Add --format flag

The flag will invoke nixpkgs-fmt

README.rst

@@ -98,6 +98,8 @@ tested.
 Additionally, the `--review` flag can be used to initiate a run of
 nixpkgs-review <https://github.com/Mic92/nixpkgs-review>, which will ensure all
 dependent packages can be built.
+In order to ensure consistent formatting, the `--format` flag will invoke
+[nixpkgs-fmt](https://github.com/nix-community/nixpkgs-fmt).
 
 
 ::
@@ -112,6 +114,8 @@ dependent packages can be built.
    $ nix-update --run nixpkgs-review
    # Run `nixpkgs-review wip` to validate dependent packages
    $ nix-update --review nixpkgs-review
+   # Format file
+   $ nix-update --format nixpkgs-review
 
 Nix-update also can optionally generate a commit message in the form
 `attribute: old_version -> new_version` with the applied version update:

default.nix

@@ -27,7 +27,7 @@ python3.pkgs.buildPythonApplication rec {
     mypy --strict nix_update
   '';
   makeWrapperArgs = [
-    "--prefix PATH" ":" (lib.makeBinPath [ nixFlakes nix-prefetch nixpkgs-review ])
+    "--prefix PATH" ":" (lib.makeBinPath [ nixFlakes nix-prefetch nixpkgs-fmt nixpkgs-review ])
   ];
   shellHook = ''
     # workaround because `python setup.py develop` breaks for me

nix_update/__init__.py

@@ -26,6 +26,7 @@ def parse_args() -> Options:
     parser.add_argument(
         "--review", action="store_true", help="Run `nixpkgs-review wip`"
     )
+    parser.add_argument("--format", action="store_true", help="Run `nixpkgs-fmt`")
     parser.add_argument(
         "--commit", action="store_true", help="Commit the updated package"
     )
@@ -59,6 +60,7 @@ def parse_args() -> Options:
         test=args.test,
         version_regex=args.version_regex,
         review=args.review,
+        format=args.format,
     )
 
 
@@ -173,6 +175,11 @@ def nixpkgs_review() -> None:
     run(cmd, check=True)
 
 
+def nixpkgs_fmt(package: Package) -> None:
+    cmd = ["nixpkgs-fmt", package.filename]
+    run(cmd, check=True)
+
+
 def main() -> None:
     options = parse_args()
     if not os.path.exists(options.import_path):
@@ -208,6 +215,9 @@ def main() -> None:
     if options.review:
         nixpkgs_review()
 
+    if options.format:
+        nixpkgs_fmt(package)
+
     if options.commit:
         assert git_dir is not None
         git_commit(git_dir, options.attribute, package)

nix_update/options.py

@@ -13,3 +13,4 @@ class Options:
     build: bool = False
     test: bool = False
     review: bool = False
+    format: bool = False