ENH: Adding default_xp context manager for xp_assert functions

src/array_api_extra/_lib/_testing.py

@@ -8,6 +8,9 @@
 from __future__ import annotations
 
 import math
+from collections.abc import Generator
+from contextlib import contextmanager
+from contextvars import ContextVar
 from types import ModuleType
 from typing import Any, cast
 
@@ -30,13 +33,53 @@
 
 __all__ = ["as_numpy_array", "xp_assert_close", "xp_assert_equal", "xp_assert_less"]
 
+_default_xp_ctxvar: ContextVar[ModuleType] = ContextVar("_default_xp")
+
+
+@contextmanager
+def default_xp(xp: ModuleType) -> Generator[None, None, None]:
+    """In all ``xp_assert_*`` function calls executed within this
+    context manager, test by default that the array namespace is
+    the provided across all arrays, unless one explicitly passes the ``xp=``
+    parameter.
+
+    Without this context manager, the default value for `xp` is the namespace
+    for the desired array (the second parameter of the tests).
+    """
+    token = _default_xp_ctxvar.set(xp)
+    try:
+        yield
+    finally:
+        _default_xp_ctxvar.reset(token)
+
+
+def _assert_matching_namespace(actual: Array, desired: Array, xp: ModuleType) -> None:
+    desired_arr_space = array_namespace(desired)
+    _msg = (
+        "Namespace of desired array does not match expectations "
+        "set by the `default_xp` context manager or by the `xp`"
+        "pytest fixture.\n"
+        f"Desired array's space: {desired_arr_space.__name__}\n"
+        f"Expected namespace: {xp.__name__}"
+    )
+    assert desired_arr_space == xp, _msg
+
+    actual_arr_space = array_namespace(actual)
+    _msg = (
+        "Namespace of actual and desired arrays do not match.\n"
+        f"Actual: {actual_arr_space.__name__}\n"
+        f"Desired: {xp.__name__}"
+    )
+    assert actual_arr_space == xp, _msg
+
 
 def _check_ns_shape_dtype(
     actual: Array,
     desired: Array,
     check_dtype: bool,
     check_shape: bool,
     check_scalar: bool,
+    xp: ModuleType | None = None,
 ) -> tuple[Array, Array, ModuleType]:  # numpydoc ignore=RT03
     """
     Assert that namespace, shape and dtype of the two arrays match.
@@ -60,8 +103,12 @@ def _check_ns_shape_dtype(
     actual_xp = array_namespace(actual)  # Raises on Python scalars and lists
     desired_xp = array_namespace(desired)
 
-    msg = f"namespaces do not match: {actual_xp} != f{desired_xp}"
-    assert actual_xp == desired_xp, msg
+    if xp is None:
+        try:
+            xp = _default_xp_ctxvar.get()
+        except LookupError:
+            xp = array_namespace(desired)
+    _assert_matching_namespace(actual, desired, xp)
 
     if is_numpy_namespace(actual_xp) and check_scalar:
         # only NumPy distinguishes between scalars and arrays; we do if check_scalar.
@@ -148,6 +195,7 @@ def xp_assert_equal(
     check_dtype: bool = True,
     check_shape: bool = True,
     check_scalar: bool = False,
+    xp: ModuleType | None = None,
 ) -> None:
     """
     Array-API compatible version of `np.testing.assert_array_equal`.
@@ -174,7 +222,7 @@ def xp_assert_equal(
     numpy.testing.assert_array_equal : Similar function for NumPy arrays.
     """
     actual, desired, xp = _check_ns_shape_dtype(
-        actual, desired, check_dtype, check_shape, check_scalar
+        actual, desired, check_dtype, check_shape, check_scalar, xp
     )
     if not _is_materializable(actual):
         return
@@ -194,6 +242,7 @@ def xp_assert_less(
     check_dtype: bool = True,
     check_shape: bool = True,
     check_scalar: bool = False,
+    xp: ModuleType | None = None,
 ) -> None:
     """
     Array-API compatible version of `np.testing.assert_array_less`.
@@ -217,7 +266,7 @@ def xp_assert_less(
     xp_assert_close : Similar function for inexact equality checks.
     numpy.testing.assert_array_equal : Similar function for NumPy arrays.
     """
-    x, y, xp = _check_ns_shape_dtype(x, y, check_dtype, check_shape, check_scalar)
+    x, y, xp = _check_ns_shape_dtype(x, y, check_dtype, check_shape, check_scalar, xp)
     if not _is_materializable(x):
         return
     x_np = as_numpy_array(x, xp=xp)
@@ -237,6 +286,7 @@ def xp_assert_close(
     check_dtype: bool = True,
     check_shape: bool = True,
     check_scalar: bool = False,
+    xp: ModuleType | None = None,
 ) -> None:
     """
     Array-API compatible version of `np.testing.assert_allclose`.
@@ -276,7 +326,7 @@ def xp_assert_close(
     Array arguments to `atol` and `rtol` must be valid input to :py:func:`float`.
     """
     actual, desired, xp = _check_ns_shape_dtype(
-        actual, desired, check_dtype, check_shape, check_scalar
+        actual, desired, check_dtype, check_shape, check_scalar, xp
     )
     if not _is_materializable(actual):
         return

tests/test_testing.py

@@ -9,6 +9,7 @@
 from array_api_extra._lib._backends import Backend
 from array_api_extra._lib._testing import (
     as_numpy_array,
+    default_xp,
     xp_assert_close,
     xp_assert_equal,
     xp_assert_less,
@@ -63,12 +64,22 @@ def test_shape_dtype(self, xp: ModuleType, func: Callable[..., None]):
     )
     @pytest.mark.parametrize("func", [xp_assert_equal, xp_assert_close, xp_assert_less])
     def test_namespace(self, xp: ModuleType, func: Callable[..., None]):
-        with pytest.raises(AssertionError, match="namespaces do not match"):
+        with pytest.raises(
+            AssertionError, match="Namespace of actual and desired arrays do not match"
+        ):
             func(xp.asarray(0), np.asarray(0))
         with pytest.raises(TypeError, match=r"array_namespace requires .* array input"):
             func(xp.asarray(0), 0)
         with pytest.raises(TypeError, match="list is not a supported array type"):
             func(xp.asarray([0]), [0])
+        with (
+            default_xp(np),
+            pytest.raises(
+                AssertionError,
+                match="Namespace of desired array does not match expectations",
+            ),
+        ):
+            func(xp.asarray(0), xp.asarray(0))
 
     @pytest.mark.parametrize("func", [xp_assert_equal, xp_assert_close, xp_assert_less])
     def test_check_shape(self, xp: ModuleType, func: Callable[..., None]):