Make check subcommand detect if only configured news file has changed

src/towncrier/check.py

@@ -41,7 +41,16 @@ def __main(comparewith, directory, config):
     try:
         files_changed = (
             _run(
-                ["git", "diff", "--name-only", comparewith + "..."], cwd=base_directory
+                [
+                    "git",
+                    "diff",
+                    "--name-only",
+                    # Only show files that were Added (A), Copied (C), Modified (M)
+                    # or Renamed (R).
+                    "--diff-filter=ACMR",
+                    comparewith + "..."
+                ],
+                cwd=base_directory
             )
             .decode(encoding)
             .strip()
@@ -55,6 +64,10 @@ def __main(comparewith, directory, config):
         click.echo("On trunk, or no diffs, so no newsfragment required.")
         sys.exit(0)
 
+    if files_changed == config["filename"]:
+        click.echo("Only the configured news file has changed.")
+        sys.exit(0)
+
     files = {
         os.path.normpath(os.path.join(base_directory, path))
         for path in files_changed.strip().splitlines()

src/towncrier/newsfragments/174.feature.rst

@@ -0,0 +1,6 @@
+Make the ``check`` subcommand detect if only the configured news file has
+changed.
+
+This should enable the ``check`` subcommand to be used as a CI lint step and
+not fail when a pull request only modifies the configured news file (i.e. when
+the news file is being assembled for the next release).

src/towncrier/test/test_check.py

@@ -3,6 +3,7 @@
 
 import os
 import sys
+from textwrap import dedent
 
 from twisted.trial.unittest import TestCase
 from click.testing import CliRunner
@@ -13,7 +14,11 @@
 
 def create_project(pyproject_path):
     with open(pyproject_path, "w") as f:
-        f.write("[tool.towncrier]\n" 'package = "foo"\n')
+        f.write(dedent("""\
+            [tool.towncrier]
+            package = "foo"
+            filename = "NEWS.rst"
+        """))
     os.mkdir("foo")
     with open("foo/__init__.py", "w") as f:
         f.write('__version__ = "1.2.3"\n')
@@ -22,6 +27,9 @@ def create_project(pyproject_path):
     with open(fragment_path, "w") as f:
         f.write("Adds levitation")
 
+    with open("NEWS.rst", "w") as f:
+        f.write("added on default branch")
+
     call(["git", "init"])
     call(["git", "config", "user.name", "user"])
     call(["git", "config", "user.email", "user@example.com"])
@@ -148,3 +156,24 @@ def test_none_stdout_encoding_works(self):
 
         self.assertEqual(0, proc.returncode)
         self.assertEqual(b"", stderr)
+
+    def test_newsfile_modified(self):
+        """
+        A modified newsfile satisfies the check.
+        """
+        runner = CliRunner()
+
+        with runner.isolated_filesystem():
+            create_project("pyproject.toml")
+
+            newsfile_path = "NEWS.rst"
+
+            with open(newsfile_path, "w") as f:
+                f.write("on other branch")
+            call(["git", "add", newsfile_path])
+            call(["git", "commit", "-m", "modify NEWS.rst"])
+
+            result = runner.invoke(_main, ["--compare-with", "master"])
+
+            self.assertEqual(result.output, 'Only the configured news file has changed.\n')
+            self.assertEqual(0, result.exit_code)