test(expr-ir): Add __dict__ test for Immutable

Bug discovered by @FBruzzesi
#3194 (comment)

Now I need to fix 200/222 failures 😂

Author: dangotbanned

narwhals/_plan/expressions/__init__.py

@@ -10,9 +10,14 @@
 from narwhals._plan.expressions import (
     aggregation,
     boolean,
+    categorical,
     functions,
+    lists,
     operators,
     selectors,
+    strings,
+    struct,
+    temporal,
 )
 from narwhals._plan.expressions.aggregation import AggExpr, OrderableAggExpr, max, min
 from narwhals._plan.expressions.expr import (
@@ -85,10 +90,12 @@
     "_ColumnSelection",
     "aggregation",
     "boolean",
+    "categorical",
     "col",
     "cols",
     "functions",
     "index_columns",
+    "lists",
     "max",
     "min",
     "named_ir",
@@ -97,4 +104,7 @@
     "over",
     "over_ordered",
     "selectors",
+    "strings",
+    "struct",
+    "temporal",
 ]

tests/plan/immutable_test.py

@@ -1,13 +1,18 @@
 from __future__ import annotations
 
+import re
 import string
 from itertools import repeat
-from typing import Any
+from typing import TYPE_CHECKING, Any, TypeVar
 
 import pytest
 
 from narwhals._plan._immutable import Immutable
 
+if TYPE_CHECKING:
+    from collections.abc import Iterator
+T_co = TypeVar("T_co", covariant=True)
+
 
 class Empty(Immutable): ...
 
@@ -147,3 +152,52 @@ def test_immutable_hash_cache() -> None:
     cached = obj.__immutable_hash_value__
     hash_cache_hit = hash(obj)
     assert hash_cache_miss == cached == hash_cache_hit
+
+
+def _collect_immutable_descendants() -> list[type[Immutable]]:
+    # NOTE: Will populate `__subclasses__` by bringing the defs into scope
+    from narwhals._plan import (
+        _expansion,
+        _expr_ir,
+        _function,
+        expressions,
+        options,
+        schema,
+        when_then,
+    )
+
+    _ = expressions, schema, options, _expansion, _expr_ir, _function, when_then
+    return sorted(set(_iter_descendants(Immutable)), key=repr)
+
+
+def _iter_descendants(*bases: type[T_co]) -> Iterator[type[T_co]]:
+    seen = set[T_co]()
+    for base in bases:
+        yield base
+        if (children := (base.__subclasses__())) and (
+            unseen := set(children).difference(seen)
+        ):
+            yield from _iter_descendants(*unseen)
+
+
+@pytest.fixture(
+    params=_collect_immutable_descendants(), ids=lambda tp: tp.__name__, scope="session"
+)
+def immutable_type(request: pytest.FixtureRequest) -> type[Immutable]:
+    return request.param  # type: ignore[no-any-return]
+
+
+def test_immutable___slots___(immutable_type: type[Immutable]) -> None:
+    featureless_instance = object.__new__(immutable_type)
+
+    # NOTE: If this fails, `__setattr__` has been overriden
+    with pytest.raises(AttributeError, match=r"immutable"):
+        featureless_instance.i_dont_exist = 999  # type: ignore[assignment]
+
+    # NOTE: If this fails, `__slots__` lose the size benefit
+    with pytest.raises(AttributeError, match=re.escape("has no attribute '__dict__'")):
+        _ = featureless_instance.__dict__
+
+    slots = immutable_type.__slots__
+    if slots:
+        assert len(slots) != 0, slots