tox-poetry-installer

A plugin for Tox that allows test environment dependencies to be installed using Poetry using its lockfile.

⚠️ This project is alpha software and should not be used in a production capacity ⚠️

Documentation

• Installation
• Getting Started
• Drawbacks
• Why would I use this? (What problems does this solve?)
• Developing
• Contributing
• Roadmap
  • Path to Beta
  • Path to Stable

Related resources:

• Poetry Python Project Manager
• Tox Automation Project
• Tox plugins

Installation

Add the plugin as a development dependency a project using Poetry:

#> poetry add tox-poetry-installer --dev

Confirm that the plugin is installed, and Tox recognizes it, by checking the Tox version:

#> poetry run tox --version
3.20.0 imported from .venv/lib64/python3.8/site-packages/tox/__init__.py
registered plugins:
    tox-poetry-installer-0.2.0 at .venv/lib64/python3.8/site-packages/tox_poetry_installer.py

If using in a CI/automation environment using Pip, ensure that the plugin is installed to the same environment as Tox:

# Calling the virtualenv's 'pip' binary directly will cause pip to install to that virtualenv
#> /path/to/my/automation/virtualenv/bin/pip install tox
#> /path/to/my/automation/virtualenv/bin/pip install tox-poetry-installer

Getting Started

After installing the plugin to a project, your Tox automation is already benefiting from the lockfile: when Tox installs your project package to one of your environments, all the dependencies of your project package will be installed using the versions specified in the lockfile. This happens automatically and requires no configuration changes.

But what about the rest of your Tox environment dependencies?

Let's use an example tox.ini file with two environments: the main testenv for running our tests and testenv:check for running some other helper checks:

[tox]
envlist = py37, static
isolated_build = true

[testenv]
description = Run the tests
deps =
    pytest == 5.3.0
commands = ...

[testenv:static]
description = Static formatting and quality enforcement
deps =
    pylint >=2.4.4,<2.6.0
    mypy == 0.770
    black --pre
commands = ...

Let's focus on the testenv:static environment first. In this project there's no reason that any of these tools should be a different version than what a human developer is using when installing from the lockfile. We can require that these dependencies be installed from the lockfile by adding the option require_locked_deps = true to the environment config, but this will cause an error:

[testenv:static]
description = Static formatting and quality enforcement
require_locked_deps = true
deps =
    pylint >=2.4.4,<2.6.0
    mypy == 0.770
    black --pre
commands = ...

Running Tox using this config gives us this error:

tox_poetry_installer.NoLockedDependencyError: Cannot install env dependency 'pylint >=2.4.4,<2.6.0': cannot specify a version for a locked env dependency

This is because we told the Tox environment to require all dependencies to be locked, but then also specified a specific version constraint for Pylint. With the require_locked_deps = true setting Tox expects all dependencies to take their version from the lockfile, so when it got conflicting information it errors. We can fix this by simply removing all version specifiers from the environment dependency list:

[testenv:static]
description = Static formatting and quality enforcement
require_locked_deps = true
deps =
    pylint
    mypy
    black
commands = ...

Now all the dependencies will be installed from the lockfile. If Poetry updates the lockfile with a new version then that updated version will be automatically installed when the Tox environment is recreated.

Now let's look at the testenv environment. Let's make the same changes to the testenv environment that we made to testenv:static above; remove the PyTest version and add require_locked_deps = true. Then imagine that we want to add a new (made up) tool the test environment called crash_override to the environment: we can add crash-override as a dependency of the test environment, but this will cause an error:

[testenv]
description = Run the tests
require_locked_deps = true
deps =
    pytest
    crash-override
commands = ...

Running Tox with this config gives us this error:

tox_poetry_installer.NoLockedDependencyError: Cannot install env dependency 'crash-override': no version of the env dependency was found in the current project's lockfile

This is because crash-override is not in our lockfile. Tox will refuse to install a dependency that isn't in the lockfile to an an environment that specifies require_locked_deps = true. We could fix this (if crash-override was a real package) by running poetry add crash-override --dev to add it to the lockfile.

Now let's combine dependencies from the lockfile ("locked dependencies") with dependencies that are specified inline in the environment configuration ("unlocked dependencies"). This isn't generally recommended of course, but it's a valid use case and fully supported by this plugin. Let's modify the testenv configuration to install PyTest from the lockfile but then install an older version of the Requests library.

The first thing to do is remove the require_locked_deps = true setting so that we can install Requests as an unlocked dependency. Then we can add our version of requests to the dependency list:

[testenv]
description = Run the tests
deps =
    pytest
    requests >=2.2.0,<2.10.0
commands = ...

However we still want pytest to be installed from the lockfile, so the final step is to tell Tox to install it from the lockfile by adding the suffix @poetry to it:

[testenv]
description = Run the tests
deps =
    pytest@poetry
    requests >=2.2.0,<2.10.0
commands = ...

Now when the testenv environment is created it will install PyTest (and all of its dependencies) from the lockfile while it will install Requests (and all of its dependencies) using the default Tox installation backend using Pip.

Drawbacks

• The following tox.ini configuration options have no effect on the dependencies installed from the Poetry lockfile (note that they will still affect unlocked dependencies):

  • install_command
  • pip_pre
  • downloadcache (deprecated)
  • download
  • indexserver
  • usedevelop

• The extras setting in tox.ini does not work. Optional dependencies of the project package will not be installed to Tox environments. (See the road map)

• The plugin currently depends on poetry<1.1.0. This can be a different version than Poetry being used for actual project development. (See the road map)

Why would I use this?

Introduction

The lockfile is a file generated by a package manager for a project that lists what dependencies are installed, the versions of those dependencies, and additional metadata that the package manager can use to recreate the local project environment. This allows developers to have confidence that a bug they are encountering that may be caused by one of their dependencies will be reproducible on another device. In addition, installing a project environment from a lockfile gives confidence that automated systems running tests or performing builds are using the same environment that a developer is.

Poetry is a project dependency manager for Python projects, and as such it creates and manages a lockfile so that its users can benefit from all the features described above. Tox is an automation tool that allows Python developers to run tests suites, perform builds, and automate tasks within self contained Python virtual environments. To make these environments useful, Tox supports installing per-environment dependencies. However, since these environments are created on the fly and Tox does not maintain a lockfile, there can be subtle differences between the dependencies a developer is using and the dependencies Tox uses.

This is where this plugin comes into play.

By default Tox uses Pip to install the PEP-508 compliant dependencies to a test environment. A more robust way to do this is to install dependencies directly from the lockfile so that the version installed to the Tox environment always matches the version Poetry specifies. This plugin overwrites the default Tox dependency installation behavior and replaces it with a Poetry-based installation using the dependency metadata from the lockfile.

The Problem

Environment dependencies for a Tox environment are usually done in PEP-508 format like the below example

# tox.ini
...

[testenv]
description = Some very cool tests
deps =
    foo == 1.2.3
    bar >=1.3,<2.0
    baz

...

Perhaps these dependencies are also useful during development, so they can be added to the Poetry environment using this command:

poetry add foo==1.2.3 bar>=1.3,<2.0 baz --dev

However there are three potential problems that could arise from each of these environment dependencies that would only appear in the Tox environment and not in the Poetry environment:

• The foo dependency is pinned to a specific version: let's imagine a security vulnerability is discovered in foo and the maintainers release version 1.2.4 to fix it. A developer can run poetry remove foo && poetry add foo^1.2 to get the new version, but the Tox environment is left unchanged. The developer environment specified by the lockfile is now patched against the vulnerability, but the Tox environment is not.

• The bar dependency specifies a dynamic range: a dynamic range allows a range of versions to be installed, but the lockfile will have an exact version specified so that the Poetry environment is reproducible; this allows versions to be updated with poetry update rather than with the remove and add used above. If the maintainers of bar release version 1.6.0 then the Tox environment will install it because it is valid for the specified version range, meanwhile the Poetry environment will continue to install the version from the lockfile until poetry update bar explicitly updates it. The development environment is now has a different version of bar than the Tox environment.

• The baz dependency is unpinned: unpinned dependencies are generally a bad idea, but here it can cause real problems. Poetry will interpret an unbound dependency using the carrot requirement but Pip (via Tox) will interpret it as a wildcard. If the latest version of baz is 1.0.0 then poetry add baz will result in a constraint of baz>=1.0.0,<2.0.0 while the Tox environment will have a constraint of baz==*. The Tox environment can now install an incompatible version of baz that cannot be easily caught using poetry update.

All of these problems can apply not only to the dependencies specified for a Tox environment, but also to the dependencies of those dependencies, and so on.

The Solution

This plugin requires that all dependencies specified for all Tox environments be unbound with no version constraint specified at all. This seems counter-intuitive given the problems outlined above, but what it allows the plugin to do is offload all version management to Poetry.

On initial inspection, the environment below appears less stable than the one presented above because it does not specify any versions for its dependencies:

# tox.ini
...

[testenv]
description = Some very cool tests
deps =
    foo
    bar
    baz

...

However with the tox-poetry-installer plugin installed this instructs Tox to install these dependencies using the Poetry lockfile so that the version installed to the Tox environment exactly matches the version Poetry is managing. When poetry update updates the lockfile with new dependency versions, Tox will automatically install these new versions without needing any changes to the configuration.

All dependencies are specified in one place (the lockfile) and dependency version management is handled by a tool dedicated to that task (Poetry).

Developing

This project requires Poetry-1.0+, see the installation instructions here.

# Clone the repository...
# ...over HTTPS
git clone https://github.com/enpaul/tox-poetry-installer.git
# ...over SSH
git clone git@github.com:enpaul/tox-poetry-installer.git

# Create a the local project virtual environment and install dependencies
cd tox-poetry-installer
poetry install

# Install pre-commit hooks
poetry run pre-commit install

# Run tests and static analysis
poetry run tox

Contributing

All project contributors and participants are expected to adhere to the Contributor Covenant Code of Conduct, Version 2.

• To report a bug, request a feature, or ask for assistance, please open an issue on the Github repository.
• To report a security concern or code of conduct violation, please contact the project author directly at ethan dot paul at enp dot one.
• To submit an update, please fork the repository and open a pull request.

Roadmap

This project is under active development and is classified as alpha software, not yet ready usage in production systems.

• Beta classification will be assigned when the initial feature set is finalized
• Stable classification will be assigned when the test suite covers an acceptable number of use cases

Path to Beta

• Verify that primary package dependencies (from the .package env) are installed correctly using the Poetry backend.
• Support the extras Tox configuration option
• Add per-environment Tox configuration option to fall back to default installation backend.
• Add detection of a changed lockfile to automatically trigger a rebuild of Tox environments when necessary.
• Add warnings when an unsupported Tox configuration option is detected while using the Poetry backend.
• Add trivial tests to ensure the project metadata is consistent between the pyproject.toml and the module constants.
• Update to use poetry-core Tox configuration option) and improve robustness of the Tox and Poetry module imports to avoid potentially breaking API changes in upstream packages.

Path to Stable

Everything in Beta plus...

• Add tests for each feature version of Tox between 2.3 and 3.20
• Add tests for Python-3.6, 3.7, and 3.8
• Add Github Actions based CI
• Add CI for CPython, PyPy, and Conda
• Add CI for Linux and Windows