"] },
]

Generative section names

Suppose you have some binary packages, and need to run tests both in 32 and 64 bits. You also want an environment to create your virtual env for the developers. This also supports ranges in the same way as generative environment lists.

INI

[testenv]
base_python =
    py311-x86: python3.11-32
    py311-x64: python3.11-64
commands = pytest

[testenv:py311-{x86,x64}-venv]
envdir =
    x86: .venv-x86
    x64: .venv-x64

> tox l
default environments:
py          -> [no description]

additional environments:
py311-x86-venv -> [no description]
py311-x64-venv -> [no description]

TOML

TOML does not support generative section names. Use env_base templates instead:

[env_base.py311-venv]
factors = [["x86", "x64"]]
base_python = { replace = "if", condition = "factor.x86", then = "python3.11-32", "else" = "python3.11-64" }
env_dir = { replace = "if", condition = "factor.x86", then = ".venv-x86", "else" = ".venv-x64" }
commands = [["pytest"]]

This generates py311-venv-x86 and py311-venv-x64, equivalent to the INI generative section.

Substitutions

Value substitution operates through the {...} string-substitution pattern. The string inside the curly braces may reference a global or per-environment config key as described above.

In substitutions, the backslash character \ will act as an escape when preceding {, }, :, [, or ], otherwise the backslash will be reproduced literally:

TOML

commands = [
  ["python", "-c", 'print("\{posargs} = \{}".format("{posargs}"))'],
  ["python", "-c", 'print("host: \{}".format("{env:HOSTNAME:host\: not set}")'],
]

INI

commands =
  python -c 'print("\{posargs} = \{}".format("{posargs}"))'
  python -c 'print("host: \{}".format("{env:HOSTNAME:host\: not set}")'

Note that any backslashes remaining after substitution may be processed by shlex during command parsing. On POSIX platforms, the backslash will escape any following character; on windows, the backslash will escape any following quote, whitespace, or backslash character (since it normally acts as a path delimiter).

Special substitutions that accept additional colon-delimited : parameters cannot have a space after the : at the beginning of line (e.g. {posargs: magic} would be parsed as factorial {posargs, having value magic).

Environment variable substitutions

If you specify a substitution string like this:

{env:KEY}

then the value will be retrieved as os.environ['KEY'] and replaced with an empty string if the environment variable does not exist.

Environment variable substitutions with default values

If you specify a substitution string like this:

{env:KEY:DEFAULTVALUE}

then the value will be retrieved as os.environ['KEY'] and replaced with DEFAULTVALUE if the environment variable does not exist.

If you specify a substitution string like this:

{env:KEY:}

then the value will be retrieved as os.environ['KEY'] and replaced with an empty string if the environment variable does not exist.

Substitutions can also be nested. In that case they are expanded starting from the innermost expression:

{env:KEY:{env:DEFAULT_OF_KEY}}

the above example is roughly equivalent to os.environ.get('KEY', os.environ['DEFAULT_OF_KEY'])

Interactive shell substitution

Added in version 3.4.0.

It’s possible to inject a config value only when tox is running in interactive shell (standard input):

{tty:ON_VALUE:OFF_VALUE}

The first value is the value to inject when the interactive terminal is available, the second value is the value to use when it’s not (optional). A good use case for this is e.g. passing in the --pdb flag for pytest.

Substitutions for positional arguments in commands

Added in version 1.0.

If you specify a substitution string like this:

{posargs:DEFAULTS}

then the value will be replaced with positional arguments as provided to the tox command:

tox arg1 arg2

In this instance, the positional argument portion will be replaced with arg1 arg2. If no positional arguments were specified, the value of DEFAULTS will be used instead. If DEFAULTS contains other substitution strings, such as {env:*}, they will be interpreted.,

Use a double -- if you also want to pass options to an underlying test command, for example:

tox run -e 3.14 -- --opt1 ARG1

will make the --opt1 ARG1 appear in all test commands where [] or {posargs} was specified. By default (see args_are_paths setting), tox rewrites each positional argument if it is a relative path and exists on the filesystem to become a path relative to the changedir setting.

Glob pattern substitution

Added in version 4.40.

You can expand glob/wildcard patterns to matching file paths:

{glob:PATTERN}

Matches are sorted and returned as a space-separated string. If no files match, the result is an empty string. You can provide a default value for when no files match:

{glob:PATTERN:DEFAULT}

Relative patterns are resolved against tox_root. Use ** for recursive matching across directories.

TOML

[env.A]
commands = [["twine", "upload", "{glob:dist/*.whl}"]]

Or using the TOML dict syntax (see pyproject.toml - native):

[env.A]
commands = [["twine", "upload", { replace = "glob", pattern = "dist/*.whl", extend = true }]]

INI

[testenv]
commands = twine upload {glob:dist/*.whl}
deps = {glob:requirements/*.txt:requirements.txt}

Other substitutions

• {:} – replaced with os.pathsep (: on POSIX, ; on Windows). Useful for building PATH-like values: set_env = PYTHONPATH={tox_root}/src{:}{tox_root}/lib.

• {/} – replaced with os.sep (/ on POSIX, \ on Windows). Useful for cross-platform paths: commands = pytest {tox_root}{/}tests{/}unit.

Substitution quick reference

The following table lists all available substitution variables. These can be used as inline replacement objects in TOML or via {name} in INI.

Substitution	Description
{tox_root} / {toxinidir}	The directory where the configuration file is located (project root).
{work_dir} / {toxworkdir}	The tox working directory (default: {tox_root}/.tox).
{temp_dir}	Temporary directory for tox (default: {work_dir}/.tmp).
{env_name} / {envname}	Name of the current tox environment (e.g. 3.13, lint).
{env_dir} / {envdir}	Directory of the current environment (default: {work_dir}/{env_name}).
{env_tmp_dir} / {envtmpdir}	Temporary directory for the current environment (default: {env_dir}/tmp).
{env_log_dir} / {envlogdir}	Log directory for the current environment (default: {env_dir}/log).
{env_bin_dir} / {envbindir}	Binary directory of the virtual environment (e.g. {env_dir}/bin).
{env_python} / {envpython}	Path to the Python executable in the virtual environment.
{env_site_packages_dir} / {envsitepackagesdir}	Pure-python site-packages directory (purelib) of the virtual environment.
{env_site_packages_dir_plat} / {envsitepackagesdir_plat}	Platform-specific site-packages directory (platlib) of the virtual environment.
{base_python} / {basepython}	The configured base Python interpreter.
{py_dot_ver}	Major.Minor version of the environment Python (e.g. 3.14).
{py_impl}	Python implementation name in lowercase (e.g. cpython, pypy).
{py_free_threaded}	True if the environment Python is a free-threaded build, else False.
{env:KEY} / {env:KEY:DEFAULT}	Value of environment variable KEY, with optional default.
{posargs} / {posargs:DEFAULTS}	Positional arguments passed after --, with optional defaults.
{tty:ON:OFF}	ON value when running in an interactive terminal, OFF otherwise.
{glob:PATTERN} / {glob:PATTERN:DEFAULT}	Expand glob pattern to matching file paths (space-separated, sorted). Relative paths resolve against tox_root.
{/}	OS path separator (/ or \).
{:}	OS path list separator (: or ;).
{[section]key} (INI only)	Value of key from [section] (cross-section reference).

For TOML-specific replacement syntax (replace = "ref", replace = "posargs", replace = "env", replace = "glob", replace = "if"), see pyproject.toml - native.