Add support for scalar arguments to xp.where (data-apis#78)

Reviewed at data-apis#78

array_api_strict/_searching_functions.py

@@ -2,7 +2,7 @@
 
 from ._array_object import Array
 from ._dtypes import _result_type, _real_numeric_dtypes, bool as _bool
-from ._flags import requires_data_dependent_shapes, requires_api_version
+from ._flags import requires_data_dependent_shapes, requires_api_version, get_array_api_strict_flags
 
 from typing import TYPE_CHECKING
 if TYPE_CHECKING:
@@ -90,20 +90,38 @@ def searchsorted(
     # x1 must be 1-D, but NumPy already requires this.
     return Array._new(np.searchsorted(x1._array, x2._array, side=side, sorter=sorter), device=x1.device)
 
-def where(condition: Array, x1: Array, x2: Array, /) -> Array:
+def where(
+    condition: Array,
+    x1: bool | int | float | complex | Array,
+    x2: bool | int | float | complex | Array, /
+) -> Array:
     """
     Array API compatible wrapper for :py:func:`np.where <numpy.where>`.
 
     See its docstring for more information.
     """
+    if get_array_api_strict_flags()['api_version'] > '2023.12':
+        num_scalars = 0
+
+        if isinstance(x1, (bool, float, complex, int)):
+            x1 = Array._new(np.asarray(x1), device=condition.device)
+            num_scalars += 1
+
+        if isinstance(x2, (bool, float, complex, int)):
+            x2 = Array._new(np.asarray(x2), device=condition.device)
+            num_scalars += 1
+
+        if num_scalars == 2:
+            raise ValueError("One of x1, x2 arguments must be an array.")
+
     # Call result type here just to raise on disallowed type combinations
     _result_type(x1.dtype, x2.dtype)
 
     if condition.dtype != _bool:
         raise TypeError("`condition` must be have a boolean data type")
 
     if len({a.device for a in (condition, x1, x2)}) > 1:
-        raise ValueError("where inputs must all be on the same device")
+        raise ValueError("Inputs to `where` must all use the same device")
 
     x1, x2 = Array._normalize_two_args(x1, x2)
     return Array._new(np.where(condition._array, x1._array, x2._array), device=x1.device)

array_api_strict/tests/test_searching_functions.py

@@ -0,0 +1,30 @@
+import pytest
+
+import array_api_strict as xp
+
+from array_api_strict import ArrayAPIStrictFlags
+from array_api_strict._flags import draft_version
+
+
+def test_where_with_scalars():
+    x = xp.asarray([1, 2, 3, 1])
+
+    # Versions up to and including 2023.12 don't support scalar arguments
+    with pytest.raises(AttributeError, match="object has no attribute 'dtype'"):
+        xp.where(x == 1, 42, 44)
+
+    # Versions after 2023.12 support scalar arguments
+    with (pytest.warns(
+              UserWarning,
+              match="The 2024.12 version of the array API specification is in draft status"
+          ),
+          ArrayAPIStrictFlags(api_version=draft_version),
+        ):
+        x_where = xp.where(x == 1, xp.asarray(42), 44)
+
+        expected = xp.asarray([42, 44, 44, 42])
+        assert xp.all(x_where == expected)
+
+        # The spec does not allow both x1 and x2 to be scalars
+        with pytest.raises(ValueError, match="One of"):
+            xp.where(x == 1, 42, 44)