Move name and description to class attributes on ColumnType

Instead of passing name= and description= as constructor arguments,
define them as class attributes on each subclass. This better reflects
that they are intrinsic to the type, not configurable per-instance.

https://claude.ai/code/session_01SvPEPqHgURTWESRp28pTC3

datasette/column_types.py

@@ -1,19 +1,17 @@
-from dataclasses import dataclass
-
-
-@dataclass(frozen=True, kw_only=True)
 class ColumnType:
-    name: str
     """
-    Unique identifier string. Lowercase, no spaces.
-    Examples: "markdown", "file", "email", "url", "point", "image".
+    Base class for column types.
+
+    Subclasses must define ``name`` and ``description`` as class attributes:
+
+    - ``name``: Unique identifier string. Lowercase, no spaces.
+      Examples: "markdown", "file", "email", "url", "point", "image".
+    - ``description``: Human-readable label for admin UI dropdowns.
+      Examples: "Markdown text", "File reference", "Email address".
     """
 
+    name: str
     description: str
-    """
-    Human-readable label for admin UI dropdowns.
-    Examples: "Markdown text", "File reference", "Email address".
-    """
 
     async def render_cell(
         self, value, column, table, database, datasette, request, config

datasette/default_column_types.py

@@ -8,6 +8,8 @@ from datasette.column_types import ColumnType
 
 
 class UrlColumnType(ColumnType):
+    name = "url"
+    description = "URL"
 
     async def render_cell(
         self, value, column, table, database, datasette, request, config
@@ -28,6 +30,8 @@ class UrlColumnType(ColumnType):
 
 
 class EmailColumnType(ColumnType):
+    name = "email"
+    description = "Email address"
 
     async def render_cell(
         self, value, column, table, database, datasette, request, config
@@ -48,6 +52,8 @@ class EmailColumnType(ColumnType):
 
 
 class JsonColumnType(ColumnType):
+    name = "json"
+    description = "JSON data"
 
     async def render_cell(
         self, value, column, table, database, datasette, request, config
@@ -76,7 +82,7 @@ class JsonColumnType(ColumnType):
 @hookimpl
 def register_column_types(datasette):
     return [
-        UrlColumnType(name="url", description="URL"),
-        EmailColumnType(name="email", description="Email address"),
-        JsonColumnType(name="json", description="JSON data"),
+        UrlColumnType(),
+        EmailColumnType(),
+        JsonColumnType(),
     ]

docs/plugin_hooks.rst

@@ -1012,6 +1012,9 @@ Return a list of :ref:`ColumnType <column_types>` instances to register custom c
 
 
     class ColorColumnType(ColumnType):
+        name = "color"
+        description = "CSS color value"
+
         async def render_cell(
             self,
             value,
@@ -1045,14 +1048,9 @@ Return a list of :ref:`ColumnType <column_types>` instances to register custom c
 
     @hookimpl
     def register_column_types(datasette):
-        return [
-            ColorColumnType(
-                name="color",
-                description="CSS color value",
-            )
-        ]
+        return [ColorColumnType()]
 
-Each ``ColumnType`` instance has the following attributes:
+Each ``ColumnType`` subclass must define the following class attributes:
 
 ``name`` - string
     Unique identifier for the column type, e.g. ``"color"``. Must be unique across all plugins.

tests/test_column_types.py

@@ -352,7 +352,11 @@ async def test_validation_allows_empty_string(ds_ct):
 
 @pytest.mark.asyncio
 async def test_column_type_base_defaults():
-    ct = ColumnType(name="test", description="Test type")
+    class TestType(ColumnType):
+        name = "test"
+        description = "Test type"
+
+    ct = TestType()
     assert await ct.render_cell("val", "col", "tbl", "db", None, None, None) is None
     assert await ct.validate("val", None, None) is None
     assert await ct.transform_value("val", None, None) == "val"
@@ -378,6 +382,9 @@ async def test_render_cell_extra_with_column_types(ds_ct):
 @pytest.mark.asyncio
 async def test_duplicate_column_type_name_raises_error():
     class DuplicateUrlType(ColumnType):
+        name = "url"
+        description = "Duplicate URL"
+
         async def render_cell(
             self, value, column, table, database, datasette, request, config
         ):
@@ -386,7 +393,7 @@ async def test_duplicate_column_type_name_raises_error():
     class _Plugin:
         @hookimpl
         def register_column_types(self, datasette):
-            return [DuplicateUrlType(name="url", description="Duplicate URL")]
+            return [DuplicateUrlType()]
 
     plugin = _Plugin()
     pm.register(plugin, name="test_duplicate_ct")
@@ -420,6 +427,9 @@ async def test_transform_value_in_json_output(tmp_path_factory):
     """A column type with transform_value should modify rows in JSON API."""
 
     class UpperColumnType(ColumnType):
+        name = "upper"
+        description = "Uppercase"
+
         async def transform_value(self, value, config, datasette):
             if isinstance(value, str):
                 return value.upper()
@@ -428,7 +438,7 @@ async def test_transform_value_in_json_output(tmp_path_factory):
     class _Plugin:
         @hookimpl
         def register_column_types(self, datasette):
-            return [UpperColumnType(name="upper", description="Uppercase")]
+            return [UpperColumnType()]
 
     plugin = _Plugin()
     pm.register(plugin, name="test_transform_ct")
@@ -469,6 +479,9 @@ async def test_column_type_render_cell_has_priority_over_plugins(tmp_path_factor
     """Column type render_cell should take priority over render_cell plugin hook."""
 
     class PriorityColumnType(ColumnType):
+        name = "priority_test"
+        description = "Priority test"
+
         async def render_cell(
             self, value, column, table, database, datasette, request, config
         ):
@@ -481,9 +494,7 @@ async def test_column_type_render_cell_has_priority_over_plugins(tmp_path_factor
     class _ColumnTypePlugin:
         @hookimpl
         def register_column_types(self, datasette):
-            return [
-                PriorityColumnType(name="priority_test", description="Priority test")
-            ]
+            return [PriorityColumnType()]
 
     class _RenderCellPlugin:
         @hookimpl