Add --check-change option, for pre-commit export hook

src/poetry_plugin_export/command.py

@@ -51,6 +51,11 @@ class ExportCommand(GroupCommand):
         ),
         option("all-extras", None, "Include all sets of extra dependencies."),
         option("with-credentials", None, "Include credentials for extra indices."),
+        option(
+            "check-change",
+            None,
+            "Return code 1 if file changed. Useful for pre-commit.",
+        ),
     ]
 
     @property
@@ -101,6 +106,15 @@ def handle(self) -> int:
             )
             return 1
 
+        check_change = self.option("check-change")
+        if check_change and not output:
+            self.line_error(
+                "<error>You must specify"
+                " `<fg=yellow;options=bold>--output</>` if"
+                " using `<fg=yellow;options=bold>--check-change</>`.</error>"
+            )
+            return 1
+
         extras: Iterable[NormalizedName]
         if self.option("all-extras"):
             extras = self.poetry.package.extras.keys()
@@ -122,6 +136,10 @@ def handle(self) -> int:
         exporter.with_hashes(not self.option("without-hashes"))
         exporter.with_credentials(self.option("with-credentials"))
         exporter.with_urls(not self.option("without-urls"))
-        exporter.export(fmt, Path.cwd(), output or self.io)
 
-        return 0
+        if check_change:
+            changed = exporter.export_and_check_change(fmt, Path.cwd(), output)
+            return 1 if changed else 0
+        else:
+            exporter.export(fmt, Path.cwd(), output or self.io)
+            return 0

src/poetry_plugin_export/exporter.py

@@ -3,6 +3,7 @@
 import urllib.parse
 
 from functools import partialmethod
+from pathlib import Path
 from typing import TYPE_CHECKING
 from typing import Iterable
 
@@ -15,7 +16,6 @@
 
 if TYPE_CHECKING:
     from collections.abc import Collection
-    from pathlib import Path
     from typing import ClassVar
 
     from packaging.utils import NormalizedName
@@ -80,6 +80,14 @@ def export(self, fmt: str, cwd: Path, output: IO | str) -> None:
 
         getattr(self, self.EXPORT_METHODS[fmt])(cwd, output)
 
+    def export_and_check_change(self, fmt: str, cwd: Path, output: str) -> bool:
+        output_path = Path(output)
+        orig_content = output_path.read_bytes() if output_path.exists() else None
+        self.export(fmt, cwd, output)
+        new_content = output_path.read_bytes()
+        changed = new_content != orig_content
+        return changed
+
     def _export_generic_txt(
         self, cwd: Path, output: IO | str, with_extras: bool, allow_editable: bool
     ) -> None: