Add a test that there are no hard dependencies

docs/dev/special-considerations.md

@@ -11,7 +11,8 @@ should be taken into account:
   PyTorch, etc., it does not hard depend on them. These libraries are not
   imported unless either an array object is passed to
   {func}`~.array_namespace()`, or the specific `array_api_compat.<namespace>`
-  sub-namespace is explicitly imported.
+  sub-namespace is explicitly imported. This is tested (as best as possible)
+  in `tests/test_no_dependencies.py`.
 
 - **Vendorability.** array-api-compat should be [vendorable](vendoring). This
   means that, for instance, all imports in the library are relative imports.

requirements-dev.txt

@@ -1,6 +1,6 @@
+array-api-strict
 dask[array]
 jax[cpu]
 numpy
 pytest
 torch
-setuptools

tests/test_no_dependencies.py

@@ -0,0 +1,72 @@
+"""
+Test that array_api_compat has no "hard" dependencies.
+
+Libraries like NumPy should only be imported if a numpy array is passed to
+array_namespace or if array_api_compat.numpy is explicitly imported.
+
+We have to test this in a subprocess because these libraries have already been
+imported from the other tests.
+"""
+
+import sys
+import subprocess
+
+from ._helpers import import_
+
+import pytest
+
+class Array:
+    # Dummy array namespace that doesn't depend on any array library
+    def __array_namespace__(self, api_version=None):
+        class Namespace:
+            pass
+        return Namespace()
+
+def _test_dependency(mod):
+    assert mod not in sys.modules
+
+    # Run various functions that shouldn't depend on mod and check that they
+    # don't import it.
+
+    import array_api_compat
+    assert mod not in sys.modules
+
+    a = Array()
+
+    # array-api-strict is an example of an array API library that isn't
+    # wrapped by array-api-compat.
+    if "strict" not in mod:
+        is_mod_array = getattr(array_api_compat, f"is_{mod.split('.')[0]}_array")
+        assert not is_mod_array(a)
+        assert mod not in sys.modules
+
+    is_array_api_obj = getattr(array_api_compat, "is_array_api_obj")
+    assert is_array_api_obj(a)
+    assert mod not in sys.modules
+
+    array_namespace = getattr(array_api_compat, "array_namespace")
+    array_namespace(Array())
+    assert mod not in sys.modules
+
+# TODO: Test that wrapper for library X doesn't depend on wrappers for library
+# Y (except most array libraries actually do themselves depend on numpy).
+
+@pytest.mark.parametrize("library", ["cupy", "numpy", "torch", "dask.array",
+                                     "jax.numpy", "array_api_strict"])
+def test_numpy_dependency(library):
+    # This unfortunately won't go through any of the pytest machinery. We
+    # reraise the exception as an AssertionError so that pytest will show it
+    # in a semi-reasonable way
+
+    # Import (in this process) to make sure 'library' is actually installed and
+    # so that cupy can be skipped.
+    import_(library)
+
+    try:
+        subprocess.run([sys.executable, '-c', f'''\
+from tests.test_no_dependencies import _test_dependency
+
+_test_dependency({library!r})'''], check=True, capture_output=True, encoding='utf-8')
+    except subprocess.CalledProcessError as e:
+        print(e.stdout, end='')
+        raise AssertionError(e.stderr)