ENH: Test for read-only arrays

array_api_compat/__init__.py

@@ -1,9 +1,9 @@
 """
 NumPy Array API compatibility library
 
-This is a small wrapper around NumPy and CuPy that is compatible with the
-Array API standard https://data-apis.org/array-api/latest/. See also NEP 47
-https://numpy.org/neps/nep-0047-array-api-standard.html.
+This is a small wrapper around NumPy, CuPy, JAX, sparse and others that are
+compatible with the Array API standard https://data-apis.org/array-api/latest/.
+See also NEP 47 https://numpy.org/neps/nep-0047-array-api-standard.html.
 
 Unlike array_api_strict, this is not a strict minimal implementation of the
 Array API, but rather just an extension of the main NumPy namespace with

array_api_compat/common/_helpers.py

@@ -787,6 +787,7 @@ def to_device(x: Array, device: Device, /, *, stream: Optional[Union[int, Any]]
         return x
     return x.to_device(device, stream=stream)
 
+
 def size(x):
     """
     Return the total number of elements of x.
@@ -801,6 +802,23 @@ def size(x):
         return None
     return math.prod(x.shape)
 
+
+def is_writeable_array(x) -> bool:
+    """
+    Return False if ``x.__setitem__`` is expected to raise; True otherwise.
+
+    Warning
+    -------
+    As there is no standard way to check if an array is writeable without actually
+    writing to it, this function blindly returns True for all unknown array types.
+    """
+    if is_numpy_array(x):
+        return x.flags.writeable
+    if is_jax_array(x) or is_pydata_sparse_array(x):
+        return False
+    return True
+
+
 __all__ = [
     "array_namespace",
     "device",
@@ -821,6 +839,7 @@ def size(x):
     "is_ndonnx_namespace",
     "is_pydata_sparse_array",
     "is_pydata_sparse_namespace",
+    "is_writeable_array",
     "size",
     "to_device",
 ]

docs/helper-functions.rst

@@ -51,6 +51,7 @@ yet.
 .. autofunction:: is_jax_array
 .. autofunction:: is_pydata_sparse_array
 .. autofunction:: is_ndonnx_array
+.. autofunction:: is_writeable_array
 .. autofunction:: is_numpy_namespace
 .. autofunction:: is_cupy_namespace
 .. autofunction:: is_torch_namespace

tests/test_common.py

@@ -5,7 +5,7 @@
     is_dask_namespace, is_jax_namespace, is_pydata_sparse_namespace,
 )
 
-from array_api_compat import is_array_api_obj, device, to_device
+from array_api_compat import device, is_array_api_obj, is_writeable_array, to_device
 
 from ._helpers import import_, wrapped_libraries, all_libraries
 
@@ -74,6 +74,24 @@ def test_xp_is_array_generics(library):
     assert matches in ([library], ["numpy"])
 
 
+@pytest.mark.parametrize("library", all_libraries)
+def test_is_writeable_array(library):
+    lib = import_(library)
+    x = lib.asarray([1, 2, 3])
+    if is_writeable_array(x):
+        x[1] = 4
+    else:
+        with pytest.raises((TypeError, ValueError)):
+            x[1] = 4
+
+
+def test_is_writeable_array_numpy():
+    x = np.asarray([1, 2, 3])
+    assert is_writeable_array(x)
+    x.flags.writeable = False
+    assert not is_writeable_array(x)
+
+
 @pytest.mark.parametrize("library", all_libraries)
 def test_device(library):
     xp = import_(library, wrapper=True)