Move nix shell to separate shell.nix

Update README for nix shell instructions, refile long lines

README.md

@@ -1,10 +1,13 @@
 # Nix Background Upgrade
-Upgrade NixOS system (and/or home-manager) in the background, with notification support.
+Upgrade NixOS system (and/or home-manager) in the background, with
+notification support.
 
-**Early stages of development, subject to change and likely not working at the current stage.**
+**Early stages of development, subject to change and likely not
+working at the current stage.**
 
 ## Install
-Clone the repository and install with pip (or preferred Python packaging system):
+Clone the repository and install with pip (or preferred Python
+packaging system):
 
 ```bash
 git clone https://git.xenia.me.uk/pixelifytica/nix-background-upgrade.git
@@ -12,9 +15,12 @@ python3 -m pip install ./nix-background-upgrade
 ```
 
 ### For development
-Project uses [Poetry](https://python-poetry.org/ "Poetry: Python packaging and dependency management tool") to manage dependencies.
+Project uses [Poetry](https://python-poetry.org/ "Poetry: Python
+packaging and dependency management tool") to manage dependencies.
 
-Install Poetry (see [documentation](https://python-poetry.org/docs/#installation) for more details), for example using `pipx`:
+Install Poetry (see
+[documentation](https://python-poetry.org/docs/#installation) for more
+details), for example using `pipx`:
 
 ```bash
 pipx install poetry
@@ -26,14 +32,23 @@ Then install project using `poetry`:
 poetry install
 ```
 
-Python code is formatted using [black](https://pypi.org/project/black/ "The uncompromising code formatter") and [isort](https://pypi.org/project/isort/ "A Python utility / library to sort Python imports").
+Python code is formatted using [black](https://pypi.org/project/black/
+"The uncompromising code formatter") and
+[isort](https://pypi.org/project/isort/ "A Python utility / library to
+sort Python imports").
 
 #### `pre-commit`
-[pre-commit](https://pre-commit.com/ "A framework for managing and maintaining multi-language pre-commit hooks") is used to ensure formatting of new/changed files. Install pre-commit hooks with:
+[pre-commit](https://pre-commit.com/ "A framework for managing and
+maintaining multi-language pre-commit hooks") is used to ensure
+formatting of new/changed files. Install pre-commit hooks with:
 
 ```bash
 pre-commit install --install-hooks
 ```
 
 ### Nix Flake
-Project provides a Nix `flake.nix` to get up and running quickly.
+Project provides a Nix `flake.nix` and `shell.nix` to get up and
+running quickly.
+
+Run `nix develop` (Flake) or `nix-shell` (non-Flake) to enter
+development shell containing project dependencies.

flake.nix

@@ -29,18 +29,10 @@
       };
     });
 
-    devShells = forAllSystems (system: let
-      inherit (poetry2nix.lib.mkPoetry2Nix {pkgs = pkgs.${system};}) mkPoetryEnv;
-    in {
-      default = pkgs.${system}.mkShellNoCC {
-        packages = with pkgs.${system}; [
-          (mkPoetryEnv {
-            projectDir = self;
-            preferWheels = true;
-            extraPackages = ps: with ps; [python-lsp-server];
-          })
-          poetry
-        ];
+    devShells = forAllSystems (system: {
+      default = import ./shell.nix {
+        pkgs = pkgs.${system};
+        inherit (poetry2nix.lib.mkPoetry2Nix {pkgs = pkgs.${system};}) mkPoetryEnv;
       };
     });
   };

shell.nix

@@ -0,0 +1,15 @@
+{
+  pkgs ? import <nixpkgs> {},
+  mkPoetryEnv ? ((builtins.getFlake "github:nix-community/poetry2nix").lib.mkPoetry2Nix {inherit pkgs;}).mkPoetryEnv,
+}:
+pkgs.mkShellNoCC {
+  packages = with pkgs; [
+    (mkPoetryEnv {
+      projectDir = ./.;
+      preferWheels = true;
+      editablePackageSources = {nix_background_upgrade = ./src;};
+      extraPackages = ps: with ps; [python-lsp-server];
+    })
+    poetry
+  ];
+}