pythonEnv: init

Add experimental one-liner command to create python environments.

```command
$ nix run github:nix-community/dream2nix#__pythonEnv requests whisper-ctranslate2
...

$ python -c "import requests; print('imported requests')"
imported requests

$ whisper-ctranslate2 --version
whisper-ctranslate2 0.4.1
```

modules/dream2nix/WIP-python-pdm/lib.nix

@@ -131,6 +131,14 @@
     else (lib.head selected).filename
   );
 
+  noValidFilenameError = filenames:
+    throw ''
+      No valid filename found in the list of filenames:
+        - ${lib.concatStringsSep "\n  - " filenames}
+      System: ${targetPlatform.system}
+      Python version: ${python3.version}
+    '';
+
   # Source selectors.
   # Prefer to select a wheel from a list of filenames.
   # Filenames should already have been filtered on environment usability.
@@ -143,7 +151,7 @@
     then wheel
     else if sdist != null
     then sdist
-    else null;
+    else noValidFilenameError filenames;
 
   # preferSdistSelector :: [String] -> String
   preferSdistSelector = filenames: let
@@ -154,7 +162,7 @@
     then sdist
     else if wheel != null
     then wheel
-    else null;
+    else noValidFilenameError filenames;
 
   # Get the dependency names out from a list of parsed deps which are
   #   required due to the current environment.

modules/flake-parts/pythonEnv/default.nix

@@ -0,0 +1,57 @@
+{self, ...}: {
+  perSystem = {
+    pkgs,
+    lib,
+    ...
+  }: let
+    python = pkgs.python3;
+    pythonAttr = "python${lib.versions.major python.version}${lib.versions.minor python.version}";
+    pdmConfig = pkgs.writeText "pdm-config.toml" ''
+      check_update = false
+      [python]
+      use_venv = false
+    '';
+    pythonEnv = pkgs.writeScriptBin "pythonWith" ''
+      #!${pkgs.bash}/bin/bash
+      set -Eeuo pipefail
+
+      export PATH="$PATH:${lib.makeBinPath [
+        pkgs.coreutils
+        pkgs.pdm
+        pkgs.yq
+      ]}"
+      export TMPDIR=$(${pkgs.coreutils}/bin/mktemp -d)
+      # trap "${pkgs.coreutils}/bin/chmod -R +w '$TMPDIR'; ${pkgs.coreutils}/bin/rm -rf '$TMPDIR'" EXIT
+      pushd $TMPDIR >/dev/null
+
+      # vscode likes to set these for whatever reason and it crashes PDM
+      unset _PYTHON_SYSCONFIGDATA_NAME _PYTHON_HOST_PLATFORM
+
+      echo ${python}/bin/python > .pdm-python
+      cat <<EOF > pyproject.toml
+      [project]
+      name = "temp"
+      version = "0.0.0"
+      requires-python = "==${python.version}"
+      dependencies = [
+      EOF
+
+      for dep in "$@"; do
+        echo "  \"$dep\"," >> pyproject.toml
+      done
+
+      echo "]" >> pyproject.toml
+      pdm -c ${pdmConfig} lock
+      popd >/dev/null
+
+      # initialize flake template
+      cat ${./flake-template.nix} > $TMPDIR/flake.nix
+      sed -i 's\__PYTHON_ATTR__\${pythonAttr}\g' $TMPDIR/flake.nix
+
+      # enter dev-shell
+      nix develop $TMPDIR -c $SHELL
+    '';
+  in {
+    packages.__pythonEnv = pythonEnv;
+  };
+}

modules/flake-parts/pythonEnv/flake-template.nix

@@ -0,0 +1,62 @@
+{
+  description = "My flake with dream2nix packages";
+
+  inputs = {
+    dream2nix.url = "github:nix-community/dream2nix/pythonEnv";
+    nixpkgs.follows = "dream2nix/nixpkgs";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+  };
+
+  outputs = inputs @ {
+    self,
+    dream2nix,
+    flake-parts,
+    nixpkgs,
+    ...
+  }:
+    flake-parts.lib.mkFlake {inherit inputs;} {
+      systems = [
+        "aarch64-darwin"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "x86_64-linux"
+      ];
+      perSystem = {
+        pkgs,
+        system,
+        ...
+      }: let
+        module = {
+          config,
+          dream2nix,
+          ...
+        }: {
+          imports = [
+            dream2nix.modules.dream2nix.WIP-python-pdm
+          ];
+          deps = {nixpkgs, ...}: {
+            python = nixpkgs.__PYTHON_ATTR__;
+          };
+          pdm.lockfile = ./pdm.lock;
+          pdm.pyproject = ./pyproject.toml;
+          mkDerivation = {
+            src = ./.;
+          };
+        };
+
+        package = dream2nix.lib.evalModules {
+          packageSets.nixpkgs = inputs.nixpkgs.legacyPackages.${system};
+          modules = [
+            module
+            {
+              paths.projectRoot = ./.;
+              paths.projectRootFile = "flake.nix";
+              paths.package = ./.;
+            }
+          ];
+        };
+      in {
+        devShells.default = package.devShell;
+      };
+    };
+}