Default check branch should be "origin/main"

src/towncrier/check.py

@@ -6,6 +6,7 @@
 import sys
 
 from subprocess import STDOUT, CalledProcessError, check_output
+from warnings import warn
 
 import click
 
@@ -18,10 +19,28 @@ def _run(args, **kwargs):
     return check_output(args, **kwargs)
 
 
+def get_default_compare_branch(base_directory, encoding):
+    branches = (
+        _run(["git", "branch", "-r"], cwd=base_directory).decode(encoding).splitlines()
+    )
+    branches = [branch.strip() for branch in branches]
+    if "origin/main" in branches:
+        return "origin/main"
+    if "origin/master" in branches:
+        warn(
+            'Using "origin/master" as default compare branch is deprecated '
+            "and will be removed in a future version.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return "origin/master"
+    return None
+
+
 @click.command(name="check")
 @click.option(
     "--compare-with",
-    default="origin/master",
+    default=None,
     metavar="BRANCH",
     help=(
         "Checks files changed running git diff --name-ony BRANCH... "
@@ -58,6 +77,14 @@ def __main(comparewith, directory, config):
     # when the attribute is present but set to None (explicitly piped output
     # and also some CI such as GitHub Actions).
     encoding = getattr(sys.stdout, "encoding", "utf8")
+    if comparewith is None:
+        comparewith = get_default_compare_branch(
+            base_directory=base_directory, encoding=encoding
+        )
+
+    if comparewith is None:
+        click.echo("Could not detect default branch. Aborting.")
+        sys.exit(1)
 
     try:
         files_changed = (

src/towncrier/newsfragments/400.removal.rst

@@ -0,0 +1,2 @@
+Default branch for `towncrier check` is now "origin/main" instead of "origin/master".
+If "origin/main" does not exist, fallback to "origin/master" with deprecation warning.

src/towncrier/test/test_check.py

@@ -6,10 +6,12 @@
 import sys
 
 from subprocess import PIPE, Popen, call
+from unittest.mock import patch
 
 from click.testing import CliRunner
 from twisted.trial.unittest import TestCase
 
+from towncrier import check
 from towncrier.check import _main as towncrier_check
 
 
@@ -271,3 +273,47 @@ def test_release_branch(self):
             # Assert
             self.assertEqual(0, result.exit_code, (result, result.output))
             self.assertIn("Checks SKIPPED: news file changes detected", result.output)
+
+    def test_get_default_compare_branch_missingf(self):
+        """
+        If there's no recognized remote origin, exit with an error.
+        """
+        runner = CliRunner()
+
+        with runner.isolated_filesystem():
+            create_project()
+
+            result = runner.invoke(towncrier_check)
+
+        self.assertEqual(1, result.exit_code)
+        self.assertEqual("Could not detect default branch. Aborting.\n", result.output)
+
+    def test_get_default_compare_branch_main(self):
+        """
+        If there's a remote branch origin/main, prefer it over everything else.
+        """
+        runner = CliRunner()
+
+        with runner.isolated_filesystem():
+            create_project()
+
+            with patch("towncrier.check._run") as m:
+                m.return_value = b"  origin/master\n  origin/main\n\n"
+                branch = check.get_default_compare_branch(".", "utf-8")
+
+        self.assertEqual("origin/main", branch)
+
+    def test_get_default_compare_branch_fallback(self):
+        """
+        If there's origin/master and no main, use it.
+        """
+        runner = CliRunner()
+
+        with runner.isolated_filesystem():
+            create_project()
+
+            with patch("towncrier.check._run") as m:
+                m.return_value = b"  origin/master\n  origin/foo\n\n"
+                branch = check.get_default_compare_branch(".", "utf-8")
+
+        self.assertEqual("origin/master", branch)