Fix nix develop

.envrc

@@ -0,0 +1,3 @@
+use flake .
+# https://github.com/nix-community/nix-direnv/issues/109
+eval "$shellHook"

.github/workflows/main.yaml

@@ -1,29 +1,22 @@
 on: [push, pull_request]
 jobs:
-  ci:
+  all-tests:
     runs-on: ubuntu-latest
-    steps:
 
-      - uses: actions/setup-python@v2
-        with:
-          python-version: '3.9'
+    steps:
 
-      - uses: snok/install-poetry@v1
+      - uses: cachix/install-nix-action@v15
         with:
-          version: 1.1.10
-          virtualenvs-create: true
-          virtualenvs-in-project: true
+          nix_path: nixpkgs=channel:nixos-unstable
 
       - uses: actions/checkout@v2
 
-      - id: cached-poetry-dependencies
-        uses: actions/cache@v2
+      - uses: actions/cache@v2
         with:
           path: |
-            .venv
-            .tox
+            build/
+            .tox/
+            $HOME/.cache/pypoetry/
           key: venv-${{ hashFiles('poetry.lock') }}-${{ hashFiles('pyproject.toml') }}
 
-      - run: poetry install
-
-      - run: poetry run ./script.py all-tests --no-interactive
+      - run: nix develop --command ./script.py all-tests --no-interactive

.gitignore

@@ -1,3 +1,6 @@
 /build/
 __pycache__/
-.tox/
+/.tox/
+/.direnv/
+
+/.cache/

CONTRIBUTING.md

@@ -1,53 +1,37 @@
 # How to setup the development environment
 
-This project manages dependencies through [Poetry][poetry] or through [Nix][nix]. These are package managers that create a "project-local" development environment with minimal system-wide changes.
+## With Nix
 
-- Poetry is a wrapper around virtualenv/pip, so it can only manage packages in PyPI. Use it like so:
+[Nix][nix] is a language-agnostic package manager that installs packages locally. [Nix Flakes][nix flakes] are a way of specifying dependencies to Nix declaraively. Currently, they are [installed separately][install nix flakes]. Use `nix develop` to get a shell or `nix develop --command ipython` to run commands in the project-specific environment.
 
-    ```
-    # install Python
-    # Version should match that in `pyproject.toml` > `tool.poetry.dependencies` > `python`
-
-    $ python -m pip --user install poetry
-    # This is the only "system-wide" command.
-
-    $ poetry install
-    # This creates a virtualenv and installs the dev dependencies.
+[nix]: https://nixos.org/
+[nix flakes]: https://nixos.wiki/wiki/Flakes
+[install nix flakes]: https://nixos.wiki/wiki/Flakes#Installing_flakes
 
-    $ poetry run ipython
-    # This runs just one command, "ipython" in this case, in the project environment.
+## With Nix and direnv
 
-    $ poetry shell
-    # This gives you a shell in the project environment.
-    ```
+In addition to Nix, I suggest also installing [direnv][direnv] and [nix-direnv][nix-direnv]. Then simply `cd`ing to the project will activate the project-specific environment.
 
-- Nix is a language-agnostic UNIX-agnostic package manager that can create project-local environments. Nix uses Poetry's config file thanks to [poetry2nix][poetry2nix], but it can install binary dependencies that are not in PyPI. However, not all Python packages work in Nix (see [poetry2nix issue #413][issue-413]). Use it like so:
+Consider adding this line in your shell's initfile so you can see when `direnv` is activated. `PS1="\$PREPEND_TO_PS1$PS1"` Note that the sigil (dollar sign) in `$PREPEND_TO_PS1` is quoted but the one in `$PS1` is not, so `PS1` is evaluated when the shell initializes, but `PREPEND_TO_PS1` is evaluated before every prompt.
 
-    ```
-    # Install nix
-    # This is the only "system-wide" command.
+[direnv]: https://direnv.net/
+[nix-direnv]: https://github.com/nix-community/nix-direnv
 
-    $ nix develop --command ipython
-    # This runs just one command in the project environment.
+## With Poetry
 
-    $ nix develop --command zsh
-    # This runs a shell in the project environment.
-    ```
+Nix can be trouble to set up, so here is how to use the project without Nix. [Poetry][poetry] is a wrapper around `pip`/`virtualenv`, and it will manage dependencies from PyPI, but *you* have to manage external dependencies, e.g. installing the right version of Python, C libraries, etc. Poetry can be installed globally with `python -m pip install poetry. `Use `poetry shell` to get a shell and `poetry run ipython` to run a command in the project-specific environment.
 
 [poetry]: https://python-poetry.org/
-[nix]: https://nixos.org/
-[poetry2nix]: https://github.com/nix-community/poetry2nix
-[issue-413]: https://github.com/nix-community/poetry2nix/issues/413
 
 # How to use development tools
 
-Once in the development environment, use `./script.py` to run black, isort, mypy, etc.
+Once in the development environment, use `./script.py` to run development tools. In the order of usefulness,
 
 - `./script.py fmt` runs code formatters (autoimport, isort, black).
 
 - `./script.py test` runs tests and code complexity analysis (mypy, pylint, pytest, coverage, radon in parallel).
 
-- `./script.py all-tests` runs the usual tests and more (proselint, rstcheck, twine, pytest, tox). It runs them in multiple Python versions. This is mostly for CI.
+- `./script.py all-tests` runs the usual tests and more (proselint, rstcheck, twine, tox (which runs mypy and pytest)). This is intended for CI.
 
 - `./script.py docs` builds the documentation locally (sphinx, proselint).
 

flake.nix

@@ -8,46 +8,65 @@
       let
         pkgs = nixpkgs.legacyPackages.${system};
         name = "ACSL.net-scraper";
+        name-pure-shell = "${name}-pure-shell";
         name-shell = "${name}-shell";
         name-test = "${name}-test";
         default-python = pkgs.python39;
         # Alternative Pythons for Tox
         alternative-pythons = [
         ];
-        poetry2nix-crypto-override = pkgs.poetry2nix.overrides.withDefaults (self: super: {
-          cryptography = super.cryptography.overridePythonAttrs(old: {
-            cargoDeps = pkgs.rustPlatform.fetchCargoTarball {
-              inherit (old) src;
-              name = "${old.pname}-${old.version}";
-              sourceRoot = "${old.pname}-${old.version}/src/rust/";
-              sha256 = "sha256-kozYXkqt1Wpqyo9GYCwN08J+zV92ZWFJY/f+rulxmeQ=";
-            };
-            cargoRoot = "src/rust";
-            nativeBuildInputs = old.nativeBuildInputs ++ (with pkgs.rustPlatform; [
-              rust.rustc
-              rust.cargo
-              cargoSetupHook
-            ]);
-          });
-        });
       in {
         packages.${name} = pkgs.poetry2nix.mkPoetryApplication {
           projectDir = ./.;
           python = default-python;
         };
-        packages.${name-shell} = pkgs.mkShell {
+
+        # There are two approaches to make a poetry project work in Nix:
+        # 1. Use Nix to install dependencies in poetry.lock.
+        # 2. Use Nix to install Poetry and use Poetry to install dependencies in poetry.lock.
+        # Option 2 is less elegant, because it uses a package manager to install a package manager.
+        # For example, to effectively cache the environment in CI, I have to cache the Nix state and the Poetry venv.
+        # But at the time of writing poetry2nix DOES NOT WORK with Python's cryptography.
+        # Cryptography is a core dependency, so it won't work at all.
+        # ${name-pure-shell} is option 1, ${name-shell} is option 2.
+        packages.${name-pure-shell} = pkgs.mkShell {
           buildInputs = alternative-pythons ++ [
             pkgs.poetry
             (pkgs.poetry2nix.mkPoetryEnv {
               projectDir = ./.;
               # default Python for shell
               python = default-python;
-              overrides = poetry2nix-crypto-override;
             })
           ];
           # TODO: write a check expression (`nix flake check`)
         };
+
+        packages.${name-shell} = pkgs.mkShell {
+          buildInputs = alternative-pythons ++ [
+            pkgs.poetry
+            pkgs.libseccomp.lib
+            pkgs.gcc-unwrapped.lib
+          ];
+          shellHook = ''
+            env_hash=$(sha1sum poetry.lock | cut -f1 -d' ')
+            if [ ! -f build/$env_hash ]; then
+                poetry install --remove-untracked
+                if [ ! -d build ]; then
+                    mkdir build
+                fi
+                touch build/$env_hash
+            fi
+            export PREPEND_TO_PS1="(${name}) "
+            export PYTHONNOUSERSITE=true
+            export VIRTUAL_ENV=$(poetry env info -p)
+            export PATH=$VIRTUAL_ENV/bin:$PATH
+            export LD_LIBRARY_PATH=LD_LIBRARY_PATH=$(nix eval --raw nixpkgs#libseccomp.lib)/lib:$(nix eval --raw nixpkgs#gcc-unwrapped.lib)/lib:$LD_LIBRARY_PATH
+          '';
+          # TODO: write a check expression (`nix flake check`)
+        };
+
         devShell = self.packages.${system}.${name-shell};
+
         defaultPackage = self.packages.${system}.${name};
       });
 }