Make towncrier create increment fragments rather than failing (#475)

* Make `towncrier create` increment fragments rather than failing
Fixes #474

* Rename change file

* Add example to tutorial of incrementing via towncrier create

* Remove the counter example from the tutorial, this isn't core functionality

* Better docstring for the new test

---------

Co-authored-by: Glyph <glyph@twistedmatrix.com>

docs/cli.rst

@@ -64,6 +64,9 @@ Create a news fragment in the directory that ``towncrier`` is configured to look
 
 ``towncrier create`` will enforce that the passed type (e.g. ``bugfix``) is valid.
 
+If the filename exists already, ``towncrier create`` will add (and then increment) a number after the fragment type until it finds a filename that does not exist yet.
+In the above example, it will generate ``123.bugfix.1.rst`` if ``123.bugfix.rst`` already exists.
+
 .. option:: --content, -c CONTENT
 
    A string to use for content.

src/towncrier/create.py

@@ -119,8 +119,17 @@ def __main(
         os.makedirs(fragments_directory)
 
     segment_file = os.path.join(fragments_directory, filename)
-    if os.path.exists(segment_file):
-        raise click.ClickException(f"{segment_file} already exists")
+
+    retry = 0
+    if filename.split(".")[-1] not in config.types:
+        filename, extra_ext = os.path.splitext(filename)
+    else:
+        extra_ext = ""
+    while os.path.exists(segment_file):
+        retry += 1
+        segment_file = os.path.join(
+            fragments_directory, f"{filename}.{retry}{extra_ext}"
+        )
 
     if edit:
         edited_content = _get_news_content_from_user(content)

src/towncrier/newsfragments/475.feature

@@ -0,0 +1,6 @@
+Make ``towncrier create`` use the fragment counter rather than failing
+on existing fragment names.
+
+For example, if there is an existing fragment named ``123.feature``,
+then ``towncrier create 123.feature`` will now create a fragment
+named ``123.feature.1``.

src/towncrier/test/test_create.py

@@ -151,20 +151,48 @@ def test_invalid_section(self):
             "Expected filename '123.foobar.rst' to be of format", result.output
         )
 
-    def test_file_exists(self):
+    @with_isolated_runner
+    def test_file_exists(self, runner: CliRunner):
         """Ensure we don't overwrite existing files."""
-        runner = CliRunner()
+        setup_simple_project()
+        frag_path = Path("foo", "newsfragments")
 
-        with runner.isolated_filesystem():
-            setup_simple_project()
+        for _ in range(3):
+            result = runner.invoke(_main, ["123.feature"])
+            self.assertEqual(result.exit_code, 0, result.output)
+
+        fragments = [f.name for f in frag_path.iterdir()]
+        self.assertEqual(
+            sorted(fragments),
+            [
+                "123.feature",
+                "123.feature.1",
+                "123.feature.2",
+            ],
+        )
 
-            self.assertEqual([], os.listdir("foo/newsfragments"))
+    @with_isolated_runner
+    def test_file_exists_with_ext(self, runner: CliRunner):
+        """
+        Ensure we don't overwrite existing files when using an extension after the
+        fragment type.
+        """
+        setup_simple_project()
+        frag_path = Path("foo", "newsfragments")
 
-            runner.invoke(_main, ["123.feature.rst"])
+        for _ in range(3):
             result = runner.invoke(_main, ["123.feature.rst"])
-
-        self.assertEqual(type(result.exception), SystemExit)
-        self.assertIn("123.feature.rst already exists", result.output)
+            self.assertEqual(result.exit_code, 0, result.output)
+
+        fragments = [f.name for f in frag_path.iterdir()]
+        self.assertEqual(
+            sorted(fragments),
+            [
+                "123.feature.1.rst",
+                "123.feature.2.rst",
+                "123.feature.rst",
+            ],
+        )
 
     @with_isolated_runner
     def test_create_orphan_fragment(self, runner: CliRunner):