tox

tox is a tool to automatically manage virtual environments, usually for tests and builds. It is used to make sure that those run in well-defined environments and is smart about caching them to reduce churn. True to its roots as a test-running tool, tox is configured in test environments. 

tox itself is a PyPI package usually installed in a virtual environment. Because tox creates ad hoc temporary virtual environments for testing, the virtual environment tox is installed in can be common to many projects. A common pattern is to create a virtual environment dedicated to tox.

$ python -m venv ~/.venvs/tox

$ ~/.venvx/tox/bin/python -m pip install tox

$ alias tox=~/.venvs/tox/bin/tox

It uses a unique ini-based configuration format. This can make writing configurations difficult since remembering the subtleties of the file format can be hard. However, while hard to tap, there is a lot of power that can certainly configure tests and build clear and concise runs.

One thing that tox lacks is a notion of dependencies between build steps. This means that those are usually managed from the outside by running specific test runs after others and sharing artifacts somewhat ad hoc.

A tox environment more or less corresponds to a section in the configuration file. By default, tox uses the tox.ini file.

[testenv:some-name]

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Note that if the name of the environment contains pyNM (for example, py36), then tox defaults to using CPython, the standard Python implementation, version N.M (3.6, in this case) as the Python interpreter for that test environment.

tox also supports name-based environment guessing for more esoteric implementations of Python. For example, PyPy, an implementation of Python in Python, is supported with the name pypyNM.

If the name does not include one of the supported short names, or if there is a need to override the default, a basepython field in the section can be used to indicate a specific Python version. By default, tox looks for Python available in the path. However, if the plug-in tox-pyenv is installed in the virtual environment that tox itself is installed in, tox will query pyenv if it cannot find the right Python on the path.