Poetry: Dependency Management for Python

Poetry helps you declare, manage and install dependencies of Python projects, ensuring you have the right stack everywhere.

It supports Python 3.7+.

The complete documentation is available on the official website.

Installation

Poetry provides a custom installer that will install poetry isolated from the rest of your system.

osx / linux / bashonwindows install instructions

curl -sSL https://install.python-poetry.org | python3 -

windows powershell install instructions

(Invoke-WebRequest -Uri https://install.python-poetry.org -UseBasicParsing).Content | python -

Warning: The previous get-poetry.py installer is now deprecated, if you are currently using it you should migrate to the new, supported, install-poetry.py installer.

The installer installs the poetry tool to Poetry's bin directory. This location depends on your system:

$HOME/.local/bin for Unix
%APPDATA%\Python\Scripts on Windows

If this directory is not on your PATH, you will need to add it manually if you want to invoke Poetry with simply poetry.

Alternatively, you can use the full path to poetry to use it.

Once Poetry is installed you can execute the following:

poetry --version

If you see something like Poetry (version 1.2.0) then you are ready to use Poetry. If you decide Poetry isn't your thing, you can completely remove it from your system by running the installer again with the --uninstall option or by setting the POETRY_UNINSTALL environment variable before executing the installer.

curl -sSL https://install.python-poetry.org | python3 - --uninstall
curl -sSL https://install.python-poetry.org | POETRY_UNINSTALL=1 python3 -

By default, Poetry is installed into the user's platform-specific home directory. If you wish to change this, you may define the POETRY_HOME environment variable:

curl -sSL https://install.python-poetry.org | POETRY_HOME=/etc/poetry python3 -

If you want to install prerelease versions, you can do so by passing --preview option to install-poetry.py or by using the POETRY_PREVIEW environment variable:

curl -sSL https://install.python-poetry.org | python3 - --preview
curl -sSL https://install.python-poetry.org | POETRY_PREVIEW=1 python3 -

Similarly, if you want to install a specific version, you can use --version option or the POETRY_VERSION environment variable:

curl -sSL https://install.python-poetry.org | python3 - --version 1.2.0
curl -sSL https://install.python-poetry.org | POETRY_VERSION=1.2.0 python3 -

You can also install Poetry for a git repository by using the --git option:

curl -sSL https://install.python-poetry.org | python3 - --git https://github.com/python-poetry/poetry.git@master

Note that the installer does not support Python < 3.7.

Updating `poetry`

Updating poetry to the latest stable version is as simple as calling the self update command.

Warning: Poetry versions installed using the now deprecated get-poetry.py installer will not be able to use this command to update to 1.2 releases or later. Migrate to using the install-poetry.py installer or pipx.

poetry self update

If you want to install prerelease versions, you can use the --preview option.

poetry self update --preview

And finally, if you want to install a specific version you can pass it as an argument to self update.

poetry self update 1.2.0

Enable tab completion for Bash, Fish, or Zsh

poetry supports generating completion scripts for Bash, Fish, and Zsh. See poetry help completions for full details, but the gist is as simple as using one of the following:

# Bash
poetry completions bash > /etc/bash_completion.d/poetry.bash-completion

# Fish
poetry completions fish > ~/.config/fish/completions/poetry.fish

# Zsh
poetry completions zsh > ~/.zfunc/_poetry

# Zsh (Oh-My-Zsh)
mkdir $ZSH_CUSTOM/plugins/poetry
poetry completions zsh > $ZSH_CUSTOM/plugins/poetry/_poetry
rm ~/.zcompdump*
# add `poetry` in the `plugins` list (https://github.com/ohmyzsh/ohmyzsh#enabling-plugins)

# Zsh (prezto)
poetry completions zsh > ~/.zprezto/modules/completion/external/src/_poetry

Note: you may need to restart your shell in order for the changes to take effect.

For zsh, you must then add the following line in your ~/.zshrc before compinit (not for homebrew setup):

fpath+=~/.zfunc

Introduction

poetry is a tool to handle dependency installation as well as building and packaging of Python packages. It only needs one file to do all of that: the new, standardized pyproject.toml.

In other words, poetry uses pyproject.toml to replace setup.py, requirements.txt, setup.cfg, MANIFEST.in and the newly added Pipfile.

[tool.poetry]
name = "my-package"
version = "0.1.0"
description = "The description of the package"

license = "MIT"

authors = [
    "Sébastien Eustace <sebastien@eustace.io>"
]

readme = "README.md"  # Markdown files are supported

repository = "https://github.com/python-poetry/poetry"
homepage = "https://github.com/python-poetry/poetry"

keywords = ["packaging", "poetry"]

[tool.poetry.dependencies]
python = "~2.7 || ^3.2"  # Compatible python versions must be declared here
toml = "^0.9"
# Dependencies with extras
requests = { version = "^2.13", extras = [ "security" ] }
# Python specific dependencies with prereleases allowed
pathlib2 = { version = "^2.2", python = "~2.7", allow-prereleases = true }
# Git dependencies
cleo = { git = "https://github.com/sdispater/cleo.git", branch = "master" }

# Optional dependencies (extras)
pendulum = { version = "^1.4", optional = true }

[tool.poetry.dev-dependencies]
pytest = "^3.0"
pytest-cov = "^2.4"

[tool.poetry.scripts]
my-script = "my_package:main"

There are some things we can notice here:

It will try to enforce semantic versioning as the best practice in version naming.
You can specify the readme, included and excluded files: no more MANIFEST.in. poetry will also use VCS ignore files (like .gitignore) to populate the exclude section.
Keywords can be specified and will act as tags on the packaging site.
The dependencies sections support caret, tilde, wildcard, inequality and multiple requirements.
You must specify the python versions for which your package is compatible.

poetry will also detect if you are inside a virtualenv and install the packages accordingly. So, poetry can be installed globally and used everywhere.

poetry also comes with a full fledged dependency resolution library.

Why?

Packaging systems and dependency management in Python are rather convoluted and hard to understand for newcomers. Even for seasoned developers it might be cumbersome at times to create all files needed in a Python project: setup.py, requirements.txt, setup.cfg, MANIFEST.in and the newly added Pipfile.

So I wanted a tool that would limit everything to a single configuration file to do: dependency management, packaging and publishing.

It takes inspiration in tools that exist in other languages, like composer (PHP) or cargo (Rust).

And, finally, I started poetry to bring another exhaustive dependency resolver to the Python community apart from Conda's.

What about Pipenv?

In short: I do not like the CLI it provides, or some of the decisions made, and I think we can make a better and more intuitive one. Here are a few things that I don't like.

Dependency resolution

The dependency resolution is erratic and will fail even if there is a solution. Let's take an example:

pipenv install oslo.utils==1.4.0

will fail with this error:

Could not find a version that matches pbr!=0.7,!=2.1.0,<1.0,>=0.6,>=2.0.0

while Poetry will get you the right set of packages:

poetry add oslo.utils=1.4.0

results in :

  - Installing pytz (2018.3)
  - Installing netifaces (0.10.6)
  - Installing netaddr (0.7.19)
  - Installing oslo.i18n (2.1.0)
  - Installing iso8601 (0.1.12)
  - Installing six (1.11.0)
  - Installing babel (2.5.3)
  - Installing pbr (0.11.1)
  - Installing oslo.utils (1.4.0)

This is possible thanks to the efficient dependency resolver at the heart of Poetry.

Here is a breakdown of what exactly happens here:

oslo.utils (1.4.0) depends on:

pbr (>=0.6,!=0.7,<1.0)
Babel (>=1.3)
six (>=1.9.0)
iso8601 (>=0.1.9)
oslo.i18n (>=1.3.0)
netaddr (>=0.7.12)
netifaces (>=0.10.4)

What interests us is pbr (>=0.6,!=0.7,<1.0).

At this point, poetry will choose pbr==0.11.1 which is the latest version that matches the constraint.

Next it will try to select oslo.i18n==3.20.0 which is the latest version that matches oslo.i18n (>=1.3.0).

However this version requires pbr (!=2.1.0,>=2.0.0) which is incompatible with pbr==0.11.1, so poetry will try to find a version of oslo.i18n that satisfies pbr (>=0.6,!=0.7,<1.0).

By analyzing the releases of oslo.i18n, it will find oslo.i18n==2.1.0 which requires pbr (>=0.11,<2.0). At this point the rest of the resolution is straightforward since there is no more conflict.

swyuuki/poetry