Add raise_if_computes for testing

cubed/core/array.py

@@ -24,6 +24,10 @@ def gensym(name="array"):
     return f"{name}-{sym_counter:03}"
 
 
+# see cubed.testing
+compute_should_raise = False
+
+
 # type representing either a CoreArray or a public-facing Array
 T_ChunkedArray = TypeVar("T_ChunkedArray", bound="CoreArray")
 
@@ -269,6 +273,8 @@ def compute(
         If True, intermediate arrays that have already been computed won't be
         recomputed. Default is False.
     """
+    if compute_should_raise:
+        raise RuntimeError("'compute' was called")
     spec = check_array_specs(arrays)  # guarantees all arrays have same spec
     plan = arrays_to_plan(*arrays)
     if executor is None:

cubed/testing.py

@@ -0,0 +1,13 @@
+from contextlib import contextmanager
+
+from cubed.core import array
+
+
+@contextmanager
+def raise_if_computes():
+    """Returns a context manager for testing that ``compute`` is not called."""
+    array.compute_should_raise = True
+    try:
+        yield
+    finally:
+        array.compute_should_raise = False

cubed/tests/test_testing.py

@@ -0,0 +1,21 @@
+import numpy as np
+import pytest
+from numpy.testing import assert_array_equal
+
+import cubed.array_api as xp
+from cubed.testing import raise_if_computes
+
+
+def test_raise_if_computes():
+    # shouldn't raise since compute has not been called
+    with raise_if_computes():
+        a = xp.ones((3, 3), chunks=(2, 2))
+        b = xp.negative(a)
+
+    # should raise since compute is called
+    with pytest.raises(RuntimeError):
+        with raise_if_computes():
+            b.compute()
+
+    # shouldn't raise since we are outside the context manager
+    assert_array_equal(b.compute(), -np.ones((3, 3)))