data-apis · lucascolley · Jan 25, 2025 · Jan 21, 2025 · Jan 25, 2025 · Jan 25, 2025
diff --git a/src/array_api_extra/_lib/_at.py b/src/array_api_extra/_lib/_at.py
@@ -9,7 +9,12 @@
 from types import ModuleType
 from typing import ClassVar, cast
 
-from ._utils._compat import array_namespace, is_jax_array, is_writeable_array
+from ._utils._compat import (
+    array_namespace,
+    is_dask_array,
+    is_jax_array,
+    is_writeable_array,
+)
 from ._utils._typing import Array, Index
 
 
@@ -141,6 +146,25 @@
     not explicitly covered by ``array-api-compat``, are not supported by update
     methods.
 
+    Boolean masks are supported on Dask and jitted JAX arrays exclusively
+    when `idx` has the same shape as `x` and `y` is 0-dimensional.
+    Note that this support is not available in JAX's native
+    ``x.at[mask].set(y)``.
+
+    This pattern::
+
+        >>> mask = m(x)
+        >>> x[mask] = f(x[mask])
+
+    Can't be replaced by `at`, as it won't work on Dask and JAX inside jax.jit::
+
+        >>> mask = m(x)
+        >>> x = xpx.at(x, mask).set(f(x[mask])  # Crash on Dask and jax.jit
+
+    You should instead use::
+
+        >>> x = xp.where(m(x), f(x), x)
+
     Examples
     --------
     Given either of these equivalent expressions::
@@ -189,6 +213,7 @@
         self,
         at_op: _AtOp,
         in_place_op: Callable[[Array, Array | object], Array] | None,
+        out_of_place_op: Callable[[Array, Array], Array] | None,
         y: Array | object,
         /,
         copy: bool | None,
@@ -210,6 +235,16 @@
 
                 x[idx] = y
 
+        out_of_place_op : Callable[[Array, Array], Array] | None
+            Out-of-place operation to apply when idx is a boolean mask and the backend
+            doesn't support in-place updates::
+
+                x = xp.where(idx, out_of_place_op(x, y), x)
+
+            If None::
+
+                x = xp.where(idx, y, x)
+
         y : array or object
             Right-hand side of the operation.
         copy : bool or None
@@ -223,6 +258,7 @@
             Updated `x`.
         """
         x, idx = self._x, self._idx
+        xp = array_namespace(x, y) if xp is None else xp
 
         if idx is _undef:
             msg = (
@@ -247,15 +283,41 @@
         else:
             writeable = is_writeable_array(x)
 
+        # JAX inside jax.jit and Dask don't support in-place updates with boolean
+        # mask. However we can handle the common special case of 0-dimensional y
+        # with where(idx, y, x) instead.
+        if (
+            (is_dask_array(idx) or is_jax_array(idx))
+            and idx.dtype == xp.bool
+            and idx.shape == x.shape
+        ):
+            y_xp = xp.asarray(y, dtype=x.dtype)
+            if y_xp.ndim == 0:
+                if out_of_place_op:
+                    # FIXME: suppress inf warnings on dask with lazywhere
+                    out = xp.where(idx, out_of_place_op(x, y_xp), x)
+                    # Undo int->float promotion on JAX after _AtOp.DIVIDE
+                    out = xp.astype(out, x.dtype, copy=False)
+                else:
+                    out = xp.where(idx, y_xp, x)
+
+                if copy:
+                    return out
+                x[()] = out
+                return x
+            # else: this will work on eager JAX and crash on jax.jit and Dask
+
         if copy:
             if is_jax_array(x):
                 # Use JAX's at[]
                 func = cast(Callable[[Array], Array], getattr(x.at[idx], at_op.value))
-                return func(y)
+                out = func(y)
+                # Undo int->float promotion on JAX after _AtOp.DIVIDE
+                return xp.astype(out, x.dtype, copy=False)
+
             # Emulate at[] behaviour for non-JAX arrays
             # with a copy followed by an update
-            if xp is None:
-                xp = array_namespace(x)
+
             x = xp.asarray(x, copy=True)
             if writeable is False:
                 # A copy of a read-only numpy array is writeable
@@ -283,7 +345,7 @@
         xp: ModuleType | None = None,
     ) -> Array:  # numpydoc ignore=PR01,RT01
         """Apply ``x[idx] = y`` and return the update array."""
-        return self._op(_AtOp.SET, None, y, copy=copy, xp=xp)
+        return self._op(_AtOp.SET, None, None, y, copy=copy, xp=xp)
 
     def add(
         self,
@@ -297,7 +359,7 @@
         # Note for this and all other methods based on _iop:
         # operator.iadd and operator.add subtly differ in behaviour, as
         # only iadd will trigger exceptions when y has an incompatible dtype.
-        return self._op(_AtOp.ADD, operator.iadd, y, copy=copy, xp=xp)
+        return self._op(_AtOp.ADD, operator.iadd, operator.add, y, copy=copy, xp=xp)
 
     def subtract(
         self,
@@ -307,7 +369,9 @@
         xp: ModuleType | None = None,
     ) -> Array:  # numpydoc ignore=PR01,RT01
         """Apply ``x[idx] -= y`` and return the updated array."""
-        return self._op(_AtOp.SUBTRACT, operator.isub, y, copy=copy, xp=xp)
+        return self._op(
+            _AtOp.SUBTRACT, operator.isub, operator.sub, y, copy=copy, xp=xp
+        )
 
     def multiply(
         self,
@@ -317,7 +381,9 @@
         xp: ModuleType | None = None,
     ) -> Array:  # numpydoc ignore=PR01,RT01
         """Apply ``x[idx] *= y`` and return the updated array."""
-        return self._op(_AtOp.MULTIPLY, operator.imul, y, copy=copy, xp=xp)
+        return self._op(
+            _AtOp.MULTIPLY, operator.imul, operator.mul, y, copy=copy, xp=xp
+        )
 
     def divide(
         self,
@@ -327,7 +393,9 @@
         xp: ModuleType | None = None,
     ) -> Array:  # numpydoc ignore=PR01,RT01
         """Apply ``x[idx] /= y`` and return the updated array."""
-        return self._op(_AtOp.DIVIDE, operator.itruediv, y, copy=copy, xp=xp)
+        return self._op(
+            _AtOp.DIVIDE, operator.itruediv, operator.truediv, y, copy=copy, xp=xp
+        )
 
     def power(
         self,
@@ -337,7 +405,7 @@
         xp: ModuleType | None = None,
     ) -> Array:  # numpydoc ignore=PR01,RT01
         """Apply ``x[idx] **= y`` and return the updated array."""
-        return self._op(_AtOp.POWER, operator.ipow, y, copy=copy, xp=xp)
+        return self._op(_AtOp.POWER, operator.ipow, operator.pow, y, copy=copy, xp=xp)
 
     def min(
         self,
@@ -349,7 +417,7 @@
         """Apply ``x[idx] = minimum(x[idx], y)`` and return the updated array."""
         xp = array_namespace(self._x) if xp is None else xp
         y = xp.asarray(y)
-        return self._op(_AtOp.MIN, xp.minimum, y, copy=copy, xp=xp)
+        return self._op(_AtOp.MIN, xp.minimum, xp.minimum, y, copy=copy, xp=xp)
 
     def max(
         self,
@@ -361,4 +429,4 @@
         """Apply ``x[idx] = maximum(x[idx], y)`` and return the updated array."""
         xp = array_namespace(self._x) if xp is None else xp
         y = xp.asarray(y)
-        return self._op(_AtOp.MAX, xp.maximum, y, copy=copy, xp=xp)
+        return self._op(_AtOp.MAX, xp.maximum, xp.maximum, y, copy=copy, xp=xp)
diff --git a/src/array_api_extra/_lib/_utils/_compat.py b/src/array_api_extra/_lib/_utils/_compat.py
@@ -8,6 +8,7 @@
         device,
         is_array_api_strict_namespace,
         is_cupy_namespace,
+        is_dask_array,
         is_dask_namespace,
         is_jax_array,
         is_jax_namespace,
@@ -23,6 +24,7 @@
         device,
         is_array_api_strict_namespace,
         is_cupy_namespace,
+        is_dask_array,
         is_dask_namespace,
         is_jax_array,
         is_jax_namespace,
@@ -38,6 +40,7 @@
     "device",
     "is_array_api_strict_namespace",
     "is_cupy_namespace",
+    "is_dask_array",
     "is_dask_namespace",
     "is_jax_array",
     "is_jax_namespace",

diff --git a/src/array_api_extra/_lib/_utils/_compat.pyi b/src/array_api_extra/_lib/_utils/_compat.pyi
@@ -25,6 +25,7 @@ def is_jax_namespace(xp: ModuleType, /) -> bool: ...
 def is_numpy_namespace(xp: ModuleType, /) -> bool: ...
 def is_pydata_sparse_namespace(xp: ModuleType, /) -> bool: ...
 def is_torch_namespace(xp: ModuleType, /) -> bool: ...
+def is_dask_array(x: object, /) -> bool: ...
 def is_jax_array(x: object, /) -> bool: ...
 def is_writeable_array(x: object, /) -> bool: ...
 def size(x: Array, /) -> int | None: ...
diff --git a/tests/test_at.py b/tests/test_at.py
@@ -1,3 +1,4 @@
+import math
 import pickle
 from collections.abc import Callable, Generator
 from contextlib import contextmanager
@@ -15,6 +16,12 @@
 from array_api_extra._lib._utils._typing import Array, Index
 from array_api_extra.testing import lazy_xp_function
 
+pytestmark = [
+    pytest.mark.skip_xp_backend(
+        Backend.SPARSE, reason="read-only backend without .at support"
+    )
+]
+
 
 def at_op(  # type: ignore[no-any-explicit]
     x: Array,
@@ -70,9 +77,6 @@ def assert_copy(array: Array, copy: bool | None) -> Generator[None, None, None]:
     xp_assert_equal(xp.all(array == array_orig), xp.asarray(copy))
 
 
-@pytest.mark.skip_xp_backend(
-    Backend.SPARSE, reason="read-only backend without .at support"
-)
 @pytest.mark.parametrize(
     ("kwargs", "expect_copy"),
     [
@@ -100,14 +104,7 @@ def assert_copy(array: Array, copy: bool | None) -> Generator[None, None, None]:
     [
         (False, False),
         (False, True),
-        pytest.param(
-            True,
-            False,
-            marks=(
-                pytest.mark.skip_xp_backend(Backend.JAX, reason="TODO special case"),
-                pytest.mark.skip_xp_backend(Backend.DASK, reason="TODO special case"),
-            ),
-        ),
+        (True, False),  # Uses xp.where(idx, y, x) on JAX and Dask
         pytest.param(
             True,
             True,
@@ -176,22 +173,72 @@ def test_alternate_index_syntax():
         at(a, 0)[0].set(4)
 
 
-@pytest.mark.parametrize("copy", [True, False])
-@pytest.mark.parametrize(
-    "op", [_AtOp.ADD, _AtOp.SUBTRACT, _AtOp.MULTIPLY, _AtOp.DIVIDE, _AtOp.POWER]
-)
-def test_iops_incompatible_dtype(op: _AtOp, copy: bool):
+@pytest.mark.parametrize("copy", [True, None])
+@pytest.mark.parametrize("bool_mask", [False, True])
+@pytest.mark.parametrize("op", list(_AtOp))
+def test_incompatible_dtype(
+    xp: ModuleType, library: Backend, op: _AtOp, copy: bool | None, bool_mask: bool
+):
     """Test that at() replicates the backend's behaviour for
     in-place operations with incompatible dtypes.
 
-    Note:
+    Behavior is backend-specific, but only two behaviors are allowed:
+    1. raise an exception, or
+    2. return the same dtype as x, disregarding y.dtype (no broadcasting).
+
+    Note that __i<op>__ and __<op>__ behave differently, and we want to
+    replicate the behavior of __i<op>__:
+
     >>> a = np.asarray([1, 2, 3])
     >>> a / 1.5
     array([0.        , 0.66666667, 1.33333333])
     >>> a /= 1.5
     UFuncTypeError: Cannot cast ufunc 'divide' output from dtype('float64')
     to dtype('int64') with casting rule 'same_kind'
     """
-    x = np.asarray([2, 4])
-    with pytest.raises(TypeError, match="Cannot cast ufunc"):
-        at_op(x, slice(None), op, 1.1, copy=copy)
+    x = xp.asarray([2, 4])
+    idx = xp.asarray([True, False]) if bool_mask else slice(None)
+    z = None
+
+    if library is Backend.JAX:
+        if bool_mask:
+            z = at_op(x, idx, op, 1.1, copy=copy)
+        else:
+            with pytest.warns(FutureWarning, match="cannot safely cast"):
+                z = at_op(x, idx, op, 1.1, copy=copy)
+
+    elif library is Backend.DASK:
+        if op in (_AtOp.MIN, _AtOp.MAX):
+            pytest.xfail(reason="need array-api-compat 1.11")
+        z = at_op(x, idx, op, 1.1, copy=copy)
+
+    elif library is Backend.ARRAY_API_STRICT and op is not _AtOp.SET:
+        with pytest.raises(Exception, match=r"cast|promote|dtype"):
+            at_op(x, idx, op, 1.1, copy=copy)
+
+    elif op in (_AtOp.SET, _AtOp.MIN, _AtOp.MAX):
+        # There is no __i<op>__ version of these operations
+        z = at_op(x, idx, op, 1.1, copy=copy)
+
+    else:
+        with pytest.raises(Exception, match=r"cast|promote|dtype"):
+            at_op(x, idx, op, 1.1, copy=copy)
+
+    assert z is None or z.dtype == x.dtype
+
+
+def test_bool_mask_nd(xp: ModuleType):
+    x = xp.asarray([[1, 2, 3], [4, 5, 6]])
+    idx = xp.asarray([[True, False, False], [False, True, True]])
+    z = at_op(x, idx, _AtOp.SET, 0)
+    xp_assert_equal(z, xp.asarray([[0, 2, 3], [4, 0, 0]]))
+
+
+@pytest.mark.skip_xp_backend(Backend.DASK, reason="FIXME need scipy's lazywhere")
+@pytest.mark.parametrize("bool_mask", [False, True])
+def test_no_inf_warnings(xp: ModuleType, bool_mask: bool):
+    x = xp.asarray([math.inf, 1.0, 2.0])
+    idx = ~xp.isinf(x) if bool_mask else slice(1, None)
+    # inf - inf -> nan with a warning
+    z = at_op(x, idx, _AtOp.SUBTRACT, math.inf)
+    xp_assert_equal(z, xp.asarray([math.inf, -math.inf, -math.inf]))
diff --git a/vendor_tests/test_vendor.py b/vendor_tests/test_vendor.py
@@ -7,6 +7,7 @@ def test_vendor_compat():
         array_namespace,
         device,
         is_cupy_namespace,
+        is_dask_array,
         is_dask_namespace,
         is_jax_array,
         is_jax_namespace,
@@ -20,6 +21,7 @@ def test_vendor_compat():
     assert array_namespace(x) is xp
     device(x)
     assert not is_cupy_namespace(xp)
+    assert not is_dask_array(x)
     assert not is_dask_namespace(xp)
     assert not is_jax_array(x)
     assert not is_jax_namespace(xp)