Review _array_expr_enabled()

crusaderky · crusaderky · commit 97de0f39b2af · 2026-01-05T19:01:44.000Z
diff --git a/dask/array/__init__.py b/dask/array/__init__.py
@@ -13,22 +13,29 @@ def _array_expr_enabled() -> builtins.bool:
 
     global ARRAY_EXPR_ENABLED
 
-    use_array_expr = dask.config.get("array.query-planning")
+    use_array_expr: builtins.bool | None = dask.config.get("array.query-planning")
+    if use_array_expr is None:
+        use_array_expr = False  # Eventually, this default will flip to True
+    if not isinstance(use_array_expr, builtins.bool):
+        # Guard against "false" string in YAML config, which would evaluate to True
+        raise TypeError(  # pragma: no cover
+            "The 'array.query-planning' config must be True, False, or None"
+        )
 
-    if ARRAY_EXPR_ENABLED is not None:
-        if (use_array_expr is True and ARRAY_EXPR_ENABLED is False) or (
-            use_array_expr is False and ARRAY_EXPR_ENABLED is True
-        ):
-            warnings.warn(
-                "The 'array.query-planning' config is now set to "
-                f"{use_array_expr}, but query planning is already "
-                f"{'enabled' if ARRAY_EXPR_ENABLED else 'disabled'}. "
-                "The query-planning config can only be changed before "
-                "`dask.array` is first imported!"
-            )
-        return ARRAY_EXPR_ENABLED
+    if ARRAY_EXPR_ENABLED is None:
+        ARRAY_EXPR_ENABLED = use_array_expr
+        return use_array_expr
 
-    return builtins.bool(use_array_expr if use_array_expr is not None else False)
+    if use_array_expr != ARRAY_EXPR_ENABLED:
+        warnings.warn(
+            "The 'array.query-planning' config is now set to "
+            f"{use_array_expr}, but query planning is already "
+            f"{'enabled' if ARRAY_EXPR_ENABLED else 'disabled'}. "
+            "The query-planning config can only be changed before "
+            "`dask.array` is first imported!",
+            RuntimeWarning,
+        )
+    return ARRAY_EXPR_ENABLED
 
 
 def array_expr_enabled() -> builtins.bool:
diff --git a/dask/array/_array_expr/tests/test_config.py b/dask/array/_array_expr/tests/test_config.py
@@ -0,0 +1,29 @@
+import pytest
+
+import dask.config
+from dask.array import _array_expr_enabled, array_expr_enabled
+
+pytestmark = pytest.mark.normal_and_array_expr
+
+
+def test_array_expr_enabled():
+    assert _array_expr_enabled() in (True, False)
+    assert array_expr_enabled() is _array_expr_enabled()
+
+
+def test_flip_config():
+    """Config changes after the initial import are ignored with a warning"""
+    prev = _array_expr_enabled()
+    with dask.config.set({"array.query-planning": not prev}):
+        with pytest.warns(RuntimeWarning, match="first imported"):
+            actual = _array_expr_enabled()
+    assert actual == prev
+
+
+def test_str_config():
+    """Prevent accidental use of 'false' string in YAML config,
+    which would evaluate to True.
+    """
+    with dask.config.set({"array.query-planning": "false"}):
+        with pytest.raises(TypeError):
+            _array_expr_enabled()
