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Use current environment instead of virtualenv for tox testenvs

Project description

The tox-current-env plugin adds these options:

tox --current-env

Runs the tox testenv’s commands in the current Python environment (that is, the environment where tox is invoked from and installed in). Unlike regular tox invocation, this installs no dependencies declared in deps. An attempt to run this with a Python version that doesn’t match will fail (if tox is invoked from an Python 3.7 environment, any non 3.7 testenv will fail).

tox --print-deps-to=FILE

Instead of running any commands, simply prints the declared dependencies in deps to the specified FILE. This is useful for preparing the current environment for tox --current-env. Use - for FILE to print to standard output.

tox --print-extras-to=FILE

Instead of running any commands, simply prints the names of the declared extras in extras to the specified FILE. This is useful for preparing the current environment for tox --current-env. Use - for FILE to print to standard output.

It is possible to use the two printing options together, as long as the FILE is different.

Invoking tox without any of the above options should behave as regular tox invocation without this plugin. Any deviation from this behavior is considered a bug.

The plugin disables tox’s way of providing a testing environment, but assumes that you supply one in some other way. Always run tox with this plugin in a fresh isolated environment, such as Python virtualenv, Linux container or chroot. See other caveats below.


Obviously, tox was created to run tests in isolated Python virtual environments. The --current-env flag totally defeats the purpose of tox. Why would anybody do that, you might ask?

Well, it turns out that tox became too popular and gained another purpose.

The Python ecosystem now has formal specifications for many pieces of package metadata like versions or dependencies. However, there is no standardization yet for declaring test dependencies or running tests. The most popular de-facto standard for that today is tox, and we expect a future standard to evolve from tox.ini. This plugin lets us use tox’s dependency lists and testing commands for environments other than Python venvs.

We hope this plugin will enable community best practices around tox configuration to grow to better accomodate non-virtualenv environments in general – for example, Linux distros, Conda, or containers.

Specifically, this plugin was created for Fedora’s needs. When we package Python software as RPM packages, we try to run the project’s test suite during package build. However, we need to test if the software works integrated into Fedora, not with packages downloaded from PyPI into a fresh environment. By running the tests in current environment, we can achieve that.

If you are interested in the RPM packaging part of this, see Fedora’s %pyproject RPM macros.


Install this via pip:

$ python -m pip install tox-current-env

Or install the development version by cloning the git repository and pip-installing locally:

$ git clone
$ cd tox-current-env
$ python -m pip install -e .


When the plugin is installed, use tox with --current-env, --print-deps-to or --print-extras-to and all the other options as usual. Assuming your tox is installed on Python 3.7:

$ tox -e py37 --current-env
py37 create: /home/pythonista/projects/holy-grail/tests/.tox/py37
py37 installed: ...list of packages from the current environment...
py37 run-test-pre: PYTHONHASHSEED='3333333333'
py37 run-test: commands...
...runs tests in current environment's Python...
___________________________________ summary ____________________________________
  py37: commands succeeded
  congratulations :)

Attempting to run the py36 environment’s test will fail:

$ tox -e py36 --current-env
py36 create: /home/pythonista/projects/holy-grail/tests/.tox/py36
ERROR: InterpreterMismatch: tox_current_env: interpreter versions do not match:
    in current env: (3, 7, 4, 'final', 0)
    requested: (3, 6, 9, 'final', 0)
___________________________________ summary ____________________________________
ERROR:  py36: InterpreterMismatch: tox_current_env: interpreter versions do not match:
    in current env: (3, 7, 4, 'final', 0)
    requested: (3, 6, 9, 'final', 0)

To get list of test dependencies, run:

$ tox -e py37 --print-deps-to -
py37 create: /home/pythonista/projects/holy-grail/tests/.tox/py37
py37 installed: can see almost anything here...
py37 run-test-pre: PYTHONHASHSEED='3333333333'
___________________________________ summary ____________________________________
  py37: commands succeeded
  congratulations :)

To get a list of names of extras, run:

$ tox -e py37 --print-extras-to -
py37 create: /home/pythonista/projects/holy-grail/tests/.tox/py37
py37 installed: can see almost anything here...
py37 run-test-pre: PYTHONHASHSEED='3333333333'
___________________________________ summary ____________________________________
  py37: commands succeeded
  congratulations :)

Caveats, warnings and limitations

tox 4

The plugin is available also for tox 4. Differences in behavior between tox 3 and 4 are these:

  • --recreate is no longer needed when you switch from the plugin back to standard tox. Tox detects it and handles the recreation automatically.

  • The plugin does not check the requested Python version nor the environment name. If you let it run for multiple environments they’ll all use the same Python.

  • Deprecated --print-deps-only option is no longer available.

Use an isolated environment

Running (especially third party software’s) tests in your system Python environment is dangerous. Always use this plugin in an isolated environment, such as a Linux container, virtual machine or chroot. You have been warned.

Do not rely on virtualenv details

In order to support the python command in the commands section, the current environment invocation of tox creates a fake virtual environment that just has a symbolic link to the Python executable. The link is named python even if the real interpreter’s name is different (such as python3.7 or pypy). Any other commands are not linked anywhere and it is the users’ responsibility to make sure such commands are in $PATH and use the correct Python. This can lead to slightly different results of tests than invoking them directly, especially if you have assumptions about sys.executable or other commands in your tests.

As a specific example, tests should invoke python -m pytest rather than assuming the pytest command is present and uses the correct version of Python.

Don’t mix current-env and regular tox runs

tox caches the virtualenvs it creates, and doesn’t distinguish between regular virtualenvs and --current-env. Don’t mix tox --current-env, tox --print-deps-to or tox --print-extras-to runs and regular tox runs (without the flags provided by this plugin). If you ever need to do this, use tox’s --recreate/-r flag to clear the cache.

The plugin should abort with a meaningful error message if this is detected, but in some corner cases (such as running tox --current-env, forcefully killing it before it finished, uninstalling the plugin, and running tox), you will get undefined results (such as installing packages from PyPI into your current environment).

Environment variables are passed by default

Since 0.0.9, all Shell environment variables are passed by default when using this plugin. The passenv tox configuration is set to *. Read the documentation for more information about passing environment variables to tox.

tox provisioning

The tested projects can specify the minimal tox version and/or additional requires needed in the environment where tox is installed. Normally, tox uses provisioning when such requirements are not met. It creates a virtual environment, installs (a newer version of) tox and the missing packages into that environment and proxies all tox invocations trough that. Unfortunately, this is undesired for tox-current-env.

  1. It is possible to invoke tox with --no-provision to prevent the provision entirely. When requirements are missing, tox fails instead of provisioning. If a path is passed as a value for --no-provision, the requirements will be serialized to the file, as JSON.

  2. The requires, if specified, are included in the results of tox --print-deps-to. This only works when they are installed (otherwise see the first point).

  3. The minimal tox version, if specified, is included in the results of tox --print-deps-to. This only works when the version requirement is satisfied (otherwise see the first point).

The recommend way to handle this is:

  1. Run tox --no-provision provision.json --print-deps-to=... or similar.

  2. If the command fails, install requirements from provision.json to the current environment and try again.

Note that the specified requirements are likely to contain other tox plugins and many of them might interfere with tox-current-env in an undesired way. If that is the case, the recommended way is to patch/sed such undesired plugins out of the configuration before running tox.

Other limitations and known bugs

The installed: line in the output of tox --print-deps-to/tox --print-extras-to shows irrelevant output (based on the content of the real or faked virtual environment).

Regardless of any Python flags used in the shebang of tox, the tests are invoked with sys.executable without any added flags (unless explicitly invoked with them in the commands section).

The current environment’s Python is tested for the major and minor version only. Different interpreters with the same Python version (such as CPython and PyPy) are treated as equal.

Only Linux is supported, with special emphasis on Fedora. This plugin might work on other Unix-like systems, but does not work on Microsoft Windows.

This is alpha quality software. Use it at your on your own risk. Pull requests with improvements are welcome.

Development, issues, support

The development happens on GitHub, at the fedora-python/tox-current-env repository. You can use the issue tracker there for any discussion or send Pull Requests.


In order to run the tests, you’ll need tox and Python from 3.6 to 3.10 installed. The integration tests assume all of them are available. On Fedora, you just need to dnf install tox.

Run tox to invoke the tests.

Running tests of this plugin with its own --current-env flag will most likely blow up.


The tox-current-env project is licensed under the so-called MIT license, full text available in the LICENSE file.

Code of Conduct

The tox-current-env project follows the Fedora’s Code of Conduct.

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