pypi2nix - generate Nix expressions for Python packages

Contents

• 1. Installation
  • System Requirements
  • Ad hoc Installation (Simple)
  • Declarative Installation (Advanced)
• 2. Usage
  • What is being generated
  • Building generated packages
  • Using generated packages
  • Command line options
• 3. When it doesn't work
  • Non-Python/system dependencies
  • Additional environment variables
  • Using requirements_override.nix
  • Shared overrides
• 4. Advanced Use
  • Creating default.nix for your project
  • Convert generated requirements.nix into nixpkgs overlay

See https://pypi2nix.readthedocs.io/en/latest/ for documentation.

pypi2nix is a command line tool that generates Nix expressions from different requirements.txt. This is useful for:

• Building a Nix derivation for a program written in Python as part of packaging it.

• Setting up a development environment to hack on a program written in Python.

  The only way we can fix bugs with pypi2nix is if you report them. Please create an issue if you discover problems.

pypi2nix will (until further notice) only work with latest unstable channel. This is due to ongoing changes in Python infrastructure happening in Nixpkgs.

The Nixpkgs manual section about Python makes good reading if you haven't seen it already.

1. Installation

pypi2nix is part of nixpkgs. If you just want to use pypi2nix on your system, it is recommended that you install it via the regular means, e.g. nix-env -iA nixos.pypi2nix on NixOS or nix-env -iA nixpkgs.pypi2nix on other systems utilizing nix.

System Requirements

Make sure Nix is installed:

% curl https://nixos.org/nix/install | sh

Currently pypi2nix is only tested against linux systems. Supported nixpkgs channels are nixos-19.09 and nixos-unstable. Due to the nature of nixos-unstable the occasional breakage of pypi2nix is to be expected. We try to provide fixes in that regard in a timely manner.

Ad hoc Installation (Simple)

For just installing the package with a command, use nix-env:

nix-env -if https://github.com/nix-community/pypi2nix/tarball/master

Declarative Installation (Advanced)

If you prefer to explicitly declare each installed package in your Nix(OS) or project configuration, you can do the following:

First, import the package from its default.nix by fetching the whole git repository with pkgs.fetchgit. Afterwards you can just add the imported attribute the list of installed software.

Below you find an example for NixOS' configuration.nix. Other methods like home-manager work similar:

let
  pypi2nix = import (pkgs.fetchgit {
    url = "https://github.com/nix-community/pypi2nix";
    # adjust rev and sha256 to desired version
    rev = "v2.0.1";
    sha256 = "sha256:0mxh3x8bck3axdfi9vh9mz1m3zvmzqkcgy6gxp8f9hhs6qg5146y";
  }) {};
in
  environment.systemPackages = [
    # your other packages
    pypi2nix
  ];

2. Usage

The easiest way to generate a Nix expressions is to invoke:

% pypi2nix -e packageA -e packageB==0.1

If you also have a requirements.txt file for your Python project you can use the -r option.

% pypi2nix -e packageA -e packageB==0.1 \
    -r requirements.txt -r requirements-dev.txt

What is being generated

Option -V tells pypi2nix which python version to be used. To see which Python versions are available consult pypi2nix --help.

Once Nix expressions are generated you should be able to see 3 new files:

• requirements_frozen.txt - full frozen set for your for your pypi2nix call. This is the output you would expect from pip freeze.
• requirements.nix is a file which contains a nix expression to for the package set that was built.
• requirements_override.nix - this is an empty file which lets you override generated nix expressions.

Building generated packages

Build one package:

% nix build -f requirements.nix packages.empy

Build all packages:

% nix build -f requirements.nix packages

Build python interpreter with all packages loaded:

% nix build -f requirements.nix interpreter
% ./result/bin/python -c "import empy"

Enter development environment:

% nix run -f requirements.nix interpreter
[user@localhost:~/dev/nixos/pypi2nix) % python -c "import empy"

Using generated packages

If you are working on a project where its dependencies are defined in requirements.txt then you can create a default.nix and add generated packages as buildInputs, as demonstrated here:

{}:
let
  python = import ./requirements.nix { inherit pkgs; };
in python.mkDerivation {
  name = "ProjectA-1.0.0";
  src = ./.;
  buildInputs = [
    python.packages."coverage"
    python.packages."flake8"
    python.packages."mock"
    python.packages."pytest"
    python.packages."pytest-asyncio"
    python.packages."pytest-cov"
    python.packages."pytest-mock"
    python.packages."pytest-xdist"
    python.packages."virtualenv"
  ];
  propagatedBuildInputs = [
    python.packages."aiohttp"
    python.packages."arrow"
    python.packages."defusedxml"
    python.packages."frozendict"
    python.packages."jsonschema"
    python.packages."taskcluster"
    python.packages."virtualenv"
  ];
  ...
}

As you can see you can access all packages via python.packages."<name>". If you want to depend on all packages you can even do:

propagatedBuildInputs = builtins.attrValues python.packages;

Command line options

-v

Increase amount and detail of information output to the user. Verbosity levels are ERROR, WARNING, INFO and DEBUG in that order. The default verbosity is INFO.

-q

Reduce amount and detail of information output to the user. See -v for more information.

-I/--nix-path TEXT

Add entries to the NIX_PATH environment variable similarly to how -I works with nix executables like nix-build. This can be useful for generating package sets based on a different nixpkgs version than the one used one the local system.

--nix-shell PATH

Path to an alternative version of the nix-shell command. The default is the first executable that will be found in the current PATH of the system.

--version

Show the current version of pypi2nix

--basename TEXT

This option determins the name the produced files. So with --basename environment you would get the files environment.nix, environment_frozen.nix and environment_override.nix.

--extra-build-inputs/-E TEXT

Extra build inputs that the required python packages need to run, e.g. libffi or libgl. In that case you would provide -E "libffi libgl". These nix packages will be available in the build environment for the wheels.

--emit-extra-build-inputs/--no-emit-extra-build-inputs

These options let you control if external build dependencies specified via -E will end up in the generated nix package set. Please note that if you select this option, your overrides need to make sure that python packages find their respective external dependencies.

--extra-env/-N TEXT

Extra environment variables that will be passed to the build environment. Note that you can use nix expressions in this string, e.g. -N 'BERKELEYDB_DIR=${pkgs.db.dev}'.

--enable-tests/-T

Specify this flag if you want to enable the check phase of all packages in the generated nix expression. Please note that this feature is highly exprimental and will probably not work for your use case.

--python-version/-V

Specify the python version you want the requirement set to be built with. The default is 3 which translates to the python3 derivation of nixpkgs.

--requirements/-r FILE

Specify a requirements file, similar as you would with pip. pypi2nix tries to be fully compatible with the file format of pip.

--editable/-e TEXT

This option allows you to specify individual requirements that get added to the requirement set, e.g. pypi2nix -e attrs, pypi2nix -e $HOME/src/myproject#egg=myproject or pypi2nix -e .#egg=myegg.

--setup-requires/-s TEXT

Allows you to specify python packages that need to be present in the build environment of other packages, a good example of this would be setuptools-scm. Note that pypi2nix tries to detect these dependencies on its own. You only need to specify this flag in cases where a package author or maintainer forgot to mention build time dependencies in their setup or neither setup.cfg nor pyproject.toml is used.

--overrides/-O URL

Allows you to specify additional overrides that conform to the general structure of requirements_override.nix. We support regular URLs with http and https scheme and also git. An example for using https would be pypi2nix -O https://myoverrides.test/overrides.nix. Reusing an overlay from a git repository would be done like so: pypi2nix -O git+https://github.com/nix-community/pypi2nix.git&path=requirement_override.nix. Please keep in mind that these overrides are incorporated in a nix expression with a precalculated hash value. So if the file changes upstream your generated package can not be built anymore.

--default-overrides/--no-default-overrides

Pull in overrides from https://github.com/nix-community/pypi2nix-overrides. This feature is enabled by default.

--wheels-cache/-W TEXT

A location where prebuilt wheels can be found. This option will ultimately be passed to pip --find-links. Only point to wheels that are built through pypi2nix on your own or a very similar system.

--build-directory TEXT

Warning A bug in pypi2nix currently prevents some packages from being built with this option set. It is recommended to not use this flag.

The directory where pypi2nix would build the python environment to generate the desired nix expression. If not specified, the build directory will be temporary and is deleted before the program exits.

--dependency-graph-output PATH

Output the dependency information of processed packages to the specified path. The output format is yaml.

3. When it doesn't work

I hope nobody is expecting pypi2nix to do always a perfect job. In Python packaging, there are just too many different cases that we will never be able to cover. What pypi2nix tries to do is to get you very close.

Sometimes pypi2nix fails entirely. If this happens, open a bug -- it's almost always a bug in pypi2nix. However, sometimes pypi2nix succeeds but the resulting requirements.nix file fails during the building of your Python package. Depending on what the problem is, this section may be helpful.

Non-Python/system dependencies

Quite a few Python packages require non-Python dependencies to be present at build time. These packages will fail to build with error messages about not being able to find foo.h or some fooconfig file. To work around this, pypi2nix has -E options which can be used to include extra non-Python dependencies.

For example, psycopg2 requires pg_config binary to be present at installation time:

% pypi2nix -v -V 2.7 -e psycopg2 -E postgresql

lxml requires libxml2 and libxslt system package:

% pypi2nix -v -V 2.7 -e lxml -E libxml2 -E libxslt

Additional environment variables

Some packages expect additional environment variables to be set:

% pypi2nix -v -V 2.7 -e bsddb3 -N 'BERKELEYDB_DIR=${pkgs.db.dev}'

Using requirements_override.nix

Some other failures might be caused because the derivation that pypi2nix wrote was incomplete. A very common situation is that pypi2nix didn't include all the dependencies of some package. As an example, execnet depends on setuptools-scm, but pypi2nix may not detect this.

When this happens, Nix will fail to build execnet, perhaps with an error message from distutils/setuptools complaining that it can't find a distribution for setuptools-scm. What's happening here is that normally execnet would fetch setuptools-scm from PyPI, but Nix disables network access to guarantee reproducability. So when you build execnet, it fails to find setuptools-scm.

For these situations, pypi2nix provides a requirements_override.nix file, which lets you override anything that it generated. You can even add new packages to the dependency set this way.

As an example, let's add setuptools-scm as a build-time dependency of execnet. Here's the requirements_override.nix:

{ pkgs, python }:

self: super: {

  "execnet" = python.overrideDerivation super."execnet" (old: {
    buildInputs = old.buildInputs ++ [ self."setuptools-scm" ];
  });

}

In a similar way, you can add or remove any Python package.

Shared overrides

In addition to the empty autogenerated requirements_overrides.nix file, you can include pre-existing overrides files. These overrides will be included the same way as your requirements_overrides.nix.

The pypi2nix author also maintains a set of "default" overrides at https://github.com/nix-community/pypi2nix-overrides/blob/master/overrides.nix -- you can include these by using the --default-overrides argument to pypi2nix. These overrides are designed in such a way that they only override dependencies that were already present in your requirements.nix.

You can also include an overrides file using the -O command line argument. pypi2nix can fetch these overrides from a local file or over certain common protocols.

http and https

pypi2nix --overrides https://raw.githubusercontent.com/nix-community/pypi2nix-overrides/master/overrides.nix

Note that the generated Nix expression will check if contents of the overrides file differs from when the Nix expression was built, and fail if this was the case (or the file does not exist anymore).

Local files

pypi2nix --override ../some/relative/path --override /some/absolute/path

Git repositories

pypi2nix --override git+https://github.com/nix-community/pypi2nix.git#path=overrides.nix

If you want to import a file from a specific git repository you have to prefix its URL with git+, quite similar to how you would do in a requirements.txt file for pip.

4. Advanced Use

Creating default.nix for your project

Nothing speaks better than an example:

{ }:

let
  pkgs = import <nixpkgs> {};
  python = import ./requirements.nix { inherit pkgs; };
in python.mkDerivation {
  name = "projectA-1.0.0";
  src = ./.;
  buildInputs = [
    python.packages."coverage"
    python.packages."flake8"
    python.packages."mock"
    python.packages."pytest"
    python.packages."pytest-asyncio"
    python.packages."pytest-cov"
    python.packages."pytest-mock"
    python.packages."pytest-xdist"
  ];
  propagatedBuildInputs = [
    python.packages."aiohttp"
    python.packages."arrow"
    python.packages."defusedxml"
    python.packages."frozendict"
    python.packages."jsonschema"
  ];
  checkPhase = ''
    export NO_TESTS_OVER_WIRE=1
    export PYTHONDONTWRITEBYTECODE=1

    flake8 src/
    py.test --cov=src -cov-report term-missing
    coverage html
  '';
}

Important to know here is that you instantiate all generated packages as python = import ./requirements.nix { inherit pkgs; }; which gives you a Python environment with all the packages generated by pypi2nix as well as some common utilities.

To create a package you use python.mkDerivation which works like the pythonPackages.buildPythonPackage function in nixpkgs. All generated packages are available as one attribute set under python.packages.

One of future goals of pypi2nix project is to also improve the UX of our Python tooling in nixpkgs. While this is very hard to do within nixpkgs it is almost trivial to experiment with this outside nixpkgs.

Convert generated requirements.nix into nixpkgs overlay

A working example is worth 1000 words.

overlay.nix:

self: super:
{
  customPython =
      (import ./requirements.nix { pkgs = self; });
}

shell.nix:

with (import <nixpkgs> { overlays = [ (import ./overlay.nix) ]; });
customPython.interpreter