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Nixpkgs: issues with stdenv.mkDerivation

This document describes some of the issues that exist with stdenv.mkDerivation and that could or should be considered when redesigning it.

Phase ordering

stdenv.mkDerivation comes with several fixed phases. It is possible to override these phases or add other phases. It is however difficult to make phases depend on one another as that requires describing their relative order

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let
  overlay = self: super: {
    python372 = let
      py = super.python37.override {
        self = py;
        sourceVersion = {
          major = "3";
          minor = "7";
          patch = "2";
          suffix = "";

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# # Python package set in Nixpkgs over time
#
# This gist is a Nix expression and generates a figure showing the size
# of the Python package set from September 2015 until now.
#
# To build the figure:
#
#   nix-build

let

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import os
import subprocess

os.environ["FOO"] = "BAR"

subprocess.run("python3 -c 'import os; print(os.environ.get(\"FOO\", \"\"))'", shell=True)

del os.environ["FOO"]

subprocess.run("python3 -c 'import os; print(os.environ.get(\"FOO\", \"\"))'", shell=True)

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let
  pkgs = import (fetchTarball channel:nixos-18.03) {};
  custom-store = "/home/freddy/nix_custom_store";
  # Run nix in a mount namespace
  nix-wrapped = pkgs.writeShellScriptBin "nix" ''
    ${pkgs.bubblewrap}/bin/bwrap \
      --unshare-all \
      --proc /proc \
      --dev /dev \
      --tmpfs /run \

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# Get nix 2.0 image from https://hub.docker.com/r/nixos/nix/
FROM nixos/nix:2.0

# Include the Nix expression we want to build
ADD default.nix default.nix

# We want the possibility for a custom store location
# Note that by using "/" there is effectively no mount namespace
# opening the doors for impurities.
ARG MOUNT_NIX_STORE

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{pkgs ? (import <nixpkgs> {})}:

pkgs.stdenv.mkDerivation {
  name = "fetchtarballtest";
  
  src = pkgs.fetchurl {
    url = https://github.com/NixOS/nixpkgs/archive/9c90ff7e7df9ceadc7335bb43cc67ba92e171d53.tar.gz;
    sha256 = "0f8ynrrj24i4g8flp978ccsb2hlwk1n321z8xwj0wakj0ak4c8i9";
  };

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#! /usr/bin/env python

"""
Filter a .bib file with keys referred to in an .aux file. Print result to STDOUT.
"""

import argparse
import bibtexparser
import re
import itertools

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# Produce a list of derivations that should likely have pythonPath set

with import <nixpkgs> {};

with builtins;
let
  # Package set to check
  pythonPackages = pkgs.python2Packages;

  # evaluate expr, returning default on error

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Python on Nix infrastructure

Introduction

Python is used throughout Nixpkgs. We use it for certain scripts, we provide Python libraries, and we provide applications. There are several methods on how to use Python on Nix, each with their pros and cons. An overview of all issues with the current Python infrastructure on Nix is available in the placeholder issue 1819.

Goal