feat: format will be defaulted from changelog file name

Author: nedbat

changelog.d/20250420_102250_nedbat.rst

@@ -0,0 +1,38 @@
+.. A new scriv changelog fragment.
+..
+.. Uncomment the section that is right (remove the leading dots).
+.. For top level release notes, leave all the headers commented out.
+..
+.. Removed
+.. .......
+..
+.. - A bullet item for the Removed category.
+..
+Added
+.....
+
+- The ``format`` setting is now set based on the ``changelog`` setting.
+  Previously, ``changelog=README.md`` would still use .rst formatting.  Now it
+  will use Markdown.
+
+..
+.. Changed
+.. .......
+..
+.. - A bullet item for the Changed category.
+..
+.. Deprecated
+.. ..........
+..
+.. - A bullet item for the Deprecated category.
+..
+.. Fixed
+.. .....
+..
+.. - A bullet item for the Fixed category.
+..
+.. Security
+.. ........
+..
+.. - A bullet item for the Security category.
+..

docs/configuration.rst

@@ -270,7 +270,8 @@ format
 The format to use for fragments and for the output changelog
 file.  Can be either "rst" or "md".
 
-Default: ``rst``
+Default:                 Derived from the changelog file name if
+provided,                 otherwise "rst".
 
 
 .. _config_fragment_directory:
@@ -386,7 +387,7 @@ source file.
 
 Default: (empty)
 
-.. [[[end]]] (checksum: 1b528f5aa913cecca1337c5dda4aa206)
+.. [[[end]]] (checksum: 6b03fa55831395ac304313bd43e01ff2)
 
 
 .. _deprecated_config:

src/scriv/config.py

@@ -20,6 +20,10 @@
 logger = logging.getLogger(__name__)
 
 
+DEFAULT_FORMAT = "rst"
+DEFAULT_CHANGELOG = "CHANGELOG.${config:format}"
+
+
 @attr.s
 class _Options:
     """
@@ -42,13 +46,17 @@ class _Options:
     # What format for fragments? reStructuredText ("rst") or Markdown ("md").
     format = attr.ib(
         type=str,
-        default="rst",
-        validator=attr.validators.in_(["rst", "md"]),
+        default=None,
+        validator=attr.validators.optional(attr.validators.in_(["rst", "md"])),
         metadata={
             "doc": """\
                 The format to use for fragments and for the output changelog
                 file.  Can be either "rst" or "md".
                 """,
+            "doc_default": f"""\
+                Derived from the changelog file name if provided,
+                otherwise "{DEFAULT_FORMAT}".
+                """,
         },
     )
 
@@ -74,12 +82,13 @@ class _Options:
 
     changelog = attr.ib(
         type=str,
-        default="CHANGELOG.${config:format}",
+        default=None,
         metadata={
             "doc": """\
                 The changelog file managed and read by scriv.  The old name
                 for this setting is :ref:`output_file <deprecated_config>`.
                 """,
+            "doc_default": f"``{DEFAULT_CHANGELOG}``",
         },
     )
 
@@ -223,6 +232,15 @@ class _Options:
         },
     )
 
+    def post_create(self):
+        if self.format is None:
+            if self.changelog is not None and "${" not in self.changelog:
+                self.format = Path(self.changelog).suffix[1:]
+            else:
+                self.format = DEFAULT_FORMAT
+        if self.changelog is None:
+            self.changelog = DEFAULT_CHANGELOG
+
 
 # Map of old config names to new config names.
 DEPRECATED_NAMES = [
@@ -255,10 +273,12 @@ class Config:
 
     """
 
-    def __init__(self, **kwargs):
+    def __init__(self, post_create_=True, **kwargs):
         """All values in _Options can be set as keywords."""
         with validator_exceptions():
             self._options = _Options(**kwargs)
+        if post_create_:
+            self._options.post_create()
 
     def __getattr__(self, name):
         """Proxy to self._options, and resolve the value."""
@@ -292,7 +312,7 @@ def read(cls) -> Config:
         The section can be named ``[scriv]`` or ``[tool.scriv]``.
 
         """
-        config = cls()
+        config = cls(post_create_=False)
         config.read_one_config("setup.cfg")
         config.read_one_config("tox.ini")
         config.read_one_toml("pyproject.toml")
@@ -301,6 +321,7 @@ def read(cls) -> Config:
         )
         with validator_exceptions():
             attr.validate(config._options)
+        config._options.post_create()
         return config
 
     def get_set_option(self, scriv_data, config_name, opt_name):
@@ -393,7 +414,8 @@ def resolve_value(self, value: str) -> str:
             "command:" read the output of a shell command.
 
         """
-        value = value.replace("${config:format}", self._options.format)
+        if self._options.format is not None:
+            value = value.replace("${config:format}", self._options.format)
         if value.startswith("file:"):
             file_name = value.partition(":")[2].strip()
             value = self.read_file_value(file_name)

tests/test_config.py

@@ -116,6 +116,7 @@ def test_reading_config(config_text, temp_dir):
     config = Config.read()
     assert config.fragment_directory == "changelog.d"
     assert config.changelog == "README.md"
+    assert config.format == "md"
     assert config.categories == ["New", "Different", "Gone", "Bad"]
     assert config.start_marker == "FIRST!"
 
@@ -329,6 +330,7 @@ def test_reading_toml_file(self, config_text, temp_dir):
         (temp_dir / "pyproject.toml").write_text(config_text)
         config = Config.read()
         assert config.changelog == "README.md"
+        assert config.format == "md"
         assert config.categories == ["New", "Different", "Gone", "Bad"]
         assert config.start_marker == "FIRST!"