Generate Nix expressions for Python packages
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README.rst

pypi2nix - generate Nix expressions for Python packages

pypi2nix is a command line tool that generates Nix expressions from different python specific sources (requirements.txt, buildout.cfg, ...).

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.

1. Installation

Make sure Nix is installed.:

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

Next clone pypi2nix repository.:

% git clone https://github.com/garbas/pypi2nix

And now install it using nix-env command.:

% cd pypi2nix
% nix-env -f release.nix -iA build."x86_64-linux"

2. Usage

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

% pypi2nix -V "3.5" -e packageA -e packageB==0.1

If you also have requirements.txt file for you python project you can use -r option.:

% pypi2nix -V "3.5" -e packageA -e packageB==0.1 \
    -r requirements.txt -r requirements-dev.txt

If your project relies on zc.buildout you can give -b option a try.:

% pypi2nix -V "2.7" -b buildout.cfg

What is being generated

Option -V tells pypi2nix which python version to be used. To see which python versions are available consule 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 you 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 is ment to override generated nix expressions.

Non-python/system dependencies

Quite few python package require non-python dependencies to be present at installation time. For this purpose pypi2nix has -E options which can be used to define this extra non-python dependencies.

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

Building generated packages

Build one package:

% nix-build requirements.nix -A packages.empy

Build all packages:

% nix-build requirements.nix -A packages

Build python interpreter with all packages loaded:

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

Enter developent environemnt:

% nix-shell requirements.nix -A interpreter
(nix-shell) % 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 as well do:

propagatedBuildInputs = builtins.attrValues python.packages;

Using requirements_override.nix

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.

When things go not as you expected, pypi2nix gives you an option to override anything that it was generated. Even add new packages this way.

An example how you would override a derivation would be adding extra build time dependencies which we can not detect with pypi2nix. As example lets add setuptools-src which got generated, but was not detected as build time dependency of execnet:

{ pkgs, python }:

self: super: {

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

}

This was you can add or remove any python package.

Including overrides

Additional to an autogenerated requirements_overrides.nix file you can include preexisting overrides files via the -O command line argument. These overrides will be included the same way as your requirements_overrides.nix. pypi2nix allows this via some common protocols. The following are some usage examples and explanations of this feature.

http and https

pypi2nix -V 3 --overrides https://raw.githubusercontent.com/garbas/nixpkgs-python/master/overrides.nix

Note that the generated nix expression will check if contents of overrides files differs from when a nix expression was build and fail of this was the case or the file does not exist anymore.

Local files
pypi2nix -V 3 --override ../some/relative/path --override /some/absolute/path
Git repositories

pypi2nix -V 3 --override git+https://github.com/garbas/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.

Creating default.nix for you project

Nothing speaks better then 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 pypi2nix generated packages and some common utilities.

To create a package you would use python.mkDerivation as you are used to that 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 tells more then 1000 words

overlay.nix:

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

shell.nix:

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

3. Existing examples

The file examples/Makefile contains specific instructions for packages like sentry, empy, lektor, awscli and rss2email.

4. Help developing pypi2nix

Clone pypi2nix repository and using nix-shell command enter development environment.:

% git clone https://github.com/garbas/pypi2nix
% cd pypi2nix
% nix-shell

Code is located in src/pypi2nix.