TableContext - table page now renders a documented Context dataclass

The table HTML view constructs a TableContext instead of an ad-hoc
dict, matching how the database and query pages already work. Fields
resolved by registered extras are declared with extra_field() so their
documentation lives on the Extra classes in table_extras.py; fields
added by the view code carry help metadata next to the view.

render_template() now converts Context dataclasses shallowly instead
of via dataclasses.asdict(), which deep-copied every value and would
fail on values like sqlite3.Row.

Keys not declared on TableContext - extras requested with ?_extra= on
the HTML page, or extra filter context from filters_from_request
plugins - are now dropped from the HTML template context rather than
passed through undocumented.

Refs #2127

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_internals_datasette.py

@@ -344,3 +344,28 @@ async def test_datasette_close_continues_past_db_error():
         ds.close()
     assert good._closed
     assert ds._internal_database._closed
+
+
+@pytest.mark.asyncio
+async def test_datasette_render_template_dataclass_values_not_deep_copied():
+    # display_rows can contain values like sqlite3.Row that cannot be
+    # deep-copied, so render_template must convert Context dataclasses
+    # shallowly - https://github.com/simonw/datasette/issues/2127
+    class RefusesDeepCopy:
+        def __deepcopy__(self, memo):
+            raise RuntimeError("deepcopy not supported")
+
+        def __str__(self):
+            return "shallow-copied-value"
+
+    @dataclasses.dataclass
+    class ExampleContext(Context):
+        title: str
+        status: int
+        error: RefusesDeepCopy
+
+    context = ExampleContext(title="Hello", status=200, error=RefusesDeepCopy())
+    ds = Datasette(memory=True)
+    await ds.invoke_startup()
+    rendered = await ds.render_template("error.html", context)
+    assert "shallow-copied-value" in rendered

tests/test_template_context.py

@@ -178,6 +178,14 @@ async def test_template_context_matches_documented_contract(
     )
 
 
+def test_table_context_fields_match_documented_contract():
+    from datasette.views.table import TableContext
+
+    assert {f.name for f in TableContext.documented_fields()} == {
+        key.name for key in PAGES["table"].documented_keys()
+    }
+
+
 def test_base_context_keys_all_have_docs():
     for key in BASE_CONTEXT_KEYS:
         assert key.doc, "Base context key {} is missing docs".format(key.name)