.. _faq:

FAQ
===

Here you'll find answers to some frequently asked questions.

Breaking changes in tox 4
-------------------------

Version 4 of tox should be mostly backwards compatible with version 3.
Exceptions and guidance for migrating are given in the :ref:`upgrading doc <upgrading>`.

New features in tox 4
---------------------
Here is a non-exhaustive list of these.

- You can now build wheel(s) instead of a source distribution during the packaging phase by using the ``wheel`` setting
  for the :ref:`package` setting. If your package is a universal wheel you'll likely want to set the
  :ref:`wheel_build_env` to ``.pkg`` to avoid building a wheel for every Python version you target.
- Editable wheel support was added as defined by :pep:`660` via the :ref:`package` setting to ``editable``.
- We redesigned our CLI interface, we no longer try to squeeze everything under single command, instead now we have
  multiple sub-commands. For backwards compatibility if you do not specify a subcommand we'll assume you want the tox 3
  legacy interface (available under the legacy subcommand), for now the list of available commands are:

  .. code-block:: bash

    subcommands:
      tox command to execute (by default legacy)

      {run,r,run-parallel,p,depends,de,list,l,devenv,d,config,c,quickstart,q,exec,e,legacy,le}
        run (r)                   run environments
        run-parallel (p)          run environments in parallel
        depends (de)              visualize tox environment dependencies
        list (l)                  list environments
        devenv (d)                sets up a development environment at ENVDIR based on the tox configuration specified
        config (c)                show tox configuration
        quickstart (q)            Command line script to quickly create a tox config file for a Python project
        exec (e)                  execute an arbitrary command within a tox environment
        legacy (le)               legacy entry-point command

  The ``exec`` and ``depends`` are brand new features. Other subcommands are a more powerful versions of previously
  existing single flags (e.g. ``-av`` is now succeeded by the ``list`` subcommand). All subcommands have a one or two
  character shortcuts for less typing on the CLI (e.g. ``tox run`` can be abbreviated to ``tox r``). For more details
  see :ref:`cli`.
- Startup times should be improved because now we no longer eagerly load all configurations for all environments, but
  instead these are performed lazily when needed. Side-effect of this is that if you have an invalid configuration will
  not be picked up until you try to use it.
- We now discover your package dependency changes (either via :pep:`621` or otherwise via :pep:`517`
  ``prepare_metadata_for_build_wheel``/``build_wheel`` metadata). If new dependencies are added these will be installed
  on the next run. If a dependency is removed we'll recreate the entire environment. This works for ``requirements``
  files within the :ref:`deps`. This means that you should never need to use ``--recreate`` flag, tox should be smart
  enough to figure out when things change and automatically apply it.
- All tox defaults can now be changed via the user level config-file (see help message output for its location, can be
  changed via ``TOX_CONFIG_FILE`` environment variable).
- All tox defaults can now be changed via an environment variable: ``TOX_`` prefix followed by the settings key,
  e.g. ``TOX_PACKAGE=wheel``.
- Any configuration can be overridden via the CLI ``-x`` or ``--override`` flag, e.g.
  ``tox run -e py311 -x testenv:py311.package=editable`` would force the packaging of environment ``py311`` to be an
  editable install independent what's in the configuration file.
- :ref:`basepython` is now a list, the first successfully detected python will be used to generate python environment.
- We now have support for inline tox plugins via the ``toxfile.py`` at the root of your project. At a later time this
  will allow using Python only configuration, as seen with nox.
- You can now group tox environments via :ref:`labels` configuration, and you can invoke all tox environments within a
  label by using the ``-m label`` CLI flag (instead of the ``-e list_of_envs``).
- You can now invoke all tox environments within a given factor via the ``-f factor`` CLI flag.

.. _faq_custom_pypi_server:

Using a custom PyPI server
--------------------------
By default tox uses pip to install Python dependencies. Therefore to change the index server you should configure pip
directly. pip accepts environment variables as configuration flags, therefore the easiest way to do this is to set the
``PIP_INDEX_URL`` environment variable:

.. code-block:: ini

  set_env =
    PIP_INDEX_URL = https://tox.wiki/pypi/simple

It's considered a best practice to allow the user to change the index server rather than hard code it, allowing them to
use for example a local cache when they are offline. Therefore, a better form of this would be:

.. code-block:: ini

  set_env =
    PIP_INDEX_URL = {env:PIP_INDEX_URL:https://tox.wiki/pypi/simple}

Here we use an environment substitution to set the index URL if not set by the user, but otherwise default to our target
URI.

Using two PyPI servers
----------------------

When you want to use two PyPI index servers because not all dependencies are found in either of them use the
``PIP_EXTRA_INDEX_URL`` environment variable:

.. code-block:: ini

  set_env =
    PIP_INDEX_URL = {env:PIP_INDEX_URL:https://tox.wiki/pypi/simple-first}
    PIP_EXTRA_INDEX_URL = {env:PIP_EXTRA_INDEX_URL:https://tox.wiki/pypi/simple-second}

If the index server defined under ``PIP_INDEX_URL`` does not contain a package, pip will attempt to resolve it also from
the URI from ``PIP_EXTRA_INDEX_URL``.

.. warning::

  Using an extra PyPI index for installing private packages may cause security issues. For example, if ``package1`` is
  registered with the default PyPI index, pip will install ``package1`` from the default PyPI index, not from the extra
  one.

Using constraint files
----------------------
`Constraint files <https://pip.pypa.io/en/stable/user_guide/#constraints-files>`_ are a type of artifact, supported by
pip, that define not what requirements to install but instead what version constraints should be applied for the
otherwise specified requirements. The constraint file must always be specified together with the requirement(s) to
install. While creating a test environment tox will invoke pip multiple times, in separate phases:

1. If :ref:`deps` is specified, it will install a set of dependencies before installing the package.
2. If the target environment contains a package (the project does not have :ref:`package` ``skip`` or
   :ref:`skip_install` is ``true``), it will:

   1. install the dependencies of the package.
   2. install the package itself.

Starting in tox 4.4.0, when ``constrain_package_deps = true`` is set in the test environment,
``{env_dir}{/}constraints.txt`` will be generated during ``install_deps`` based on the package specifications listed
under ``deps``. These constraints are subsequently passed to pip during the ``install_package_deps`` stage, causing an
error to be raised when the package dependencies conflict with the test environment dependencies.

For stronger guarantees, set ``use_frozen_constraints = true`` in the test environment to generate the constraints file
based on the exact versions enumerated by the ``list_dependencies_command`` (``pip freeze``).  When using frozen
constraints, if the package deps are incompatible with any previously installed dependency, an error will be raised.
To use constraints with url, path, or "extras" (``.[tests]``) deps, then you must ``use_frozen_constraints``, as these
types of deps are not valid constraints as specified (see pypa/pip#8210).

Note constraint files are a subset of requirement files. Therefore, it's valid to pass a constraint file wherever you
can specify a requirement file.

.. _platform-specification:

Platform specification
----------------------

Assuming the following layout:

.. code-block:: shell

    tox.ini      # see below for content
    setup.py     # a classic distutils/setuptools setup.py file

and the following ``tox.ini`` content:

.. code-block:: ini

    [tox]
    min_version = 2.0  # platform specification support is available since version 2.0
    envlist = py{310,39}-{lin,mac,win}

    [testenv]
    # environment will be skipped if regular expression does not match against the sys.platform string
    platform = lin: linux
               mac: darwin
               win: win32

    # you can specify dependencies and their versions based on platform filtered environments
    deps = lin,mac: platformdirs==3
           win: platformdirs==2

    # upon tox invocation you will be greeted according to your platform
    commands=
       lin: python -c 'print("Hello, Linus!")'
       mac: python -c 'print("Hello, Tim!")'
       win: python -c 'print("Hello, Satya!")'

You can invoke ``tox`` in the directory where your ``tox.ini`` resides. ``tox`` creates two virtualenv environments
with the ``python3.10`` and ``python3.9`` interpreters, respectively, and will then run the specified command according
to platform you invoke ``tox`` at.

Ignoring the exit code of a given command
-----------------------------------------

When multiple commands are defined within the :ref:`commands` configuration field tox will run them sequentially until
one of them fails (by exiting with non zero exit code) or all of them are run. If you want to ignore the status code of
a given command add a ``-`` prefix to that line (similar syntax to how the GNU ``make`` handles this):

.. code-block:: ini

   [testenv]
   commands =
     - python -c 'import sys; sys.exit(1)'
     python --version

You can also choose to provide a ``!`` prefix instead to purposely invert the exit code, making the line fail if the
command returned exit code 0. Any other exit code is considered a success.

.. code-block:: ini

   [testenv]
   commands =
     ! python -c 'import sys; sys.exit(1)'
     python --version


Customizing virtual environment creation
----------------------------------------

By default tox uses the :pypi:`virtualenv` to create Python virtual environments to run your tools in. To change how tox
creates virtual environments you can set environment variables to customize virtualenv. For example, to provision a
given pip version in the virtual environment you can set ``VIRTUALENV_PIP`` or to enable system site packages use the
``VIRTUALENV_SYSTEM_SITE_PACKAGES``:


.. code-block:: ini


   [testenv]
   setenv =
     VIRTUALENV_PIP==22.1
     VIRTUALENV_SYSTEM_SITE_PACKAGES=true

Consult the :pypi:`virtualenv` project for supported values (any CLI flag for virtualenv, in all upper case, prefixed by
the ``VIRTUALENV_`` key).

Building documentation with Sphinx
----------------------------------

It's possible to orchestrate the projects documentation with tox. The advantage of this is that now generating the
documentation can be part of the CI, and whenever any validations/checks/operations fail while generating the
documentation you'll catch it within tox.

We don't recommend using the Make and Batch file generated by Sphinx, as this makes your documentation generation
platform specific. A better solution is to use tox to setup a documentation build environment and invoke sphinx inside
it. This solution is cross platform.

For example if the sphinx file structure is under the ``docs`` folder the following configuration will generate the
documentation under ``.tox/docs_out/index.html`` and print out a link to the generated documentation:

.. code-block:: ini

    [testenv:docs]
    description = build documentation
    basepython = python3.10
    deps =
      sphinx>=4
    commands =
      sphinx-build -d "{envtmpdir}{/}doctree" docs "{toxworkdir}{/}docs_out" --color -b html
      python -c 'print(r"documentation available under file://{toxworkdir}{/}docs_out{/}index.html")'

Note here we also require Python 3.10, allowing us to use f-strings within the sphinx ``conf.py``.

Building documentation with mkdocs
----------------------------------

It's possible to orchestrate the projects documentation with tox. The advantage of this is that now generating the
documentation can be part of the CI, and whenever any validations/checks/operations fail while generating the
documentation you'll catch it within tox.

It's best to define one environment to write/generate the documentation, and another to deploy it. Use the config
substitution logic to avoid duplication:

.. code-block:: ini

    [testenv:docs]
    description = Run a development server for working on documentation
    deps =
      mkdocs>=1.3
      mkdocs-material
    commands =
      mkdocs build --clean
      python -c 'print("###### Starting local server. Press Control+C to stop server ######")'
      mkdocs serve -a localhost:8080

    [testenv:docs-deploy]
    description = built fresh docs and deploy them
    deps = {[testenv:docs]deps}
    commands = mkdocs gh-deploy --clean

Understanding ``InvocationError`` exit codes
--------------------------------------------

When a command executed by tox fails, it always has a non-zero exit code and an ``InvocationError`` exception is raised:

.. code-block:: shell

    ERROR: InvocationError for command
           '<command defined in tox.ini>' (exited with code 1)

Generally always check the documentation for the command executed to understand what the code means. For example for
:pypi:`pytest` you'd read `here <https://docs.pytest.org/en/latest/reference/exit-codes.html#exit-codes>`_. On unix
systems, there are some rather `common exit codes <http://www.faqs.org/docs/abs/HTML/exitcodes.html>`_. This is why for
exit codes larger than 128, if a signal with number equal to ``<exit code> - 128`` is found in the :py:mod:`signal`
module, an additional hint is given:

.. code-block:: shell

    ERROR: InvocationError for command
           '<command>' (exited with code 139)
    Note: this might indicate a fatal error signal (139 - 128 = 11: SIGSEGV)


The signal numbers (e.g. 11 for a segmentation fault) can be found in the "Standard signals" section of the
`signal man page <https://man7.org/linux/man-pages/man7/signal.7.html>`_. Their meaning is described in
`POSIX signals <https://en.wikipedia.org/wiki/Signal_(IPC)#POSIX_signals>`_. Beware that programs may issue custom exit
codes with any value, so their documentation should be consulted.


Sometimes, no exit code is given at all. An example may be found in
:gh:`pytest-qt issue #170 <pytest-dev/pytest-qt/issues/170>`, where Qt was calling
`abort() <https://unix.org/version2/sample/abort.html>`_ instead of ``exit()``.

Access full logs
----------------

If you want to access the full logs you need to write ``-q`` and ``-v`` as individual tox arguments and avoid combining
them into a single one.

Running within a Docker container
---------------------------------

If you want to run tox within a Docker container you can use `31z4/tox <https://hub.docker.com/r/31z4/tox>`_.
This Docker image neatly packages tox along with common build dependencies (e.g., ``make``, ``gcc``, etc) and currently
`active CPython versions <https://devguide.python.org/versions/#status-of-python-versions>`_. See more details in
its `GitHub repository <https://github.com/31z4/tox-docker>`_.

The recommended way of using the image is to mount the directory that contains your tox configuration files and your
code as a volume. Assuming your project is within the current directory of the host, use the following command to run
tox without any flags:

.. code-block:: shell

    docker run -v `pwd`:/tests -it --rm 31z4/tox

Because an entry point of the image is ``tox``, you can easily pass subcommands and flags:

.. code-block:: shell

    docker run -v `pwd`:/tests -it --rm 31z4/tox run-parallel -e black,py311

Note, that the image is configured with a working directory at ``/tests``.

If you want to install additional Python versions/implementations or Ubuntu packages you can create a derivative image.
Just make sure you switch the user to ``root`` when needed and switch back to ``tox`` afterwards:

.. code-block:: Dockerfile

    FROM 31z4/tox

    USER root

    RUN set -eux; \
        apt-get update; \
        DEBIAN_FRONTEND=noninteractive \
        apt-get install -y --no-install-recommends \
            python3.12; \
        rm -rf /var/lib/apt/lists/*

    USER tox


.. _eol-version-support:

Testing end-of-life Python versions
-----------------------------------

``tox`` uses ``virtualenv`` under its hood for managing virtual environments.
`Virtualenv 20.22.0 <https://virtualenv.pypa.io/en/latest/changelog.html#v20-22-0-2023-04-19>`_ dropped support for all
Python versions smaller or equal to Python 3.6.

If you need to test against e.g. Python 2.7, 3.5 or 3.6, you need to add the following ``requires`` statement to your
``tox.ini`` configuration files.

.. code-block:: ini

    [tox]
    requires = virtualenv<20.22.0

In case you need to do this for many repositories, we recommend to use
`all-repos <https://github.com/asottile/all-repos>`_.

Support for Python 3.7 was dropped in `virtualenv 20.27.0 <https://virtualenv.pypa.io/en/latest/changelog.html#v20-27-0-2024-10-17>`_.
In order to test against Python 3.7, you can limit the version with ``requires = virtualenv<20.27.0`` instead.


Testing with Pytest
-------------------

Running ``pytest`` from ``tox`` can be configured like this:

.. code-block:: ini

    [tox]
    envlist = py311, py312

    [testenv]
    commands = pytest

If required, ``tox`` positional arguments can be passed through to ``pytest``:

.. code-block:: ini

    [testenv]
    commands = pytest {posargs}

When running ``tox`` in parallel mode (:ref:`tox-run-parallel-(p)`), care should be taken to ensure concurrent
``pytest`` invocations are fully isolated.

This can be achieved by setting ``pytest``'s base temporary directory to a unique temporary directory for each virtual
environment as provided by ``tox``:

.. code-block:: ini

    [testenv]
    commands = pytest --basetemp="{env_tmp_dir}"

Setting the ``pytest`` ``--basetemp`` argument also causes all temporary ``pytest`` files to be deleted immediately
after the tests are completed. To restore the default ``pytest`` behavior to retain temporary files for the most recent
``pytest`` invocations, the system's temporary directory location could be configured like this instead:

.. code-block:: ini

    [tox]
    set_env =
      TEMP = {env_tmp_dir}

    [testenv]
    commands = pytest
